Individual Differences and their Implications for Theories of Language Development

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INDIVIDUAL DIFFERENCES AND THEIR IMPLICATIONS
FOR THEORIES OF LANGUAGE DEVELOPMENT
Elizabeth Bates
University of California, San Diego
Philip S. Dale
University of Washington
Donna Thal
San Diego State University
Chapter 4 for Paul Fletcher & Brian MacWhinney (Eds.), Handbook of Child Language.
Oxford: Basil Blackwell, 1995.
Introduction
Like every other aspect of human development,
language development is characterized by variation.
Historically this variation has been largely ignored by
students of child language, who have concentrated on
the remarkable similarities in sequence of development
that are usually observed across children acquiring a
given. language Individual differences in rate of
development and individual differences in learning
style have been left to applied practitioners such as
speech pathologists and special educators. We believe
it is no accident that these professionals, concerned
with such important questions as the definition of
abnormality, the relationship of language to nonverbal
cognition, and the role of environmental variables, have
found it essential to focus on variation.
It is our contention that quantitative and qualitative
variations within and across components of early
language are also relevant, indeed essential, if we want
to understand the mechanisms that underlie normal
language development. Far from simply reflecting
noise in our measuring instruments or variability in
low-level aspects of physiological maturation, the
variations that we will document here are substantial,
stable, and have their own developmental course.
Because this variation is substantial, it is critical for
defining the boundary between normal and abnormal
development; because it is stable, it provides a window
onto the correlates and (by inference) the causes of
developmental change; and because it has its own
developmental course, it can be used to pinpoint critical
developmental transitions that form the basis for
theories of learning and change.
Although we are well aware of the clinical
applications that hinge on an adequate assessment of
normal variation (i.e. one cannot define “abnormal”
without an adequate definition of “normal”), our
primary goal here will be an exploration of the
implications of individual differences for theories of
normal language. We will concentrate on the early
stages of language learning, from the onset of word
comprehension (around eight to ten months of age) to
the onset of grammar (from 20–36 months). This is the
period in which the most dramatic changes in language
ability are observed, phenomena which have been
amply documented in small- and large-sample studies.
It is also a period characterized by dramatic events in
postnatal brain development (e.g. synaptogenesis),
which means that biological factors may play a
particularly important role in those aspects of language
that change at the same time (Bates, Thal, and
Janowsky, 1992). For these reasons, we stand a good
chance of discovering something interesting about the
interplay of biology and environment. The chapter is
divided into four parts, as follows:
1. Variations in rate within components of early
language. In this section we will review evidence for
variations in speed of development in word
comprehension, word production, first word
combinations, and the first stages of grammar. As we
shall see, there are enormous individual differences in
onset time and rate of growth in each of these
components, variations large enough to challenge and
constrain the notion of a universal bioprogram
(Bickerton, 1984) or a universal maturational timetable

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for early language development (Lenneberg, 1967).
Such maturational accounts are insufficient, because the
linguistic variations that we observe in perfectly healthy
children are so much larger than the variations that are
usually observed in other maturational milestones like
crawling or walking. At the same time, environmental
variables (at least those that have been examined to
date) appear to account for only a modest proportion of
the variance observed in early comprehension and
production of language. At the risk of inviting
accusations of radical centrism, we conclude that the
variations observed in early language development are
so large that they require substantial contributions from
both genetic and environmental factors, with special
emphasis on their interaction.
2. Dissociations between components of early
language. Having demonstrated large-scale variation in
rate of development within individual components, we
can go on to ask about the degree of association or
dissociation in rate of development that is observed
between those components. In this section, we will
look for evidence of developmental asynchrony
between comprehension and production, and between
lexical production and grammar. The purpose of this
investigation is to locate the seams and joints of the
language processor, i.e. components that can develop at
different rates because they depend on different
cognitive and/or neural mechanisms. Hence this sec-
tion has implications for the hotly contested issues of
modularity and the autonomy or interdependence of
linguistic and cognitive systems (Fodor, 1983).
3. Variations in learning style. Continuing our
search for the seams and joints of the language
processor, we will move on in this section to a brief
review of evidence for qualitative variations in learning
style (aka. “referential v. expressive style”, “nominal v.
pronominal style”, “analytic v. holistic style”). We end
by concluding that stylistic variation is the emergent
property of quantitative variations in the information-
processing mechanisms that all children must have for
successful language learning.
4. Atypical populations: variation at the extremes
of the normal range. Finally, we will review evidence
on the same three themes (rate, dissociations and style)
in the early stages of development for several quite
different atypical populations: early talkers (a
nonclinical but very unusual group), late talkers (many
of whom go on to qualify for a diagnosis of specific
language impairment), children with focal brain injury
(to provide insights into the neural mechanisms that
underlie individual differences in early language), and
children with contrasting forms of mental retardation
(i.e. Williams Syndrome, where language eventually
moves ahead of many other cognitive domains; Down
Syndrome, where language levels often fall behind
mental as well as chronological age). This will be a
very brief review of a large topic, but it will help to
round out our understanding of the mechanisms
involved in early language development, across the
period from first words to grammar (Bates, Bretherton
and Snyder, 1988). We will conclude that most of the
variations observed in atypical populations represent
extensions of the variations that are also observed in the
normal range.
1. Variations in Rate
We will start with variations in rate of development
within individual components, a form of variation that
is (at least in principle) easy to define and quantify. In
fact, this apparently simple form of measurement poses
a substantial methodological problem. Estimations of
variability, even more than estimations of central
tendency, require a substantial sample size. For
obvious reasons, this is generally not possible for
studies of child language, which are exceptionally time
and labor intensive. The great majority of research
studies have included fewer than 25 subjects; and many
of the most influential have been far smaller, i.e. single-
case studies (e.g. Leopold, 1949) or studies of three or
four children (e.g. Bloom, 1970; Brown, 1973). Even
studies nominally focussed on individual differences
have continued the tradition of small samples or single-
case studies (e.g. Peters, 1983). As illuminating as
these studies have been in defining those patterns of
individual variation that are possible, the extent and
nature of such variation will remain controversial until
large samples are available. For this reason, we will
concentrate here on a single study with a uniquely large
sample of more than 1,800 children: the norming study
for the MacArthur Communicative Development
Inventories (Fenson, Dale, Reznick, Thal, Bates,
Hartung, Pethick, and Reilly, 1993; Bates, Marchman,
Thal, Fenson, Dale, Reznick, Reilly, and Hartung,
1994; Marchman and Bates, 1994; Fenson, Bates, Dale,
Thal and Reznick, 1994). These results are based on
two parental report instruments (CDI:Infants, for
children 8–16 months, and CDI:Toddlers, for children
16–30 months) that have been developed over a period
of more than 15 years. A variety of studies have
demonstrated the reliability and validity of this
instrument and its immediate precedessors (Dale, Bates,
Reznick, and Morisset, 1989; Dale, 1991; Camaioni,
Caselli, Longobardi, and Volterra, 1991). For example,
the vocabulary checklists correlate positively and
significantly with laboratory assessments (both standard
tests and free speech) with coefficients ranging from
+0.40 to +0.80; the grammatical complexity scale
correlates with laboratory measures of Mean Length of
Utterance at +0.88 at 20 months and +0.76 at 24
months. In hindsight, this high validity is hardly
surprising. Parents have a far larger dataset than
researchers or clinicians can ever hope to assemble; it is
also far more representative of the child's ability, as it is
based on the child's behavior in a wide range of

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situations which call for an equally wide range of
language skills.
There are, of course, limitations to the kind of
information that can be obtained with parent report. As
Bates et al. (1994) acknowledge,
We can say nothing here about phonological
development (e.g. segmental v. suprasegmental
approaches to the analysis of speech), nor about the
frequency with which children use particular
vocabulary types (i.e. type/token relations). We
cannot distinguish between imitations and
spontaneous speech, nor can we specify the range
of contexts in which individual lexical items are
used (e.g. flexible and productive use v.
memorized frames). However, we can provide an
exceptionally clear view of developmental changes
from 8 to 30 months of age, and we can establish
the boundaries of variation.......within and across
levels of development.
As long we keep these variations firmly in mind, and
make no attempts to generalize beyond the factors that
can be studied reliably and accurately with parental
report, a data base of this kind can be extraordinarily
useful.
Methodology
Parents of 1,803 children between eight and 30 months
participated in a norming study for these inventories,
conducted in San Diego, Seattle, and New Haven
(Fenson et al., 1993). Parents of 673 children between
8–16 months completed the CDI:Infants; parents of
another 1,130 children, between 16–30 months,
completed the CDI:Toddlers. A minimum of 30 males
and 30 females are represented at each age level.
Children with serious health problems or extensive
exposure to a language other than English were
excluded from the study, and are not included in the
numbers just listed. The sample includes a wide
socioeconomic range, although it is heavily weighted
toward families in the middle class (e.g. parents with at
least a high school education). For the present
discussion, we focus on two core subscales from the
CDI:Infants (word comprehension, word production)
and three subscales from the CDI:Toddlers (word
production, onset of word combinations, and
grammatical complexity).
Parents of 500 children in the original sample also
completed a second inventory approximately six weeks
later. Parents of another 503 children completed a
second inventory approximately 6.5 months later. Of
this latter group, 62 parents of children in the Infant
sample completed the CDI:Infants a second time;
parents of 217 children in the Infant sample completed
the CDI:Toddlers; and parents of 224 children in the
Toddler sample completed the CDI:Toddlers a second
time. This information was used to assess the cross-age
stability of parental report.
The CDI:Infants includes a 396-item vocabulary
checklist organized into 19 semantic categories. Ten of
these categories comprise nouns (animal names,
vehicles, toys, food and drink, clothing, body parts,
furniture and rooms, small household items, outside
things and places to go, and people). Additional
categories are included for sound effects and animal
sounds, games and routines, verbs, adjectives,
pronouns, question words, prepositions and locations,
quantifiers, and words about time. Parents are asked to
indicate which words the child understands
(comprehension) and which words the child says and
understands (production). Note that we have excluded
a third theoretically plausible category, i.e. words that
the child says but does not understand. This decision
reflects our discovery and acknowledgment of an
important limitation of parental report. In earlier
versions of the CDI, we asked parents to distinguish
between words that the child imitates without
comprehension, and words that are produced
spontaneously and productively. Our results made it
clear that parents find it difficult to make a distinction
of this kind; indeed, most parents operate under the
assumption that production reflects understanding. We
have built that parental assumption into the final
version of the CDI, but we realize that there is no way
to win on this matter. Degree of productivity is a subtle
dimension that must be studied with a different
methodology, including in-depth observations of
language use and context and detailed interviews with
parents that elicit information about the contexts in
which words are used (see Snyder, Bates and
Bretherton, 1981 and Bates et al., 1988, for results
using the interview technique).
The CDI:Toddlers includes a 680-word vocabulary
checklist, organized into 22 semantic categories. The
larger number of categories on the toddler form is a
result of two sections (helping verbs, and connecting
words) and the division of outside things and places
into separate sections. As vocabulary becomes larger, it
is no longer possible for parents to monitor
comprehension vocabulary; they are asked only to
indicate use (production). The second part of the
toddler form is designed to assess morphological and
syntactic development. Only two measures from this
part will be discussed here. Parents are asked if their
child is combining words; they can respond “not yet,”
“sometimes,” and “often.” If they respond
“sometimes” or “often,” they proceed to a set of 37
forced-choice recognition items in which they choose
the member of each pair that best reflects their child's
current level of language use (“In each of the following
pairs, please mark the one that sounds MOST like the
way your child talks right now”). The 37 items include
contrasts in the use of bound morphemes (e.g. "Daddy
car" v. “Daddy's car”), functors (e.g. “Kitty sleeping” v.

4
“Kitty is sleeping”), and early-emerging complex
sentence forms (e.g. “Baby crying” v. “Baby crying cuz
she's sad”).
Stability of individual differences
Before turning to evidence for the substantial variation
in rate of development observed in early child
language, we should review evidence for the
stability/reliability of this variation from the six-month
longitudinal data collected in the norming study. To
evaluate continuity, we applied a very stringent
multiple regression analysis, controlling age (a rough
index of maturational status), gender (reflecting a
combination of biological and cultural factors), and six
family and social class variables (birth order, SES,
mother's education, father's education, mother's
occupation, and father's occupation), before the Time 1
measure was entered as a predictor of the corresponding
Time 2 measure. Five such analyses were performed:
Infant–Infant Comprehension; Infant-Infant Production;
Infant-Toddler Production; Toddler–Toddler
Production; and Toddler–Toddler Sentence
Complexity. In every case the earlier measure was a
highly significant (p < .001) predictor of the later
measure, accounting for an additional 16.6 percent to
31.1 percent of the variance on the final step. Thus, the
individual differences discussed below are unusually
robust, compared with other psychometric studies in the
same age range (McCall, Eichorn, and Hogarty, 1977).
It also follows from these analyses that a substantial
portion of the variation described below cannot be
explained by such traditional factors as age, gender, and
social class.
Vocabulary comprehension
Figure 4.1 shows the mean developmental function for
word comprehension between 8 and 16 months,
together with functions that describe children who are
1.28 standard deviations above or below the mean.
This contrast (which we will use in most of the graphs
that follow) illustrates the developmental zone occupied
by approximately 80 percent of the sample (so that the
probability of falling outside this region is p < .10 at
either tail of the distribution).
For most children, robust evidence of word
comprehension first appears between eight and ten
months of age. At the eight-month entry point, the
mean number of words that parents report in
comprehension is 36, although the median (a more
conservative measure) is only 17. At ten months of age
the mean has surged to 67 words, with a median of 41.
By the 16-month exit point for the CDI: Infants scale,
children have a mean receptive vocabulary of 191, with
a median of 169. These median scores are consistent
with other estimates of the onset of word
comprehension and its early growth (Benedict, 1979;
Rescorla, 1981; Bates et al., 1988). However, the means
are relatively high, reflecting much more variability in
the onset and rate of receptive vocabulary growth than
might have been expected from previous research. For
example, the 1.28 standard deviation range at ten
months of age goes from a low of zero words to a high
of 144. By 16 months the corresponding range is from
78 to 303 words. An indication of the magnitude of
individual differences is the fact that the overall
correlation between word comprehension and age is
positive and significant (r = 0.60, p < 0.001), but
accounts for only 36 percent of the variance. The
remainder of the variance must be a combination (exact
recipe unknown) of true individual variation and the
noise and error of parental report.
Some evidence for the reality of this variation
comes from recent electrophysiological studies of
comprehension. Mills, Coffey, and Neville (1993)
tested a group of ten-month-old children, half of them
“early comprehenders” reported to understand at least
five to ten words from a short laboratory checklist, and
half “early noncomprehenders” reported as not
understanding these common words. Event-related
brain potentials (ERPs) were recorded while the
children listed to familiar and unfamiliar words.
Significant differences between familiar and unfamiliar
words were observed for children in the "early
comprehenders" group, but not for children in the
“noncomprehenders” group. Thus the parental report of
comprehension was correlated with an
electrophysiological measure of recognition. Of course
this does not mean that high-performing ten-month-olds
understand the meaning of familiar words. The ERP in
this experiment is an index of recognition, nothing
more. However, it does suggest that parents who report
high comprehension are aware of their child's selective
sensitivity to speech.
The very high levels of word comprehension
reported for children at the upper end of the distribution
at eight to ten months may also reflect a much less
interesting factor. In particular, some parents may
adopt a different and more liberal definition of
“understands” than we had in mind, inferring
comprehension from nothing more than evidence for
high attention and positive affect. There is some
evidence that this overestimation may be more
characteristic of parents with low education (see Fenson
et al., 1994). Nevertheless, taken together, the Mills et
al. (1993) electrophysiological data and the cross-age
stability cited earlier demonstrate that a high proportion
of the variance in early word comprehension is
authentic.
Vocabulary production
Figures 4.2 and 4.3 show the mean developmental func-
tion and range of variation (1.28 standard deviations in

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either direction) for word production between eight and
16 months (on the CDI: Infants), and between 16–30
months (on the CDI: Toddlers). The mean and median
functions are consistent with previous small-sample
studies, showing that expressive language does not get
off the ground for most children before 12 months of
age. For most children, there is slow growth in
vocabulary production from a mean of 1.8 words at
eight months (with a median of 0), to a mean of ten
words at 12 months (with a median of 6), to a mean of
64 words at 16 months (and a median of 40)1. By 30
months, the mean score has increased to 534 words,
with a median of 573 (notice that the median is now
higher than the mean, indicating that the distribution is
now skewed by "late talkers" at the low end of the
distribution — see section 5). The overall correlation
between age and vocabulary is 0.47 (p < 0.001) for the
Infant data, accounting for just 22 percent of the
variance. For the Toddler data, the age correlation is
0.68 (p < 0.001), accounting for 46 percent of the
variance. Thus there is a substantial amount of age-
independent variation.
Around these central tendencies, variation in
vocabulary production has a complex and interesting
time course. There is relatively little individual
variance at or before 12 months. The 1.28 standard
deviation window at one year of age extends from a low
of zero words to a high of 24. However, after 13
months there is a dramatic increase in variability, due
primarily to rapid growth in children at the high end of
the distribution. At 16 months, for example, children in
the top tenth percentile have reported productive
vocabularies of at least 154 words, while children in the
lowest 10th percentile are still producing no words at
all. This highly skewed distribution continues to
characterize variation in expressive vocabularies
throughout the 16–30-month range, until ceiling effects
are operative. For example, at the two-year point (24
months), the mean for reported expressive vocabulary
on this measure is 312 words, but the 1.28 standard
deviation range goes from a low of 89 to a high of 534
(see also Huttenlocher, Haight, Bryk, Seltzer, and
Lyons, 1991).
It seems likely that the nonlinearities in figures 4.2
and 4.3 have something to do with a controversial
phenomenon called the "vocabulary burst". Some time
during the second year of life, many children
experience a marked acceleration in rate of word
learning (Nelson, 1973; Dromi, 1987; Gopnik and
Meltzoff, 1987; Goldfield, and Reznick, 1990). For
most of the children that have been studied to date, the
growth curve starts to rise somewhere between 50–100
words. The single best example in the literature comes
1Although the mean is higher at 16 months for the
CDI:Toddlers list than for the same age on the CDI:Infants
list, the medians are quite similar: 44 and 40 words,
respectively. The skewed nature of the distribution is
responsible for the differences in means.
from Dromi's detailed and exhaustive longitudinal
study of vocabulary development in a single child (see
also van Geert, 1991). A number of contrasting
proposals have been offered to account for this
nonlinear shift. For example, it has been argued that
children achieve a sudden insight into the idea that
things have names (Dore, 1974), or alternatively, that
all objects ought to have a name (Baldwin and
Markman, 1989). Others have attributed this shift to a
more general change in cognition, including
developmental changes in the ability to categorize
objects (Gopnik and Meltzoff, 1987) and/or
developmental shifts in representational capacity
(Shore, 1986; Brownell, 1988). Yet another class of
explanations revolve around reorganizations in the
phonetic segmentation (Plunkett, 1993) and/or
articulatory ability (Menn, 1976). All of these accounts
are interesting, but they suffer from two related
problems: lack of universality, and absence of an
inflection point. Let us address each of these in turn,
and then see what we can glean from the large cross-
sectional data base currently at our disposal.
The first problem with the various explanations of
the vocabulary burst cited above revolves around the
fact that not all children display a growth spurt of this
kind. Reznick and Goldfield, among others, have
argued from longitudinal data that the burst is not
universal. In some children, vocabulary grows so
continuously that it is difficult to identify a single point
or narrow region of acceleration (see below). In other
cases, vocabulary development is characterized by a
series of small bursts, a stairstep pattern that is difficult
to reconcile with most single-factor theories of "the
burst".
A second problem comes from the discontinuity
implied by the word "burst", and by most of the
theories that seek to explain accelerations in word
learning. Van Geert (1991) and Bates and Carnevale
(1993) have noted that individual growth curves
described in longitudinal studies of vocabulary
development during the second year (see especially
Dromi, 1987) are best fit by a smoothly accelerating
exponential function, or by related nonlinear functions
such as the quadratic or the logistic. By appropriate
variation in their parameters, such nonlinear models
could lead to growth curves marked by apparently
intense bursts, weaker bursts, or no significant
acceleration at all. The key insight here is that there is
no inflection point in the exponential portion of the so-
called vocabulary burst, i.e. no single "take-off point"
of the kind assumed by most of the theories cited
above. And yet, there does appear to be a region of
acceleration during the second year that cries out for
explanation.
Because the CDI data base is essentially cross-
sectional (one cannot construct a growth curve from
two points), it cannot provide direct information about
the existence, incidence or prevalence of the legendary

6
burst. If there are a large number of individual stepwise
functions, they are masked by summation across
individual children. However, these data can provide
two indirect sources of information about the timing
and nature of nonlinear changes in rate of development.
First, let us assume that accelerations in rate of
development become most evident between 50–100
words. If this is the case, then some children in our
sample are on their way to a vocabulary burst between
14–16 months of age (see figure 4.3) — well before the
17–19 month transition predicted by many theories. By
the same argument, the data in figure 4.3 suggest that
some children in the third year of life are still well
below the 50–100-word boundary where the putative
burst typically appears. Thus, if a vocabulary burst
does occur, its timing is characterized by wide
individual variation. For some children, it may occur as
early as 14 months, for some as late as 24–26 months.
A second set of facts derives from analyses of
stability over time in the MacArthur Norming Study.
Reznick and Goldfield (1992) have reported
longitudinal data from the 500 parents who completed a
second inventory approximately six weeks after the
initial questionnaire. Correlations between initial and
later scores were computed separately for each age
group, based on the child's age at first administration.
Correlations for the CDI:Todders exceeded r = 0.90 at
every age. Correlations for the CDI:Infants were in the
0.8–0.9 range for both comprehension and production,
with a single exception: at 12 months, the test-retest
correlation for infant vocabulary production dropped to
r = 0.60. Analyses of our six-month longitudinal data
were consistent with these results, suggesting a
discontinuity in individual differences occurring at
approximately one year of age. The lower reliability
coefficient at 12 months may reflect a general
reorganization of infant cognition at the 12-month
boundary (see McCall, Eichorn and Hogarty, 1977 for a
similar 12-month discontinuity in longitudinal studies
using the Bayley Scales of Infant Development).
Alternatively, it may reflect a discontinuity in parental
perceptions of infant language. That is, the emergence
of meaningful speech at 12 months may cause some
parents to reevaluate the criteria that they used before
this point to define a "real word" (e.g. "I thought he said
'mama' before, but what he is doing now is really
different"). If this observation is correct, then the most
important discontinuity in early word production is the
one that occurs between 12–13 months of age — and
not the one that is supposed to occur later in the second
year. The putative vocabulary burst from 16–20
months may be the inevitable product of a growth
function that is set in motion at the end of the first year,
i.e. from 10–12 months.
These statistical observations are consistent with
some informal observations of Dale and Thal (personal
communication), both of whom have studied early
talkers. It is difficult to locate children below the age
of 12 months with a demonstrable productive
vocabulary of, say, 10 or more words. It is much easier
to identify children at 14–16 months with very large
vocabularies of 100 words or more. We suspect that
there may be a "bottleneck" or "gate" into referential
vocabulary which cannot be substantially accelerated.
Once through that milestone, however, exceptional
ability may lead to a flowering of vocabulary. It is an
intriguing, but speculative, hypothesis that this
phenomenon may be biologically specified.
Combining words
For English, with its relatively modest inflectional
morphology and general absence of case markings, the
initial step in grammatical development for most
children is combining words (this generalization is not
entirely accurate even for English, and is of course
highly inaccurate for languages such as Japanese and
Hungarian.) In the MacArthur study, parents were
given three options for describing their child's
combinatorial language: “not yet,” “sometimes,” and
“often.” Parents appeared to interpret these terms quite
consistently, even though they were given without
precise quantitative specification. Figure 4.4 shows the
regular progression in parents' response to this question.
There is a gap of approximately three to four months
between the two criteria. It might have been expected
that “sometimes” would be more easily quantified by
parents (“greater than zero”) than “often,” hence more
reliably judged. Contrary to this expectation, the
“often” criterion was more highly correlated with both
age (0.58) and total vocabulary size (0.73) than was
“sometimes” (0.47 and 0.57 for age and vocabulary,
respectively). Fenson et al. have suggested that parents
choose “often” in response to their children's first use of
genuinely productive combinatorial language, in which
individual words from a particular syntactic or semantic
category may be combined flexibly with a variety of
words from one or more other categories to express a
consistent semantic relationship. In contrast, the choice
of “sometimes” may reflect the appearance of
nonproductive or rote combinations that do not reflect a
generalized and semantically consistent word-
combining skill. Such rote combinations are likely to
have a stronger component of individual stylistic
variation than does productive combinatorial language
(see below), and are therefore less strongly correlated
with other measures of language development.
Whichever criterion is used, it is apparent that there
is wide variation in the onset of combinatorial
language. At 18 months, approximately 11 percent of
parents report that their child is often combining words,
and another 46 percent report that their child is
sometimes combining words. Although this question
was not included in the CDI:Infants, an extrapolation of
figure 4.4 suggests that a subset of children are
combining words prior to 16 months. We assume that

7
these early combinations are formulaic in nature,
although there are sporadic reports in the literature of
rare but novel word combinations in children as young
as 14 months (e.g. the expression "Wadoo baba —
Water bottle" uttered by a 14-month-old child after
throwing a playmate's bottle in a wading pool — Bates
et al., 1988). By 25 months, nearly all parents report
some combinations, but 19 percent are still not
reporting combinations 'often.' Thus the much-used
clinical criterion of failure to combine words by age
two years (e.g. Rescorla, 1989) corresponds roughly to
the lowest 10 percent of the CDI distribution.
Sentence complexity
Finally, we turn to grammatical development, as in-
dexed by parental response to the 37 forced-choice
recognition items. Figure 4.5 illustrates the devel-
opmental function and standard deviations for this
scale. The 37 items include bound morphemes, functor
words, and early-emerging complex sentence forms.
The mean developmental function of figure 4.5 is
concordant with observational studies of early gram-
matical development, but there is (once again) sub-
stantial variation from the earliest period. Children
functioning 1.28 standard deviations above the mean at
16 months are at a level which will not be achieved by
children one standard deviation below the mean until 28
months. In other words, there is a full calendar year
separating children at the high v. low end of the dis-
tribution for sentence complexity.
To clarify the range of variation, and make it more
meaningful, we can convert this scale to a more familiar
one, namely, Mean Length of Utterance (MLU). MLU
is widely used as a measure of early grammatical
development, despite a number of limitations (Crystal,
1974; Klee and Fitzgerald, 1985). For purposes of
characterizing variation, the two most significant
limitations of MLU are the absence of good normative
data, and the potential for achieving similar MLU
values with quite different syntactic abilities (and
conversely, achieving different MLU values with
similar syntactic abilities). Here we attempt to
characterize variation in MLU with a much larger
sample than has been previously available, taking
advantage of the high correlation between sentence
complexity and MLU. A correlation of 0.84 (p <
0.001) between sentence complexity and observed
MLU was obtained by Dale for a sample of 44 children
at 20–24 months (Fenson et al., 1994). Based on this
high correlation, a linear regression formula for
estimating MLU can be derived from the parent
measure:2
2 To ensure normality and linearity of relationship, both
complexity and MLU were log-transformed. Note also that
the intercept in this formula is greater than 1.0, due to the fact
that a child may receive a complexity score of zero because
MLU = e(.174 * (ln (C + 1)) + .7299) - 1.
This estimation formula was then applied to
sentence complexity normative data (n = 1,130) from
the CDI:Toddlers, yielding the developmental function
for estimated MLU illustrated in figure 4.6. The mean
growth in MLU produced with this method is startlingly
close to the estimated norms provided by Miller and
Chapman (1981) on the basis of a much smaller sample
of 123 children. And, like figures 4.4 and 4.5, it indi-
cates that normal children of the same chronological
age can vary from six months to a year in their
"grammatical age".
To summarize so far, substantial variation is ob-
served in early vocabulary comprehension. Equivalent
variability does not emerge in vocabulary production
until after 12 months, an age which appears to mark a
discontinuity. Vocabulary growth is positively acceler-
ated after this point, although it is not yet clear whether
this is best characterized as a "burst" or a smoothly
accelerating curve. If it is the latter, then it is safe to
say that the "true discontinuity" in vocabulary devel-
opment is the one that starts around 12 months of age.
The nonlinearities that are observed after this point
represent nothing more (or less) than the predictable
course of growth along an exponential function that was
set in place around the 12-month mark. There is also a
great deal of variability in the onset of word com-
binations, and the growth of MLU following the
transition to grammar. To determine whether this later
"burst" in grammar is a true discontinuity, or a
continuation of the earlier "burst" in vocabulary, we
turn to the analyses of associations and dissociations
between components of early language.
2. Finding the Fault Lines: Dissociations
Between Components of Early Language
In the previous section, we presented evidence showing
massive variation in rate of development for healthy,
normal children, in every area of early communication
and language from 8–30 months of age. The existence
of such variation leads to another question: are these
variations in rate consistent across all areas of
development, or can we document significant
dissociations in rate of development between the major
components of early language? As we pointed out in
the introduction, the existence and nature of
dissociations between components of early language is
relevant to the vexed issue of modularity.
Fodor (1983) has proposed a distinction between
vertical modules that operate on a particular
information type and respect the boundaries between
the child is still in the single-word stage, or because the child
is combining words, but still producing the less advanced
form in each sentence pair.

8
domains (e.g. a putative module for face perception),
and horizontal modules that cut across information
types (e.g. short-term memory, or particular forms of
attention). In Fodor's view, the human language faculty
is an excellent candidate for vertical modularity, i.e. an
innate, encapsulated, special-purpose processor that
operates on language and language alone. Fodor makes
no claims about further modular sudivisions within this
language faculty, although he acknowledges Noam
Chomsky's proposal (e.g. Chomsky, 1986 & 1988) that
language itself is made up of distinct subdivisions that
fit the criteria for vertical modules (e.g. phonology v.
grammar, grammar v. the lexicon — see also Pinker,
1991). In testing for the fault lines of early language
development we will examine the case for and against
one potential set of horizontal modules (comprehension
v. production) and another potential set of vertical
modules (grammar v. vocabulary). We note that Fodor
might view both of these as instances of vertical
modularity, since they deal primarily with the
processing of language. However, we will review
limited evidence to suggest that the comprehension/
production dissociation implicates abilities outside the
boundaries of language proper.
Comprehension v. production
Every pediatrician has had worried inquiries about
children who have barely started to talk, even though
they appear to understand much of the speech that is
addressed to them. Cases of this sort are also well
attested in the child language literature, in virtually
every study that has investigated comprehension and
production in the same set of children (Goldin-
Meadow, Seligman, and Gelman, 1976; Bates, Benigni,
Bretherton, Camaioni, and Volterra, 1979; Benedict,
1979; Oviatt, 1980; Snyder, Bates, & Bretherton, 1981;
Mills, Coffey, and Neville, 1993 and in press).
Most of these studies have concentrated on
comprehension and production of single words, but
studies that have investigated comprehension and
production of grammatical forms in the same set of
children yield a similar conclusion. In a few cases that
dissociation actually seems to run in the opposite
direction, with children demonstrating poor
comprehension of sentence forms that are present in
their own spontaneous speech. However, most
researchers agree that dissociations of the latter sort
reflect cases in which (a) the comprehension test itself
involves complex task demands that obscure the child's
actual knowledge of grammatical structure (Chapman
and Miller, 1975; Crain, 1992), and/or (b) children
appear to display production of grammar in advance of
comprehension because they are using the grammatical
forms in question in unanalyzed formulae that have
been acquired through rote memory (Bates et al., 1988).
When these confounds are eliminated, then the
comprehension/production profiles observed for early
grammar match those that have been reported for
comprehension and production of single words. That
is, some children appear to understand far more than
they are able to say (Hirsh-Pasek and Golinkoff, 1991).
Furthermore, these profiles appear to be relatively
stable over time. In a longitudinal study tracking
development from first words to grammar, Bates et al.
(1988) report significant correlations between the
comprehension/production profiles that children display
at the lexical level from 13–20 months, and the
comprehension/production profiles that the same
children display at the grammatical level at 28 months
of age.
The incidence and magnitude of the dissociation
between comprehension and production has been
demonstrated at the lexical level in two successive
large-sample studies within the MacArthur CDI project
described above. Figure 4.7 illustrates the relationship
between number of words in comprehension (horizontal
axis) and number of words in production (vertical axis)
for children between eight and sixteen months of age in
the CDI norming study. This graph shows a
characteristic fan effect, created by a substantial
number of children in each sample who seem to
understand far more words than they produce —
including some children who are producing no words at
all despite receptive vocabularies of more than 200
words! Hence the comprehension/production dissocia-
tion appears to be a robust and pervasive phenomenon
in the early stages of language development. But it is a
phenomenon that is still in search of an explanation. To
find that explanation, we need to consider some of the
cognitive and neural correlates of this profile.
First, let us consider the limited evidence that is
currently available on the nonlinguistic measures that
correlate with comprehension and production, respect-
ively. Since the 1970s, a large number of studies have
investigated the cognitive predecessors and correlates
of early language milestones, with a particular focus on
the correlates of first words and first word combinations
in production. Candidates include aspects of tool use
and causality (Bates, Camaioni and Volterra, 1975;
Bates, Benigni, Bretherton, Camaioni, and Volterra,
1979; Harding and Golinkoff, 1979), categorization
(Sugarman, 1983; Gopnik and Meltzoff, 1986, 1987;
Mervis and Bertrand, 1993), block construction (Shore,
1986), and symbolic play (Nicolich, 1977; Snyder,
1978; McCune-Nicolich, 1981; McCune-Nicolich and
Bruskin, 1982; Shore, O'Connell, and Bates, 1984;
Kelly and Dale, 1989; Shore, Bates, Bretherton,
Beeghly, and O'Connell, 1990; Rescorla and Goossens,
1992). In most of these studies, no effort was made to
parcel out the contributions of comprehension v.
production to the observed correlations. However, as
we shall see in more detail in section 4 on abnormal
variations, comprehension and production map onto
different aspects of nonlinguistic development. In
particular, when there is a dissociation between these

9
two modalities of language, comprehension appears to
"win custody" of most of the cognitive correlates of
early language. For example, Bates, Thal, Whitesell,
Fenson, and Oakes (1989) have shown that variation at
14 months in communicative and symbolic gesture
correlates more strongly with lexical comprehension
than it does with lexical production. A similar pattern
appears in the MacArthur CDI parental report data for
children from 8–16 months of age. The finding that
nonlinguistic measures correlate better with compre-
hension than they do with production can be
paraphrased as follows:
Most cognitive variables correlate with what the child
knows about language (indexed by comprehension), as
opposed to what the child does (indexed by produc-
tion).
A different kind of evidence about the correlates
and fellow travelers of comprehension and production
comes from recent electrophysiological studies of
normally developing children. Mills, Coffey, and
Neville (1993, and in press) have examined the event-
related brain potentials elicited by auditorily presented
words in children between 13 and 20 months of age.
Although this is clearly a test of receptive processing
(i.e. passive reaction to familiar words), Mills et al.
report specific patterns of brain activity that distinguish
between the child's current level of comprehension
(indexed by bilateral waves over posterior cortex) and
the same child's current level of production (indexed by
waves that are larger over left anterior cortex). Hence,
even though these event-related potentials are all
elicited in the receptive modality, they suggest that
different neural systems mediate comprehension and
(latent) production.
To summarize this section, there are robust
dissociations between comprehension and production in
the early stages of language learning. These
dissociations are observed across the transition from
first words to grammar, and the profiles of individual
children tend to be preserved at both the lexical and the
grammatical level. Studies with normally developing
children also suggest that comprehension and
production draw on different cognitive resources, and
are mediated by different neural systems. In particular,
rate of progress in comprehension appears to be
associated with a wide range of nonlinguistic measures,
mediated (at least in the early stages) by bilateral brain
mechanisms. By contrast, variations in production have
fewer nonlinguistic correlates (when levels of
comprehension are controlled), and may involve a
special form of mediation by anterior regions of the left
hemisphere. We suggest that this dissociation between
comprehension and production constitutes a form of
horizontal modularity, i.e. a dissociation that cuts across
linguistic and (perhaps) nonlinguistic information types.
Grammar v. the lexicon
In contrast with the comprehension/production profiles
described above, any dissociations that we might
observe between grammatical and lexical development
could be used to argue in favor of vertical modularity,
i.e. a dissociation that respects the boundaries between
well-specified linguistic domains. However, in our
investigation of the relationship between grammar and
the lexicon in the first stages of language learning, we
have to distiguish between two forms of dissociation:
temporal asynchrony within individual children, and
dissociations in rate of development across individual
children. As we shall see, there is solid evidence for
the former but very little evidence for the latter.
The temporal asynchrony between grammatical
and lexical development is one of the best-established
facts in developmental psycholinguistics. In every
language that has been studied to date, children do not
combine or inflect words productively until they have
passed through a prolonged single-word stage, where
content words are used with few (if any) inflectional
contrasts. Even in languages such as Turkish and
Japanese, in which some inflections appear in the one-
word stage, this development occurs after a prolonged
noninflected stage. During the one-word stage, there
are also significant changes in the composition of the
lexicon, changes that signal a shift from reference, to
predication, to grammar. Figure 4.8 from Bates et al.
(1994) illustrates this change in vocabulary composition
from 16–30 months of age, based on cross-sectional
results from the MacArthur norming study. In this
figure, children are divided by vocabulary level rather
than age, as follows: 0–50 words, 51–100 words,
101–200, 201–300, 301–400, 401–500, 501–600, and >
600. Although these 100-word groupings are rather
arbitrary, figure 4.8 shows a systematic and elegant
relationship between vocabulary size and vocabulary
composition. Figure 4.8 illustrates the average
proportion of total vocabulary made up of common
nouns (i.e. names for common objects), predicates
(verbs and adjectives), and function words (pronouns,
prepositions, articles, quantifiers, conjunctions, modal
and auxiliary verbs) at each vocabulary level. The
horizontal lines on the graph represent the total
proportion of common nouns, predicates and function
words on the list as a whole (i.e. the checklist ceiling
for each vocabulary type). If vocabulary development
drew randomly (or evenly) from each of these lexical
categories, then the average proportion scores at each
vocabulary level should follow these horizontal lines.
Clearly, this is not the case. Common nouns
predominate in the first stages of language learning,
reaching a peak around 100 words and then dropping
sharply down to the checklist ceiling. Predicates show
a slow but steady proportional increase across the entire
period of development. Function words show no

10
proportional growth at all, averaging approximately 6
percent of total vocabulary until somewhere around the
400-word mark; after that point, function words
increase rapidly until they reach the 14 percent
checklist ceiling. In other words, each of these
vocabulary types shows a different growth function,
and each one has its own season.
These temporal asynchronies suggest that different
mechanisms may be involved in lexical and
grammatical development. But is this necessarily the
case? Temporal asynchronies of this kind can also be
observed when a single learning device is applied to a
complex task with several levels of difficulty. That is,
grammatical function words, grammatical inflections
and word combinations may come in later than
individual content words, for two related reasons:
1. Grammatical forms may come in later than
content words because they tend to be short, fast,
unstressed, and phonologically reduced. In other
words, they are relatively difficult to perceive. In fact,
several studies have shown that function words excised
from continuous speech are recognized less than 30
percent of the time when they are presented out of
context to competent adults (Pickett and Pollack, 1963;
Pollack and Pickett, 1963; Goodman, Nusbaum, Lee
and Broihier, 1990), compared with recognition rates
from 50–80 percent for content words (see also Gerken,
1994; Goodman and Nusbaum, 1994). Hence it appears
that adults have to depend on context to assist in the
perception of closed-class items. If this is the case, then
it suggests the following hypothesis for language
development:
Closed-class vocabulary will not be acquired until
children have built up a content word lexicon that is
large enough to "bootstrap" perception of unstressed
grammatical forms.
2. Most function words are relational in nature, i.e.
their purpose is to set up a relationship between other
items in the sentence. O'Grady (1987) has proposed a
formal taxonomy of lexical items into three logical
types, based on the number of elements and
relationships that must be presupposed for that item to
work in its intended fashion. Primaries are elements
with a "stand alone" function or meaning. Secondaries
are items that depend upon a relation with at least one
primary. Tertiaries are items that presuppose or depend
upon at least one secondary relationship. Within
O'Grady's system, most nouns are primaries, most verbs
and adjectives are classified as secondaries, and most
(though not all) function words are classified as
tertiaries. The developmental results outlined in figure
4.8 are quite consonant with O'Grady's theory: nouns
are acquired first because they must be acquired first
(although they may coexist with routines like "bye-bye"
that also stand alone), verbs and adjectives tend to come
in later because they presuppose the prior existence of
nouns, and closed-class elements cannot be acquired
until some requisite number of primary and secondary
elements are in place.
In short, the temporal asynchrony that is invariably
observed in the acquisition of lexical and grammatical
functions may be an inevitable by-product of phonetic
and semantic differences among these linguistic types.
Indeed, if proposals by Goodman et al. (1990) and
O'Grady (1987) are correct, this temporal asynchrony
may reflect a powerful cause-and-effect relationship.
To the extent that this is true, we should expect to find
robust correlations between lexical and grammatical
development across individual children. That is, in
fact, exactly what we find (Bates et al., 1988; Bates et
al., 1994; Dale, 1991; Marchman and Bates, 1994). In
their longitudinal study of 27 children from 10–28
months of age, Bates et al. (1988) report a correlation of
+0.83 between vocabulary size at 20 months
(determined by parental report) and mean length of
utterance at 28 months (based on videotapes of free
speech in home and laboratory settings). This
correlation falls at the reliability ceiling for MLU (i.e.
split-half and test-retest correlations for samples of
MLU do not exceed +0.83). Because no measure can
correlate any higher with another variable than it
correlates with itself (i.e. "Spearman's Law"), we can
conclude that the relationship between 20-month
vocabulary and 28-month MLU is one that approaches
statistical identity.
The developmental nature of this relationship
between vocabulary size and grammar is illustrated in
figures 4.9 and 4.10 from the MacArthur CDI norming
study. Figure 4.9 shows the number of children who
are reported by their parents to produce at least some
word combinations, as a function of the same eight
vocabulary groupings described above (figure 4.8).
Figure 4.9 shows that there is a strong correlational
relationship between vocabulary size and the appear-
ance of multiword speech. For most children, word
combinations start to appear when vocabularies fall
between 50 and 200 words. Figure 4.10 displays an
equally strong but later-emerging relationship between
vocabulary size and sentence complexity (a parent
report measure that, as we noted earlier, correlated
around +0.84 with mean length of utterance based on
laboratory observations). For most children, sentence
complexity accelerates markedly (following a smooth
exponential function) when total vocabulary exceeds
400 words. Note that there is a significant nonlinear
component to the grammar/vocabulary relationship in
both these graphs. Marchman and Bates report similar
but more specific nonlinear relationships between verb
vocabulary (i.e. the number of specific regular and
irregular verbs reported on the vocabulary checklist)
and the emergence of verb morphology (including
correct regulars, correct irregulars, and the appearance
of overgeneralizations like "falled").

11
As described in more detail by Plunkett (this
volume; see also Plunkett and Marchman, 1991, and
1993; Marchman, 1993) and MacWhinney (this
volume; see also MacWhinney 1989 and 1991a, b),
these functions are quite similar to the nonlinear "mass
action" effects reported in studies of language learning
in neural networks. In these connectionist models, the
same learning mechanism is responsible for acquisition
of vocabulary items and acquisition of the past tense
markings for specific present tense forms. In other
words, there is no modular distinction between lexical
and grammatical learning. Nevertheless, the models
display a temporal asynchrony between lexical
development and the subsequent emergence of
grammatical marking. To illustrate, consider the
incremental learning of past-tense marking in a recent
simulation by Plunkett and Marchman (1993; cf.
Plunkett, this volume). In the early stages of learning,
the system appears to learn each mapping from present
to past tense by rote, with no generalization to novel
lexical forms (i.e. it flunks the wug test) and no
overgeneralization errors. As instances of present/past-
tense mapping accumulate, some dramatic nonlinear
changes are observed: the rate of learning accelerates
markedly, overgeneralization errors start to appear, and
the system starts to provide a default mapping to novel
items (i.e. it passes the wug test). Superficially, the
network behaves as though it has switched from one
mode of learning (rote) to another (rule). And yet there
are no structural discontinuities in the system itself, or
in the one-verb-at-a-time nature of the input. Instead,
the behavioral discontinuities observed in these
simulations result from the operation of a single
nonlinear learning device. Simply put, grammatical
generalizations (i.e. rulelike behaviors) do not arise
until the system has acquired enough instances to
support those generalizations. When the requisite
number of items has been acquired, dramatic changes
can take place, even within a single system.
These neural network simulations provide a
possible explanation for the strong nonlinear
correlations between lexical and grammatical
development observed in human children from 16–30
months of age. Indeed, it could be argued that lexical
and grammatical development are paced by the same
learning mechanisms — at least within this age range.
However, before we accept such a strong conclusion we
need to ask whether dissociations between grammar
and vocabulary are ever observed. Recall the fan effect
observed in figure 4.7, which illustrates the relation
between comprehension and production. Figure 4.10
presents comparable information on individual
variability in the relationship between vocabulary size
and sentence complexity. Note that there is no fan
effect at all in figure 4.10, i.e. no children with very
large expressive vocabularies but no evidence of
grammar. In other words, there are no lexical/
grammatical dissociations in normally developing
children from 16–30 months of age. We will return to
this point later, in section 4 on variations outside the
normal range.
However, this does not mean that grammar and
semantics are "the same thing". As Gerken (1994) and
O'Grady (1987) have pointed out, function words and
content words have different perceptual, semantic and
logical properties. Hence they cannot be processed in
exactly the same way, even though they may be
acquired by the same kind of learning device. In fact,
new electrophysiological studies by Mills, Neville and
colleagues suggest that content words and function
words are mediated by different areas of cortex (Mills,
1993). By the time normally developing children are
three years of age, grammatical function words elicit
specific patterns of activity over left anterior cortex that
are not observed in response to content words (see also
Greenfield, 1991). These patterns may be the hallmark
of fluent, connected speech, a skill that requires forms
of information processing that are not required for the
comprehension or production of single words.
To summarize so far, studies of normal children
provide robust evidence for a dissociation between
comprehension and production. By contrast, there is
very little evidence for a dissociation between lexical
and grammatical development for children who fall
within the normal range. To be sure, temporal
asynchronies are reliably observed in the passage from
first words to grammar. However, these temporal
asynchronies may reflect a powerful cause-and-effect
relationship between grammar and the lexicon,
reflected in the strong nonlinear correlations illustrated
in figures 4.9 and 4.10. We suggest that grammatical
development depends upon the establishment of a
critical lexical base. Indeed, different grammatical
events may each depend upon a different lexical base
(e.g. word combinations emerge in the 50–100-word
range; verb morphology emerges in the 400–600-word
range — see Marchman and Bates, 1994). These mass-
action effects are consonant with simulations of
grammatical learning in neural networks, which
suggests in turn that grammatical and lexical
development may be achieved with the same kind of
learning device (Plunkett, this volume). Nevertheless,
there are clearcut semantic and perceptual differences
between grammatical and lexical forms, which may
require a different mix of processors for rapid and
efficient language use.
3. Individual Differences in Style
Both the previous sections, which have focused on rate
differences among children in specific components of
language, and on potential dissociations, or
asynchronies, among those components, have implicitly
assumed a uniform sequence of acquisition within each
of these components. The credibility of this assumption

12
is the fruit of several decades of research on child
language. It is not the whole story, however. An
appreciation of stylistic variation, encompassing
differences in sequence and in qualitative aspects of
language development, was delayed for many years by
a focus on universal sequences of acquisition —
especially in grammar — as a window onto innate
structures and processes, and by an apparently innocent
methodological decision to select relatively talkative,
intelligible, and nonimitative children for early studies
(Goldfield and Snow, 1985). As the domain of child
language studies expanded to include semantic and
pragmatic development, and a wider range of children
were observed, the universal fact of stylistic variation
has become obvious.
Three phases in the study of stylistic
variation
At a highly oversimplified level of abstraction, we may
distinguish three phases in the study of qualitative
variation in early child language. The first might be
captured with the phrase "gee whiz.” Following
Nelson's (1973) seminal monograph on the composition
of early vocabulary, a stream of research projects
attempted to document variation in specific aspects of
language development. Nearly all of them were
successful. Comprehensive reviews of this research are
available in Wells (1985), Goldfield and Snow (1985),
and Bates et al. (1988). Here we note only that whether
the components of language are conceived as horizontal
or vertical (cf. previous section), variation may
observed within each of them. We list just a few
examples organized by linguistic content area. Within
phonology, differences have been noted in emphasis on
segmental v. suprasegmental features, and in the
consistency of pronunciation across word tokens.
Within semantics, there are differences in the
proportions of nouns in early vocabularies, and in the
use of semantically empty "dummy" words. Within
grammar, there are differences in use of nouns v.
pronouns in early sentences, and in degree of
overgeneralization of morphological rules. And within
pragmatics, children differ in their variety of speech
acts, and in the balance of declarative and imperative
utterances. Alternatively, differences may be organized
by processing modules. Children differ in the degree to
which comprehension is superior to production, in their
rate of imitation and whether it is ahead of or behind
spontaneous speech, and whether early utterances are
limited to single words or are reproductions of longer
phrases and sentences.
The general argument in most of these studies has
been to draw inferences from product to process. For
example, the higher proportion of nouns in children
with referential vocabularies has been interpreted as
evidence for an interest in language as a tool for talking
about objects and categorizing them, whereas children
with expressive vocabularies are assumed to be more
socially oriented, and to be acquiring language to talk
about themselves and others. Bloom, Lightbown, and
Hood's (1975) observation of variability in the use of
nouns v. pronouns in early word combinations was
interpreted as evidence for two qualitatively distinct
strategies, one more "pivot-like" (Braine, 1963) in
using all-purpose pronouns in combination with lexical
items, the other based on combining content words
from large categories. These process differences in turn
were often attributed to differences among children
specific to the domain of language. For example,
Nelson (1981) proposed that children have different
hypotheses as to the essential function of language.
Bloom et al. (1975) suggested that nominal and
pronominal children were distinguished by a
concentration on the word-order properties of language
or on morphology (i.e. closed-class items), respectively.
A second phase of study of language acquisition
styles may be viewed as the "grand synthesis" phase.
This phase began with Nelson's (1973, 1981) reanalysis
of her own data, which showed a substantial correlation
between her original classification of children as
referential or expressive based on their early
vocabularies, and Bloom et al.'s distinction between
nominal and pronominal basis of early syntax. This
report was followed by a flurry of studies, some data-
based and some more purely speculative, arguing for a
cohesion of the various dimensions of stylistic
variation. Table 1, from Bates et al. (1988), represents
a synthesis of these arguments, attempting to combine
virtually all of the reported dimensions. In this "unitary
dimension" or "two-strand" theory of individual
differences, infants who are word oriented during the
babbling period grow into children with referential
vocabularies at the first-word level, and into children
who display nominal style in their first word
combinations, followed by high rates of morphological
overgeneralization during the acquisition of grammar.
Conversely, children who are intonation oriented during
the babbling period grow into children with expressive
vocabularies in the one-word stage, and then show a
formulaic, pronominal style in first word combinations,
followed by a pattern of grammatical learning
characterized by undergeneralization and inconsistent
application of rules. At the highest level of generality,
these styles have been labelled "analytic" v. "holistic"
or "rote," respectively.
Perhaps not surprisingly, given the psychologically
loaded terminology of "analytic" v. "rote," the first
strand usually included the descriptor "fast learner."
And indeed, it also included certain demographic
variables — female, firstborn, high SES — assumed to
be associated with more rapid language learning. In
fact, the primary evidence for this claim was restricted
to just two of the aspects of style listed in the table:
proportion of nouns, and error rate. Several studies,
such as that of Horgan (1981), observed a positive

13
relationship between the use of nouns and the overall
rate of language development, a finding to which we
return below. In the second set of studies, Horgan
(1981) and Ramer (1976) demonstrated a positive
relationship between rate of errors, including word
order errors, and overall rate of development.
As the dimensions of stylistic difference were
reduced in number, but broadened in scope, it became
increasingly appropriate to look for explanations
outside of language proper: the nature of maternal
input, the child's temperament, global (IQ) or specific
(play, information-processing style, etc.) aspects of
cognitive functioning, neurological differences, and
others. In addition, new versions of language-based
explanations were offered, such as the proposal of
Gleitman and Wanner (1982) that learners differ
significantly in their emphasis on the open-class lexicon
(nouns, verbs, adjectives) and closed-class lexicon
(function words). Some of these alternative accounts
are summarized in table 2 (adapted from Bates et al.,
1988).
It is essential here to distinguish processing
mechanisms, such as analysis, or rote memory, from
more distal factors, such as gender or maternal input,
which may lead to differences in the degree to which
children rely on one or the other of these mechanisms.
These constitute two qualitatively different levels of
explanatory factors. We suggest that it may be a more
fruitful research program to characterize first the
processing differences, and then seek the more distal
explanations. It is too easy to be guided (or mis-
guided!) by implicit or explicit stereotypes about these
external factors to inferences about mechanisms which
may not be valid, e.g. that higher-SES children have
larger vocabularies, that girls have more precise
pronunciation, or that the left-hemisphere superiority
for analytic processing will lead to a fundamental
difference among children in degree of analysis. For
this reason, we will have little to say in the present
chapter about the exogenous factors that push
processing mechanisms apart. Our focus will be on the
nature of the mechanisms that underlie a dissociation in
styles.
The third phase of the study of stylistic variation
may be dubbed "sober reconsideration." It is
characterized by a critical examination, on empirical,
conceptual, and methodological grounds, of the grand
synthesis. Evaluation of a theory of individual
differences necessarily requires a substantial number of
subjects, measures, and time points. For example,
Bloom et al. noted that their two "nominal" children
were both girls, whereas the two "pronominal" children
were both boys. This is an intriguing observation, but
assuming a null hypothesis that any single child has a
50/50 chance of developing either style, there is a 1 in 8
probability that a group of four children would divide in
a gender-consistent fashion. A few studies are
appearing which at least approximate the requirements
above, and the results are generally more complex than
the grand synthesis would predict. Bates et al. (1988)
followed a group of 27 children through four age levels:
10, 13, 20, and 28 months. Correlational and factor
analyses based on the full longitudinal data set were
most consistent with a model consisting of three
language acquisition mechanisms, not two: compre-
hension, analyzed production, and rote production, with
a complex relationship over time between compre-
hension and analyzed production. For example, MLU
at 28 months was predicted by early measures of
comprehension and analyzed production, not by MLU
at 20 months, which was associated instead with earlier
measures of rote production (for further evidence
supporting this partitioning, see Shore, 1986; Dixon and
Shore, 1992, 1993). In other words, the simple two-
strand account (figure 4.11a, from Bates et al., 1988)
must give way to a more complex, dynamic account of
associations and dissociations over time (figure 4.11b,
from Bates et al., 1988).
Even more important than the accumulation of
larger data sets on individual variation is the need to
embed characterizations of individual variation into a
larger model of developmental change. Stylistic
variables are not nearly as distinct from developmental
ones as the literature would lead one to believe.
Children differ in their use of nouns and pronouns in
early sentences, as Bloom et al. demonstrated; but
children also generally increase their use of pronouns
during this period. Children vary in their rate of
spontaneous imitation, but there is also an inverted U-
shaped developmental trend for imitation, first rising
and then declining. And so on down the line.
Paradoxically, only if we can separate these two types
of variance can we hope to understand their
relationship. Otherwise we may only be demonstrating
one more time that "good things go together."
The validation of stylistic differences
What constitutes adequate evidence for the character-
ization of a dimension of stylistic variation? We
suggest there are at least three necessary criteria: The
first is the requirement of synchronic generality and
discrimination. There are two parts to this requirement,
sometimes called convergent and discriminant validity
in the psychometric literature. The first is based on the
view that a single measure of variability is unlikely to
be interpretable or interesting on its own. It might
simply be error variance; alternatively it might be
genuine, but so limited in scope as to have few
implications. Stylistic dimensions which have multiple
measures, especially measures based on different types
of linguistic performance or content, provide a more
substantial basis for interpretation. The second
requirement, of discrimination, simply reflects the
observation that when everything is changing at the

14
same time, correlations are not interesting.
Methodologically, this first criterion requires
correlations among the proposed system of variables,
contrasting with smaller correlations between the
stylistic variables and other measures. In practice, the
demonstration of these correlations and their
interpretation is far more difficult than it sounds. There
are three recurring substantial difficulties with the
correlational approach. One is that the size of a
correlation is not determined solely by the relationship
between the underlying constructs, but also by the
inherent reliability of the measures. As Spearman first
noted, measures cannot correlate more highly with each
other than they do with themselves. Many child
language measures, especially stylistic ones, have only
modest reliabilities, which will depress correlations
among them. A second difficulty is that the
relationships among stylistic measures may not be
linear, as assumed by the test of significance of the
ubiquitous Pearson coefficient. The relationship may
be nonlinear, e.g. a discontinuity or step function, or
even nonmonotonic, as when the extremes of a measure
have more in common with each other than with the
middle of the distribution. The third difficulty concerns
the underlying nature of the variables, which may have
different meaning at differing time points. As will be
discussed below, the frequency of closed-class words in
a child's vocabulary has quite a different meaning at 20
months than at 28 months. Hence, evaluation of a
dimension of stylistic variation that includes closed-
class word usage must consider carefully the most
appropriate age for assessment.
The second criterion for identification of a stylistic
dimension is longitudinal continuity. No matter how
substantial a dimension of variability may be at a single
moment in time, if it has no enduring predictive
significance, it is unlikely to play a major role in
understanding language development. This is not a
requirement for "dimension permanence". Early use of
nominals and pronominals in word combinations
appears to be a stable characteristic, associated with
other aspects of style, though eventually it fades as
children become fluent with both approaches to
sentence construction. It appears to reflect a genuine
difference in first approach to syntax. The three strands
identified by Bates et al. (1988) discussed above were
supported by synchronic generality and longitudinal
continuity. Demonstrating longitudinal continuity is
subject to the same difficulties as those listed above
under synchronic generality. The third difficulty, the
changing meaning of measures, is particularly
challenging and important. Kagan (1971) and others
have pointed out that there are two types of longitudinal
continuity: homotypic continuity (cross-age correlations
within the same content domain, such as vocabulary
totals) and heterotypic continuity (e.g. correlations
between early measures of vocabulary and later
measures of grammar). Heterotypic continuity may be
more common, and more interesting, theoretically, than
homotypic continuity, as it suggests more abstract,
underlying explanatory mechanisms. Conversely, what
appears to be the same measure at two ages may not be
correlated, i.e. a homotypic discontinuity. For example,
proximity seeking in a two-year-old is a good measure
of attachment to a parent, and is positively correlated
with other measures of social and emotional
development; proximity seeking in a four-year-old is
more likely to be a sign of dependency, with negative
correlations to other measures. Thus proximity seeking
at the two ages is either uncorrelated or negatively
correlated. A similar conclusion holds for closed-class
usage: This measure at 20 months correlates
significantly and negatively with the "same" measure of
closed-class use at 28 months (Bates et al., 1988).
Homotypic discontinuity does not eliminate the
significance of such measures for understanding
individual differences in children; it suggests only that
the measures have a qualitatively different relationship
to stylistic variation at the two different times.
The third criterion for establishing a dimension of
stylistic variation concerns the nature of the measure
which is evaluated for synchronic and longitudinal
generality. In particular, stylistic variables must be
unconfounded with developmental rate, in order to be
interpretable. We exemplify both the need for this
unconfounding and one approach for doing so with a
recent study by Bates et al. (1994). As discussed
earlier, a number of studies have reported that
referential style, that is, a high proportion of common
nouns early in development, is associated with faster
rates of development. However, the percentage of
nouns in total vocabulary itself shows a monotonic
increase longitudinally during the period of acquisition
of the first 50 or 100 words. Hence the correlation
between rate of development and referential vocabulary
observed at a given age style may simply reflect the
developmental change in noun use. Those children
with larger vocabularies will have a higher proportion
of nouns as a consequence of developmental change.
As Pine and Lieven (1990) have pointed out, age-based
measures and correlations based on them are not
appropriate for evaluating stylistic variation.
To address this issue, Bates et al. (1994) used the
large norming sample from the MacArthur
Communicative Development Inventories discussed
earlier in this chapter. Their first goal was to identify
developmental changes in vocabulary composition
across the 8–30-month period. As noted in section 2,
they identified three "waves of reorganization": (1) an
initial increase in percent common nouns between
1–100 words, followed by a slow proportional decrease;
(2) a slow linear increase in predicates, i.e. verbs and
adjectives, with the largest gains occurring between
100–400 words; (3) no proportional change in closed-
class vocabulary (pronouns, prepositions, question
words, quantifiers, articles, auxiliary verbs, and

15
connectives) between 1–400 words, followed by a sharp
increase after 400 words. These changes were
summarized as "a shift in emphasis from reference, to
predication, to grammar."
The second goal of their project was to characterize
stylistic variation in vocabulary composition indepen-
dent of these developmental changes. Figure 4.12 plots
the observed changes in percent common nouns as a
function of the vocabulary levels adopted in figures
4.8–4.10. In addition to values for children at the mean,
figure 4.12 also includes referential-style scores for
children who are 1.28 standard deviations above or
below the mean at each vocabulary level. It is clear that
there is very substantial variation in "nouniness", even
when total size is held (approximately) constant. This
variation is greatest for children with vocabularies
below 50 words, replicating the original observation in
Nelson (1973). At this vocabulary level, children who
are 1.28 standard deviations above the mean (i.e.
"referential style") have vocabularies that comprise
more than 58.5 percent common nouns; children who
are 1.28 standard deviations below the mean (i.e.
"expressive style") have fewer than 18 percent common
nouns.
Variation in closed-class vocabulary showed a
different pattern, illustrated in figure 4.13. As noted
earlier, there is little developmental change in these
proportion scores before 400 words, although there is
still some variation around the mean (ranging from no
closed-class words at all to more than 15 percent).
After vocabularies pass the 400-word point, there are
increases in the relative size of the closed class for all
children, including those at the low end of the
distribution. In other words, everybody has to get
around to learning those little words eventually,
regardless of style (see also Bloom et al., 1975). Hence
developmental and stylistic variation in closed-class
proportion scores are confounded after the 400-word
point.
With percentile-based measures of vocabulary
composition, it was possible for Bates et al. to
investigate the correlates of stylistic variation
unconfounded with developmental variation, their third
goal. Referential vocabulary style for children between
8–16 months with at least 10 words was modestly, but
significantly, correlated with age (0.15), gender (0.11,
girls higher), SES, mother's education (0.18) and
father's education (0.19). Note that the positive
correlation with age means that children high in
referential style are actually older than those with
proportionally fewer common nouns, that is, not
precocious in overall vocabulary growth. This finding
is strikingly confirmed by studies of the longitudinal
predictiveness of referential style. Referential style at
8–16 months was not significantly correlated with total
vocabulary, percent common nouns, percent closed-
class words, or grammatical complexity 6.5 months
later. Thus, the result of shifting from age-based raw
scores to percentiles based on vocabulary level is to
cast substantial doubt on the hypothesis that referential
style is a harbinger of language precocity. The lack of
correlation, positive or negative, with later closed-class
use is also inconsistent with the view that referential
children are more likely to display a "telegraphic" style
in early word combinations, while expressive children
are more likely to adopt a pronominal/holistic style.
The modest correlations of referential style with social
class and maternal education suggest that
environmental factors may contribute some of the
variance in early "nouniness."
A second set of analyses investigated the correlates
of closed-class proportion scores. As should be clear
from figure 4.13, variation in closed-class use appears
to change in nature when vocabulary reaches
approximately 400 words. Prior to this point there is no
increase in closed-class proportion scores as a function
of vocabulary size, suggesting that variation is due to
stylistic rather than developmental factors; after this
point, the same measure is strongly correlated with
vocabulary size, suggesting that variation is strongly
influenced by developmental status. For this reason,
Bates et al. (1994) conducted correlational analyses
separately for these two periods. Children were divided
into three groups: (1) children who were below that
400-word level at both time points (i.e. continuity in
closed-class style during the nonproductive phase), (2)
children who were above 400 words at both ages (i.e.
continuity in closed-class development within the
productive phase), and (3) the largest and most
informative group, children who were below 400
words at Time 1 but above 400 words at Time 2 (to test
for continuity or lack thereof around the 400-word
border).
For children below the 400-word level at both time
periods, early closed-class usage was negatively
associated with later vocabulary (-0.19) but positively
associated with later closed-class usage (0.46). This
suggests some continuity in style within this early
period, but it also suggests that the style dimension is
unrelated to other aspects of progress in language. For
children above the 400-word level at both time periods,
closed-class scores were strongly correlated with one
another and with progress in vocabulary. This suggests
that variance in closed-class use after the 400-word
mark is a stable index of productive grammar, and it
also shows (from another perspective) that the
emergence of productive grammar is paced by lexical
growth (see section 2).
A more striking finding comes from the largest
group of children, those below 400 words at Time 1 and
above 400 words at Time 2. In this group, there was no
relationship between early closed-class use and the
emergence of productive grammar. This is an excellent
example of homotypic discontinuity, i.e. a measure that
looks like the same thing has a very different meaning
at Time 1 and Time 2. There was also no continuity

16
between early closed-class usage and later vocabulary
scores.
So what exactly is the basis of early closed-class
use? Bates et al. do report positive correlations below
the 400-word point between closed-class proportion
scores and age. That is, children who use a relatively
high proportion of 'little words' at an early stage of
development tend to be somewhat older than children
who avoid those words at a comparable vocabulary
level. Hence their superior ability to detect and
reproduce function words may be due to their more
mature memories and/or perceptual abilities. As Bates
et al. conclude:
We need not assume that early use of function
words is "bad for children." Instead, we suggest
that children who are developing slowly (for
reasons we do not yet understand) arrive at the
"same" stage of lexical development with an
information-processing system that is somewhat
more mature in other respects from that of younger
lexically matched controls." (Bates et al., 1994.)
These longitudinal findings provide still more
evidence for the point made earlier, that the same
measure may have quite different meaning at different
points in development. Closed-class use after 400
words is an excellent indicator of grammatical
development, but in early development it reflects a
particular stylistic approach.
A supplementary, but potentially very valuable,
approach for evaluating dimensions of stylistic
variation is the training study. By introducing novel
concepts and words in a controlled fashion, we can
observe the acquisition of a piece of language in "real
time" (Nelson and Bonvillian, 1978), and test our
hypotheses concerning processing differences in
language acquisition. Bates et al. (1988), for example,
introduced a novel object (“fiffin”) and action
(“glooping”) to 20-month-old children at home, and
also administered several tests 2–3 days later in the
laboratory. Of particular interest was the tendency to
imitate the label or the action when it was first
presented, and the child's later comprehension in the
laboratory. These measures were then related to a two-
strand model of individual differences posited for early
development: comprehension + analyzed production
(something like referential style), and rote production
(something like expressive/holistic style).
Comprehension of the new label was predicted by other
measures of the first strand, e.g. comprehension,
whereas imitation was predicted by the second strand.
(As Bates et al. (1988) point out, the link to imitation
here is limited to imitation of a new label when it is first
presented.) Thus the hypothesized differences extended
to novel concepts and labels. Interpretation of their
results and those of others, such as Nelson, Baker,
Denninger, Bonvillian, and Kaplan (1985), is made
more problematic because of the curvilinear
relationship of imitation to development, observed in
many domains. It is likely that the phenomenon of
imitation itself would also benefit from an analysis
which separates developmental from individual
variation. In any case, training studies can provide
some of the most direct evidence for both the nature
and generality of stylistic dimensions.
Rethinking”analytic” and “holistic”
The most sweeping characterization of individual
differences in early language remains the pair of terms
"analytic" v. "holistic." We conclude with a suggestion
that the equation of holistic with rote may be a
substantial oversimplification. Children who differ
along this dimension might better be characterized as
differing in the size of the unit they are able or prefer to
manage (Peters, 1977). Plunkett (1993) has pointed out
that articulatory fluency and articulatory precision may
be inversely related, and that a learner who has
extracted a longer expression as a chunk may produce it
fluently, at the expense of precision. There is an
analogy between this inverse variation and the
speed–accuracy tradeoffs so often seen in cognitive
performance. What appear to be qualitative differences
among subjects may simply reflect selection of a
different point along the speed–accuracy continuum. A
holistic strategy corresponds to emphasizing speed by
selecting a larger unit for processing; an analytic
strategy corresponds to emphasizing accuracy, by
selecting a smaller unit. Such is distinction is consonant
with the observation of Vihman (1981) and others that
some children utilize a small but consistent repertoire of
segmental phonemes in prespeech and early meaningful
speech, while others use a larger but less consistent
repertoire.
There are, in fact, at least two hypotheses here
about the relationship of analytic processes and the
preferred size of units, to be distinguished and
evaluated in future research. First, it is possible that all
children are essentially analytic in their approach to
language, but the preferred size of units varies, so some
children appear more holistic than others.
Alternatively, the dimensions of analysis v. rote
processes, and of size of unit, are distinct dimensions of
individual differences. Some children have good
enough memories, and are sufficiently analytic in
approach, that they can pick up larger units and
combine them productively, as well as later analyzing
them into their subparts. Wong-Fillmore's (1979) well-
known observations of differences among child second-
language learners are relevant here. Her most
successful learners began by acquiring phrases and
sentences of immediate functional use, and then
systematically analyzed them into parts for
recombination. The two precocious children M and S,
to be discussed in the next section, also exemplify

17
highly analytic children with different preferred size of
units. Differences in preferred size of units might have
continuing consequences for language development.
Plunkett suggests that the lack of articulatory precision
associated with larger, i.e. formulaic, units might lead
to impoverished representations for these longer units,
which in turn might hinder the recognition of new
linguistic units in the speech signal. He predicts that
the vocabulary spurt will be relatively delayed for these
children (but cf. previous section on the vocabulary
spurt).
To summarize, we have learned some sobering
lessons about individual differences in style of language
learning, especially with regard to the confound
between developmental and stylistic aspects of
vocabulary composition (i.e. referential style; closed-
class usage). It is increasingly clear that these
dimensions of variation cut across different aspects of
language processing (i.e. heterotypic continuity), and it
is also clear that the “same” measure means different
things at different points in time (i.e. homotypic
discontinuity). The relevant underlying dimension may
involve factors like the size of the unit that children
prefer to use at a given point in time, the trade-off
between speed and accuracy in linguistic performance,
variations in perceptual acuity and/or variations in
memory capacity within a given language level. For all
these reasons, it is likely that no single measure will
suffice if we want to divide children into meaningful
groups. It may also be the case that no single language
will suffice if we really want to understand the causal
mechanisms responsible for stylistic variation. The
dimensions of variation that have proven relevant in the
acquisition of English might be quite different in
another language, e.g. a language like Turkish with a
regular and perceptually salient set of inflectional
markers (Slobin, 1985a), or a language like Greenlandic
Eskimo where a complete sentence may consist of
nothing more than a single word with six or eight
grammatical inflections (Fortescue and Lennert Olsen,
1992). There is still a great deal to be learned about the
nature and meaning of stylistic variation in early
language, but we are in a much better position to meet
this challenge than we were 20 years ago.
4. Atypical Children: Variation at the
Extremes
Up to this point, we have focused on variation in
normal language development. We have described
differences in rate of development, dissociations
between comprehension and production (but not
between lexical and grammatical aspects of language),
and differences in cognitive style that are reflected in
language learning in normal children between 8 and 30
months of age. In the final section we turn to studies of
a number of atypical populations that have added useful
data to the investigation of language acquisition. These
include children with delayed onset of expressive
vocabulary (late talkers), children with precocious onset
of expressive vocabulary (early talkers), children with
pre- or perinatal focal brain injury, and children with
cognitive deficits resulting from Williams and Down
Syndrome. Within each subgroup, our discussion is
organized into the same three divisions that we used to
describe variation in normal children: variations in rate,
dissociations between components, and variations in
style. As we shall see, the available data on children at
the extremes adds significantly to our understanding of
the nature and causes of normal variation.
Late talkers
Only recently have researchers begun to focus on
toddlers with delayed onset of language. Because there
is a great deal of variability in early language
development, and many of these children will appear
normal within a year or so, researchers have been
hesitant to call these children language impaired. Thus,
they have come to be called "late talkers" by most of
the researchers who are studying them (Thal and Bates,
1988a; Caulfield, Fischel, DeBaryshe, and Whitehurst,
1989; Fischel, Whitehurst, Caulfield, and DeBaryshe,
1989; Rescorla and Schwartz, 1990; Paul, 1991;
Whitehurst, Fischel, Arnold, and Lonigan, 1992).
However, in many samples there is a disturbing
continuity in language delay for a substantial proportion
of these toddlers (Rescorla and Schwartz, 1990;
Scarborough and Dobrich, 1990; Paul, Spangle-Looney,
and Dahm, 1991; Thal, Tobias, and Morrison, 1991).
Variations in rate
By definition, late talkers are delayed in rate of
expressive language development. Researchers have
shown that rate of development remains delayed in
approximately 40 to 50 percent of their late-talking
subjects, and that other factors are correlated with
continued delay. Paul et al. (1991), for example,
reported that children from families with lower
socioeconomic status were most likely to remain
delayed. Rescorla and Schwartz (1990) noted that
those children who were older when initially identified
as late talkers, and those with the largest lag in
expressive vocabulary were most likely to remain
delayed. Fischel, Whitehurst, and Connors (1987)
reported that the factors identified by Rescorla and
Schwartz were correlated with short-term outcome.
However, in a follow-up one year later they reported
that all children had caught up in expressive vocabulary
(Whitehurst et al., 1992).
Our own studies of three different groups of
children also have varying results. In all cases late
talkers were defined as children who were in the lowest
10 percent for vocabulary production on the MacArthur

18
Communicative Development Inventory (CDI). In our
first cohort, 40 percent (four out of ten) of the children
were delayed in vocabulary production and MLU one
year after being identified (Thal, Marchman, Stiles,
Aram, Trauner, Nass, and Bates, 1991). All of those
children had been delayed in vocabulary
comprehension as well as vocabulary production.
Children who had normal comprehension at the first
time point had normal production vocabulary and MLU
one year later. In the second cohort, nine of 17 children
with specific expressive language delay were compared
to age- and language-matched controls on MLU and
IPSyn (Scarborough, 1990) scores one year later (Thal,
Cleveland, and Oroz, in preparation). Late talkers
remained delayed in MLU (i.e. they were significantly
lower than age-matched controls) but had normal IPSyn
scores (i.e. they were significantly higher than
language-matched controls and no different from age-
matched controls). This suggests that their continued
delay may have more to do with fluency (i.e. utterance
length) than complexity (i.e. diversity of
morphosyntactic structures). In a third cohort, children
who served in the longitudinal validation of the CDI in
the norming study were used (Thal, Bates, and Fenson,
1994). Of 33 toddlers in the lowest 10 percent for
vocabulary production who received the CDI again six
to eight months later, 19 (or 57 percent) were still
delayed in vocabulary production at the second
assessment.
Although the data vary to some extent, the majority
of the studies indicate continuity in the delayed rate of
language development in a large of late talkers over a
six-month to two-year period. We are aware of only
one study in which the development of children
identified as delayed in expressive language at 24
months of age has been followed for a longer period of
time (Rescorla, 1993). Data from that study
demonstrated normal vocabulary development by age
three or four years, but continued delays in subtle
aspects of language use for proportion many children
through age five. At ages six, seven, and eight most of
the specific expressive language-delayed children in
this study performed normally on standardized tests of
language development. However, they were still
significantly different from age-matched controls in a
number of areas including verbal short-term memory,
sentence formulation, word retrieval, auditory
processing of complex information, and elaborated
verbal expression. It is also worth noting that although
the late talkers did perform in the average range on
most of these measures (at or near the 50th percentile),
their age-, sex- and SES-matched controls were
performing significantly above the national average (in
the 70–80th percentile). In short, the jury is still out on
the long-range consequences of an initial delay in word
production. Some children do qualify for a diagnosis of
specific language impairment, but a majority (of still
unspecified size) go on to perform within the normal
range.
Dissociation between comprehension and
production
The most striking dissociation in our studies of late
talkers has been the dissociation between comprehen-
sion and production in those late talkers with specific
expressive language delay. The mean production
vocabulary (based on the CDI) for normal
comprehenders in the first group noted above was 23.33
(range 2–64), whereas the mean comprehension
vocabulary was 285.8 (range 224–371). In the second
cohort the mean for production was 25.1 (1–59) and
comprehension was 268.5 (182–405). In both samples
comprehension vocabulary (even the lowest end of the
range) was well above the average comprehension
expected for children with production vocabularies in
that range. Hence the late-talker population provides an
extreme variant of the comprehension/production
dissociations that we described for children in the
normal range. The fact that so many of our late talkers
do catch up further down the line may be related to
their unimpaired receptive language skills. Conversely,
their initial delays in expressive language could reflect
a temporary bottleneck in processing that can be
overcome further down the line.
Dissociation between lexicon and grammar
Vocabulary and grammar appear to be strongly
associated in our studies of late talkers. In the first
cohort, subjects with normal comprehension had
normal vocabulary production and MLU one year later.
Those with delayed comprehension had delayed
vocabulary production and MLU one year later. In our
second cohort, late talkers were significantly below
age-matched controls on MLU, but they performed
within the normal range on a measure of syntactic
diversity.
Although these results may be typical of late
talkers in the earliest stages of language acquisition, we
know that some language-impaired children with
normal expressive vocabulary have particular difficulty
with grammatical morphology. Johnston and Kamhi
(1984), for example, found that language-impaired five-
year-olds had more difficulty with auxiliary, catenative,
and infinitive verb structures than MLU-matched
controls. Others have noted that many children with
specific language impairment are especially weak in
use of grammatical morphology, with grammatical
morphology lagging behind other areas of language
(Johnston and Schery, 1976; Steckol and Leonard,
1979; Kahn and James, 1983; Bliss, 1989; Leonard,
Bortolini, Caselli, McGregor, and Sabbadini, 1992).
Thus, with prolonged delay it appears that dissociations
within language itself can and do develop. It may be

19
the case that early grammatical learning is inextricably
tied to lexical level, but dissociations develop later at
the point where normal children develop a more fluent
and automatized ability to use grammar in real time —
in line with our conclusions for variability in the normal
range at the end of section 2.
Variations in style
Earlier studies have suggested that the so-called
"holistic style" may be more common in children who
are developing on a slow schedule (Horgan, 1981;
Bates et al., 1988), perhaps because these children have
more mature perceptual systems and/or better memory
for longer strings of sound compared with younger
children at the same level of language development.
When vocabulary composition was examined in the
cohorts described above, no differences were found
between late talkers and vocabulary-matched controls
in proportion of open- and/or closed-class words,
suggesting that late talkers are not atypical on these
dimensions. Hence, the correlation that has been
observed between age and "holistic style" may be
restricted to children within the normal range of
language development.
Early talkers
Rate of development
Linguistic precocity has been the focus of only a few
studies (Robinson, Dale, and Landesman, 1990; Crain-
Thoreson and Dale, 1992; Dale, Robinson and Crain-
Thoreson, 1992; Thal and Bates, 1988b; Thal, Bates,
and Zappia, in preparation). In our studies, we have
defined early talkers as children between the ages of 11
and 21 months of age who are in the top ten percent for
vocabulary production for their age on the CDI. By
definition, they are ahead of their peers, demonstrating
considerable variability in the rate of language
development in this early period.
We know relatively little about the long-term
stability of early precocity in language. However, at
least one study suggests that early talkers tend to stay
ahead of their peers. Robinson, Dale, and Landesman
(1990) identified children as precocious at 20 months
on the basis of vocabulary, MLU, or verbal reasoning
(language items on the Bayley Scales of Infant
Development) at a level two standard deviations above
the mean. Their precocity was maintained for the five
years of the project. For example, at 24 months the
mean MLU for this group was 3.14, equivalent to that
of average children at 36 months; at 30 months the
mean PPVT-R vocabulary age equivalent was 42.6
months; and at age 61/2, their Stanford-Binet IV
Vocabulary age equivalent was nine years, ten months.
Each of these scores is approximately two standard
deviations above the mean, suggesting that early talkers
maintain a significant advantage throughout early
childhood.
Dissociations between comprehension and
production
Many of these early talkers appear to be equally
precocious in comprehension and production.
However, as in the late talkers described above, there
are also a number of children who display a
dissociation between comprehension and production.
In this case, the dissociation takes a different form:
Production does not exceed comprehension, but the
disparity between receptive and expressive vocabulary
displayed by most normal children (and adults) is
markedly reduced. In essence, these early talkers might
be described as "saying everything they know". As a
result they are normal in comprehension (i.e. around the
50th percentile for their age) but abnormally high in
production (in the upper 10th percentile for their age).
Hence there is strong evidence for a dissociation
between comprehension and production at both
extremes of the normal distribution.
Dissociations between lexicon and grammar
Some interesting relationships between grammar and
the lexicon have been observed in some of our early
talkers. We have studied four of the children with
extremely precocious language development in some
detail (Thal and Bates, 1988b; Thal, Bates, and Zappia,
in preparation). In two of these children, grammar did
appear to lag behind lexicon, suggesting some degree of
dissociation, at least in extreme cases. However,
detailed examination of the data reveals an unexpected
association between vocabulary development and
inflectional morphology in these two single-word-stage
children. Table 3 (from Thal, Bates, and Zappia, in
preparation) provides examples of the utterances
produced by two of the children who form a contrasting
pair: S (17 months old with an expressive vocabulary of
603 words in the CDI) and M (21 months old with an
expressive vocabulary of 696 words on the CDI). With
an MLU of 2.13, S demonstrates the correlation
between grammar and vocabulary that we have come to
expect. M, on the other hand, has just begun to
combine words (MLU 1.12). At first glance it appears
that grammar and vocabulary are dissociated in this
child. However, M does produce contrasting
grammatical inflections in her single-word speech.
This is a very odd phenomenon for children acquiring
English, but typical of children acquiring a highly
inflected language such as Turkish (Slobin, 1985a). In
fact, the amount and type of grammatical morphology
observed in M's speech falls well within the range that
we would expect for children with more than 500 words
(Marchman and Bates, 1994 ). Thus, the seeming
dissociation between lexicon and grammar does not

20
exist. The difference between these children must be
sought elsewhere, which brings us to the next point.
Variations in style
In the early-talker group identified within the CDI
norming study, we found the same range of variation in
referential and/or closed-class style evidenced by
children further down in the distribution. This mirrors
our results (or lack thereof) for late talkers, suggesting
that vocabulary style is not uniquely associated with
slow or fast rates of language learning.
However, we did find some interesting evidence
for stylistic variation in the case studies of very early
talkers described above. Whereas M produced
carefully articulated single words, S was observed to
use longer utterances that often appeared formulaic in
nature. Her parents indicated that she could remember
and produce a number of songs and idiomatic
expressions (e.g. "No way, José," or "You little
monkey!"). An examination of the sample utterances
for S in table 3 supports this idea. She appears to make
use of partially analyzed formulae and "frame-slot"
structures in her spontaneous speech (e.g. "Where —
went?"). In fact, to the surprise and amusement of her
mother and the experimenter, S produced a novel
juxtaposition of two established formulae during one of
the experimental sessions: "No way, you monkey!"
Thal et al. tentatively conclude that the apparent
dissociation between grammar and lexicon illustrated
by M and S actually reflects a style dimension based on
the length of the unit that can be retained in memory.
Both M and S appear to produce analyzed utterances,
and both seem to use a slot-filler approach to
combining words. Examples of S's formulaic
combinations were noted above. M's few two-word
utterances were typical pivot-style utterances made with
two pronouns (see table 3) including "hold it", "keep
it", "pour it", and "hide it". What was different between
the two children was not so much analytic versus
holistic processing but, rather, the size of the units
which they were able to combine.
Children with focal brain injury
For a number of years, we have studied the first stages
of language development in children with unilateral
damage to the right or left hemisphere, injuries that
were acquired prenatally or before 6 months of age (i.e.
before the onset of meaningful speech). It has been
known for some time that children with this etiology go
on (more often than not) to achieve normal or near-
normal levels of language ability, despite damage to
areas that often result in irreversible aphasia when they
occur in an adult (Lenneberg, 1967; Hecaen, 1976;
Rasmussen and Milner, 1977; Woods and Teuber,
1978). Hence this group of children provides
compelling evidence for the plasticity of neural
organization for language in our species. However, this
does not mean that the newborn brain is equipotential
for language (for reviews, see Satz, Strauss and
Whitaker, 1990; Stiles and Thal, 1993; Aram and
Eisenberg, this volume). Current evidence suggests a
compromise between two theoretical extremes (i.e.
irreversible determinism v. total equipotentiality).
There are initial biases, which affect the timing and
nature of early language, but eventually most of these
children find alternative modes of behavioral and/or
neural organization that are sufficient to sustain
adequate (if not optimal) language development.
In this section, we will review evidence on the
early stages of language in children with focal brain
injury with two purposes in mind: (1) to uncover further
evidence for variations in rate and style and
dissociations between components, at the extremes of
normal development, and (2) to provide some insights
into the neural bases of the patterns that have been
observed in other populations.
Rate of development
As a group, children with focal brain injury are delayed
in phonology (Marchman, Miller, and Bates, 1991), and
in lexical development (Thal et al., 1991) and MLU
(research in progress). These group delays have been
found to continue through 44 months of age (our most
recent data point). However, there are exceptions
among individual children, including some who fall
within the normal range from the very beginning. In
the largest sample that we have studied to date, 38
children between 16–31 months of age have been
assessed with the vocabulary scale of the CDI:
Toddlers. For the group as a whole, including those
with left- and right-hemisphere damage, 16 out of 38
children (approximately one third of the sample) have
vocabulary scores in the bottom 10th percentile.
Although this is far more than we would expect by
chance (p < 0.001), it also means that most children
with focal brain injury fail to meet the criterion used to
define late talkers. When attention is restricted only to
the children with left-hemisphere damage, 13 out of
25 children (one half of the sample) fall in the bottom
10th percentile. Obviously left-hemisphere damage is a
serious risk factor for early language delay, but it is also
true that half of even this 'high risk' sample falls within
the normal range, including a substantial number of
children with scores well above the median.
We are now trying to determine those sites within
the left hemisphere that are most often associated with a
serious initial language delay, to determine whether and
to what extent the “risk sites” map onto findings from
brain-behavior mapping in adult aphasics. So far it is
clear that these results will be approximate at best.
Some sites may be more important than others (e.g.
sites surrounding Broca's and Wernicke's areas).
However, we have seen individual cases of children
with injuries that involve all of the tissue in and around
the classical language areas, who nevertheless display

21
normal levels of language comprehension and
production (see also Dall'Oglio, Bates, Volterra, Di
Capua, and Pezzini, 1992; Feldman, Holland, Kemp,
and Janosky, 1994; Vargha-Khadem, Isaacs, Van der
Werf, Robb, and Wilson, 1992). These findings on the
earliest stages of development in children with focal
brain injury fits with a well-established finding on the
long-term sequelae in this population: more often than
not, these children go on to develop language skills that
are well within the normal range, testimony to the
extraordinary plasticity of the human brain for its most
important cognitive skill.
Dissociations between comprehension and
production
Within the focal lesion population, we find some of the
same strong dissociations between comprehension and
production reported for the late-talker sample. At the
same time, we also find children who are impaired in
both modalities, and children who appear to have
escaped unimpaired in both receptive and expressive
language.
One of the first questions that we have asked of
this population pertains to the neural sites associated
with deficits in comprehension v. production. The
answers to date are mixed (Thal et al., 1991; Wulfeck,
Trauner, and Tallal, 1991), but they do suggest some
differentiation in the neural mechanisms that underlie
development in these two modalities. Comprehension
deficits occur with damage to either hemisphere, and
are (if anything) somewhat more common in children
with unilateral right-hemisphere damage. Hence, we
tentatively conclude that early comprehension is a
"whole-brain activity", i.e. a bilaterally distributed
process that may require more right-hemisphere
mediation during the early stages of learning than we
see in adults with equivalent forms of brain injury.
This is, in fact, quite compatible with the
electrophysioogical studies of comprehension and
production in normal children that we reported in
section 2 (Mills et al., 1993, and in press).
Production deficits do appear to be somewhat more
likely with damage to the left hemisphere, although the
intrahemispheric sites associated with initial delays are
variable from one study to another. In the study by
Thal et al, delays in expressive vocabulary were equally
likely between 12–16 months with left- or right-
hemisphere damage, and with damage to both anterior
and posterior sites on the left. In other words, brain
damage is certainly not a good thing to have, and it is
hard to get expressive language off the ground with any
form of cortical or subcortical damage. By contrast,
between 16–36 months, there was a clear association
between continued expressive delays and damage to
left-hemisphere sites. In the Thal et al. sample, these
delays in recovery of expressive language were greater
in children with left posterior damage — a peculiar
reversal of the usual story for adult aphasics (where
damage to frontal sites around Broca's area tends to
result in nonfluent aphasia). However, in our more
recent studies with an expanded sample, delays in
recovery from expressive deficits are associated with
specific sites in both the anterior and posterior
quadrants of the left hemisphere.
These results are complicated by a number of
factors, including (a) the absence of a solid data base on
normal brain development to anchor analyses of
damage to specific lesion sites (see Bates et al., 1992,
for a discussion of this point), and (b) confounding
factors like seizure activity and seizure medications that
plague research on children with focal brain injury. For
the moment, we must be content with only one tentative
conclusion: Dissociations between comprehension and
production do occur in the focal lesion sample, and they
appear to be associated with different patterns of focal
brain injury. However, the brain-behavior correlations
observed in early language development do not map
consistently onto the correlations that are observed in
adults with analogous lesions.
Dissociations between lexicon and grammar
In contrast with the strong dissociations between
comprehension and production that appear in the focal
lesion population, we find (once again) virtually no
evidence for a dissociation between grammar and the
lexicon — at least not within the period from 20–36
months when the foundations of grammar are laid
down. Indeed, within our latest sample of toddlers with
focal brain injury, the correlation between vocabulary
and grammar scores falls between +.70 – +.92,
depending on the subset of children who are included or
excluded from the sample. Thus we may conclude that
early grammatical development is lexically driven in
this population as well. However, we leave open the
possibility that a dissociation may appear at later stages
of development, when grammar must become a fluent
and automatized skill.
Variations in style
One of the first questions that we are often asked when
we lecture on individual variations in style revolves
around the potential neural bases of the analytic/holistic
contrast. For example, it has been argued that the right
hemisphere of the normal adult brain plays an important
role in the integration of perceptual information
(including but not restricted to visual–spatial patterns)
into a global configuration, while the left hemisphere
plays a more important role in the extraction of
perceptual detail (Robertson and Delis, 1986; Lamb,
Robertson, and Knight, 1990; Dukette and Stiles,
1991). By analogy to this analytic/holistic division in
perceptual analysis, it has been suggested to us that
children who evidence a holistic approach to language
may make greater use of right-hemisphere processes,

22
while children who adopt an analytic style rely to a
greater extent on left-hemisphere processes.
A different neural basis for stylistic variation has
been suggested for children with left-hemisphere
damage. In particular, it has been pointed out that
children at the extreme analytic end of the normal
distribution produce telegraphic speech with a high
proportion of content words and very few function
words (e.g. "Mommy sock") — very much like the
speech that is characteristic of adult patients with
Broca's aphasia. By the same token, children at the
extreme holistic end of the normal distribution tend to
produce utterances with a relatively high ratio of
function words to content words (e.g. "I wan' dat") —
very much like the speech that is characteristic of adult
patients with Wernicke's aphasia. To the extent that (a)
Broca's aphasia is correlated with left frontal injury, and
(b) Wernicke's aphasia is correlated with left posterior
injury, it has been proposed that the analytic/holistic
dimension in early language learning could reflect
differential use or rate of maturation in anterior v.
posterior regions of the left hemisphere, with "little
words" handled by the regions around Broca's area and
content words mediated by left posterior cortex.
Our evidence on this point is meager and
inconclusive on both these points, but to the extent that
it can be used at all, it suggests exactly the opposite
conclusion on both counts. In general, we find the
same mix of vocabulary composition scores in our focal
lesion population that we find in the normal range, and
in both our late-talker and early-talker groups. Nor do
we find a strong association between language style and
lesion type, organized by side (left v. right) or site (left
anterior v. left posterior). However, the study by Thal
et al. found significantly less use of closed-class
morphemes in children with injuries involving left
posterior cortex — in direct contradiction to both the
interhemispheric hypothesis (right hemisphere =
holistic; left hemisphere = analytic) and the
intrahemispheric hypothesis (left anterior = function
words; left posterior = content words). A more
definitive answer regarding the cortical basis of analytic
v. holistic style will await studies that use a broader
range of measures, including laboratory analyses of
imitativeness, phonological precision, and speech rate.
Williams Syndrome and Down Syndrome
A final source of information on the extremes of
variation comes from children with two dramatically
different forms of mental retardation, Williams
Syndrome and Down Syndrome. Children with Down
Syndrome are markedly delayed in the acquisition of
language, but more importantly, their language abilities
at virtually every stage (including the adult steady state)
fall below the levels that we would expect based upon
their mental age (Miller, 1987; Bellugi, Marks, Bihrle,
and Sabo, 1988; Bellugi, Sabo, and Vaid, 1988; Miller,
1988; Bellugi, Bihrle, Jernigan, Trauner, and Doherty,
1990; Jernigan and Bellugi, 1990; Reilly, Klima, and
Bellugi, 1991; Bellugi, Bihrle, Neville, Jernigan, and
Doherty, 1992; Wang, Doherty, Hesselink, and Bellugi,
1992; Wang, Hesselink, Jernigan, Doherty, and Bellugi,
1992; Mervis and Bertrand, 1993; Bellugi, Wang, and
Jernigan, 1994; Miller, in press a & b). Furthermore,
they appear to be especially impaired in the use of
bound and free grammatical morphemes, constituting a
form of congenital agrammatism that is even more
severe than the selective impairments in grammatical
morphology reported for children with Specific
Language Impairment (see above). By contrast, older
children and adults with Williams Syndrome display
levels of linguistic knowledge and language use that are
surprisingly good when they are compared with the low
levels of performance that the same individuals show
on most measures of visual–spatial cognition, problem-
solving and reasoning (Bellugi, Delis and Marks, 1989;
Bellugi et al., 1992; Bihrle, Karmiloff-Smith, and
Grant, 1993; Carey, Johnson, and Levine, 1993; Mervis
and Bertrand, 1993). to here
Before we go into details on the early stages of
language learning in these two groups, we should point
out that older children and adults with Williams
Syndrome cannot be characterized as "language
savants", where savants are defined as retarded
individuals who show skills far above normal in a
single domain. Indeed, those studies that have used
normal controls have shown that the linguistic
performance of Williams subjects is invariably below
their chronological age — which is, of course, not
surprising for subjects with an IQ score around 50.
When they are compared with younger normals
matched for mental age, the picture is mixed: they
perform above mental-age controls on some measures
(e.g. the Peabody Picture Vocabulary Test, word
fluency, and a number of measures of grammar) while
they look very much like mental-age controls on others
(e.g. the Wisc R). The excitement that research on
Williams Syndrome has engendered in recent years
hinges not on their absolute levels of performance, but
on the striking profiles of sparing, deviance and delay
that they display across different linguistic and
nonlinguistic tasks — a profile that challenges many
existing theories of the relationship between language
and cognition. On some linguistic measures their
performance is not only superior to mental-age controls,
but qualitatively different from normals of any age. For
example, on word fluency tasks (e.g. "Name all the
animals you can think of") they tend to produce low-
frequency items like "ibex" and "brontosaurus" that are
never produced by normals or by other individuals with
mental retardation. Even more compelling evidence for
qualitative variation comes from a story-telling task
(Reilly, Klima, and Bellugi, 1991), where Williams
Syndrome children and adolescents were compared
with normal controls and with IQ-matched subjects

23
with Down Syndrome. In retelling the same story to
the same audience, normal children and Down
Syndrome individuals tend to produce succinct stories
with relatively flat affect, stories that get shorter and
less interesting with every retelling. By contrast, the
Williams Syndrome subjects produce novel, colorful,
and emotional descriptions on every trial, with no loss
of enthusiasm. Aside from their command of the
grammar, their stories abound with rich prosody and
"audience hookers" like "You know what?", or "And
then guess what happened!". This level and style of
performance has not been observed in any other normal
or clinical group. The peculiar patterns of sparing and
impairment that are observed within the language
domain in Williams Syndrome are complemented by
equally interesting patterns of sparing and impairment
outside of language. For example, although they show
severe impairments on most visual-spatial tasks, they
are remarkably good at face recognition (Bellugi, Wang
and Jernigan, 1994; see also Bertrand, Mervis, Rice and
Adamson, 1993), surpassing their age-matched controls
on some measures. And although they perform poorly
on virtually all reasoning and problem-solving tasks,
they are far less impaired on so-called "theory of mind"
tasks that require an ability to reason about the
intentions and plans of other human beings (Karmiloff-
Smith and Grant, 1993).
Taken together, these contrasts between Down
Syndrome and Williams Syndrome provide an
important challenge to interactive theories of language
and cognition, and appear at first glance to provide
considerable support for the autonomy of language
from other cognitive systems (Karmiloff-Smith, 1992;
Bellugi, Wang and Jernigan, 1994). However, when we
compare the early stages of language development in
these two clinical groups, a somewhat more subtle
picture emerges.
Rate of development. A number of studies have
begun to explore the early stages of language
development in children with Williams syndrome. In
every case these children are significantly delayed and
the developmental period is prolonged (Thal, Bates and
Bellugi, 1989; Mervis and Bertrand, 1993). In other
words, despite their ultimate proficiency with language,
children with Williams Syndrome are late talkers. This
has become exceedingly clear in a recent study by
Bellugi, Bates and colleagues (research in progress),
who have used the MacArthur CDI to obtain early
language data from more than 130 children with
Williams and Down Syndrome, between one and six
years of age.
In the period of development covered by the CDI:
Infants (equivalent to normal children between 8–16
months), both the Williams and the Down samples are
delayed by many months or years on both word
comprehension and word production. The predicted
separation between the two clinical groups does not
emerge until the period of development covered by the
CDI:Toddlers (equivalent to normal children between
16–30 months), around the time when grammar begins
(see below). At this point, both groups are approxi-
mately two years older than normal language-matched
controls.
The late appearance of language in Williams
Syndrome sets interesting limits on the degree of
dissociation that can be observed between linguistic and
nonlinguistic skills. We tentatively suggest that the
similarities between Williams and Down Syndrome in
the first stages of language development, and the late
onset of language (both vocabulary and grammar) in
both these groups, are evidence in favor of a revised
version of the cognitive prerequisites to language view
proposed in the 1970s (for reviews, see Bates and
Snyder, 1987; Bates and Thal, 1991; Bates, Thal and
Marchman, 1991). Specifically, we propose the
following hypothesis:
Language cannot get off the ground until some
minimal set of cognitive infrastructures are finally in
place, including cognitive structures that are necessary
though (perhaps) not sufficient for the establishment of
reference and predication.
Dissociations between comprehension and
production. Although there is considerable variability
within both these clinical groups, we do not find more
comprehension/production dissociations in either group
than we would expect by chance (i.e. no more than we
would expect if these children were drawn from the
normal population). Furthermore, there are no
significant differences between the two groups in
comprehension/production profiles. The general
picture appears to be one of delay rather than deviance,
at least along this dimension.
Dissociations between grammar and the lexicon.
In contrast with their findings for the Infant scale,
Bellugi and colleagues report a sharp divergence
between Williams Syndrome and Down Syndrome
children during the period in which grammar is finally
under way. However, the direction of this disparity is
interesting, and perhaps somewhat surprising. Within
the Williams group, grammatical development appears
to be paced by vocabulary size, in the normal fashion.
In fact, when these children are compared with lexically
matched normal controls from the CDI sample, the
relationship between grammar and vocabulary size is
identical, following the nonlinear accelerating function
described in Section II (see figure 4.10). In short, there
is no evidence for a dissociation between grammatical
and lexical development in the Williams group — at
least not in this early phase of grammatical
development.
By contrast, data for the Down Syndrome sample
provide our first evidence to date for a significant
dissociation between grammar and the lexicon. In
particular, Down Syndrome children score significantly
below the grammatical levels displayed by normal
children (or Williams Syndrome children) matched for

24
vocabulary size. We tentatively conclude that lexical
size is a necessary but not sufficient condition for the
acquisition of grammatical function words, the onset of
word combinations, and growth in sentence complexity.
This finding is compatible with reports on the selective
impairment of grammar displayed by older Down
Syndrome children, although the basis of the
impairment is still unknown. It could be due to
impairment of some domain-specific grammatical
processor (e.g. Pinker, 1991), or, alternatively, it may
derive from aspects of information processing that are
only indirectly related to grammar. For example, Wang
and Bellugi (1993) have presented evidence from older
Williams and Down Syndrome individuals suggesting
that there may be a double dissociation in the two
groups between auditory digit span (significantly better
in Williams Syndrome) and visual short-term memory
on the Corsi sequential block touching task
(significantly better in Down Syndrome). If Down
Syndrome individuals suffer from a selective
impairment in aspects of auditory processing (super-
imposed on their more general cognitive deficits), then
it is not unreasonable to infer that they may be
selectively impaired in the ability to detect, store and/or
retrieve aspects of the auditory input that are low in
phonological salience or stress (Leonard et al., 1992)
and low in visual imagery (Goodglass and Menn,
1985).
It would be premature to conclude from these data
that grammar is perfectly normal in Williams Syndrome
but impaired in Down Syndrome. Studies of older
Williams Syndrome individuals have uncovered subtle
but significant deficits in certain aspects of grammatical
morphology, including prepositions (Rubba and Klima,
1991) and grammatical gender (a study of French-
speaking Williams Syndrome individuals by Karmiloff-
Smith and Grant, 1993). There is still a great deal to be
learned about the patterns of sparing and impairment
that occur inside and outside of language in this
puzzling syndrome (Carey and Grant, 1993; Mervis and
Bertrand, 1993; Bellugi, Wang and Jernigan, 1994).
For our purposes here, we may conclude that grammar
does not develop ahead of the lexicon in the early
phases of development, in any of the syndromes that we
have studied to date including Williams Syndrome.
However, the Down Syndrome data show that
dissociations in the opposite direction can and do occur,
with grammar falling behind vocabulary growth.
Variations in Style. Older children and adults with
Williams Syndrome produce language that is novel and
creative, although it is occasionally bizarre or
inappropriate from a pragmatic point of view. They
also show better receptive language skills (including
judgments of grammaticality and interpretation of
reversible passives) than we would expect on a
formulaic/echolalic interpretation of their spontaneous
speech. We can, therefore, reject the hypothesis that
Williams Syndrome children are little tape-recorders,
spewing back sentences that they heard the day before
(i.e. "cocktail party syndrome" — Tew, 1975). In other
words, Williams Syndrome does not constitute an
extreme variant of holistic/formulaic style as it was
originally conceived.
On the other hand, it should be clear from our
review in Section III that the formulaic/echolalic view
also fails to capture the speech produced by normal
children at the holistic end of the analytic/holistic
distribution. To be sure, there was an initial effort to
characterize this dimension as a continuum from
spontaneous production to imitative speech, and there
is a small amount of evidence suggesting that holistic-
style children imitate more often than children at the
analytic/telegraphic end of the distribution (e.g.
immediate imitation of nonsense words at 20 months of
age in Bates et al., 1988). However, other studies have
shown that children who are high in referential style
imitate more often when they are presented with a
series of new names for novel objects (Leonard et al.,
1991). In short, imitativeness may be a correlate or
fellow traveler along this mysterious dimension, but it
does not define the set of distinctions reviewed in
Section III. We have proposed instead that the
distinction between analytic and holistic style has more
to do with factors like auditory short-term memory and
the size of the unit a child can manage at a given point
in time, degree of perceptual acuity (which helps some
children to tune into inflections and function words at a
relatively early point in lexical development), and/or a
trade-off between speed and accuracy in early speech
production. Armed with this refinement of the
analytic/holistic dimension, let us reconsider the
applicability of this dimension to language development
in children with Williams and Down Syndrome.
In the Bellugi et al. study of Williams and Down
Syndrome using the MacArthur CDI, parental reports
have been analyzed to determine whether the two
groups differ significantly in vocabulary composition,
including referential style (controlling for vocabulary
level) and closed-class style (in the period before 400
words, i.e. before the emergence of productive
grammar). Results suggest that there is considerable
variability within both groups along both of these
dimensions, but no more than we would expect if
children were drawn from the normal population (i.e.
no more than we find in language-matched controls),
and there were no significant differences between the
two groups. In other words, there is no evidence in
support of the idea that Down Syndrome children
display a variant of analytic style while William
Syndrome children display an extreme version of
holistic style. On the other hand, as we pointed out in
Section III, vocabulary composition scores offer an
indirect and inadequate view of this important
dimension of variation. More interesting information
would come from studies that include
repetition/imitation of nonsense words, phonological

25
precision and consis-tency, frequency of certain
construction types in spontaneous speech, auditory
short-term memory, and so forth. In these respects, the
predicted analogy may still hold (see also Thal, Bates
and Bellugi, 1988). As we noted above, Williams
Syndrome individuals perform significantly better than
Down Syndrome children in an auditory memory task.
Karmiloff-Smith and Grant, 1993) have also shown that
older individuals with Williams Syndrome are actually
more proficient than age-matched normal controls in a
task that requires immediate repetition of novel words
— even though they perform more poorly than controls
in the assignment of grammatical gender for the same
new words. In other words, we think the
analytic/holistic dissociation may be fruitfully pursued
in research on Williams and Down Syndrome, but only
with the caveats that we have described here.
There is, of course, another possibility: the peculiar
profiles of linguistic and nonlinguistic ability displayed
by older children and adults with Williams Syndrome
may reflect a qualitatively different form of brain
organization for language, unlike anything that we have
seen in any other normal or abnormal population
(Jernigan and Bellugi, 1991; Jernigan, Bellugi, Sowell,
Doherty and Hesselink, 1993). If this proves to be the
case, then evidence from Williams Syndrome will have
greatly expanded our understanding of variation and
plasticity in language development.
CONCLUSION
We have reviewed evidence for individual
differences in early language development in normal
children, and in several contrasting clinical populations.
Some consistent themes emerge from this survey.
First, there is enormous variability in rate of
development during the passage from first words to
grammar, in each and every one of our measures.
These variations are so stable that they cannot be
ascribed to noise, and so large that they are hard to
explain with any single causal factor, maturational or
environmental. We do not pretend to have exhausted
the universe of possible biological and/or demographic
causes for this variation, but we think it likely that a
combination of factors will be required to explain
individual differences of this magnitude (see also
Huttenlocher et al., 1991; Hampson and Nelson, 1993).
This includes a need for cross-linguistic studies of
developmental variation, to complement cross-
linguistic studies that compare the sequences and
central tendencies that are observed in different
language families (Slobin, 1985b, Volumes I and II;
Slobin, 1993, Vol. III).
Second, we find robust evidence for a dissociation
between comprehension and production in several
normal and abnormal populations. A possible
explanation for this dissociation comes from
independent evidence suggesting that the two
modalities are correlated with different nonlinguistic
factors (in particular, comprehension is strongly
associated with several different measures of cognitive
development during the first and second year), and may
be mediated by different neural systems (bilateral
mechanisms for word comprehension, specialized left-
hemisphere mechanisms for word production).
By contrast, we find a remarkably strong
association between lexical and grammatical develop-
ment in the period from 16–30 months of age (or its
developmental equivalent in clinical populations). For
example, there is a powerful nonlinear relationship
between vocabulary size and the appearance of word
combinations (usually between 50–100 words), and an
equally strong exponential relationship between
vocabulary size and increases in utterance length and
sentence complexity (with a "take-off point" some-
where between 300–600 words). This relationship is
also observed in late talkers, early talkers, children with
focal brain injury, and children with Williams
Syndrome. The robust nonlinear association between
grammar and vocabulary size is predicted by recent
connectionist models of morphological development
(Plunkett and Marchman, 1993), and it is consistent
with theories that underscore the role of perceptual
and/or semantic bootstrapping or mass-action effects
(Goodman and Nusbaum, 1994 ; O'Grady, 1987). We
should point out, however, that this finding does not
preclude the possibility of a grammar/semantics
dissociation at some later point in development, when
grammar achieves the status of a fluent, automatic skill
(e.g. Marchman, Bates, Burkhardt and Good, 1991).
Down Syndrome children are the only group that
we have seen so far in which there is a clear
dissociation between grammar and vocabulary during
this stage of language learning. For these children,
early grammar falls far behind vocabulary growth,
suggesting that lexical size is a necessary but not
sufficient condition for the extraction of grammatical
regularities. The nature of the limiting factor that
creates the grammatical deficit in Down Syndrome is
still unknown, but it may be related to limitations in
auditory short-term memory (Wang and Bellugi, 1993).
We also reviewed the history and current status of
a peculiar dimension of qualitative or stylistic variation
that travels under many names. The most common
name for this dimension of variation is "analytic v.
holistic style", although our review suggests that this
may be an unfortunate choice of terms. To be sure, this
dimension of individual differences cuts across specific
ages and content domains (i.e. phonology, vocabulary,
grammar), and it does seem to have something to do
with the size and precision of the unit that children can
handle (or prefer to use). But it has proven frustratingly
difficult to operationalize this dimension (if it is indeed
a single dimension). There are some serious confounds
between developmental and stylistic variance in the
vocabulary composition scores that have been used to
assess language styles in previous studies. Propensity
for imitation is clearly an associate of the style

26
dimension, but it does not define that dimension.
Furthermore, as several studies have shown (Thal and
Bates, 1988b; Pine and Lieven, 1990; Thal, Bates and
Zappia, in preparation), holistic style is not the same
thing as frozen or formulaic speech, and can be a route
into grammatical productivity. Our findings for
children with focal brain injury offer little help in
uncovering the neural bases of this dimension. The
most obvious neural hypotheses receive no support in
our results to date (e.g. identification of holistic style
with the right hemisphere; identification of telegraphic
style with limitations in Broca's area). The contrasting
profiles presented by Williams Syndrome and Down
Syndrome hold some promise in unravelling the basis
of this mysterious contrast in learning styles, but most
of the important work still lies before us. We have
proposed that future work in this direction should focus
not on simple measures of vocabulary composition, but
on processing dimensions like auditory short-term
memory, perceptual acuity, unit size, and a speed-
accuracy trade-off in real-time language use. Needless
to say, these are difficult variables to study in children
under 3 years of age.
One conclusion seems uncontroversial: The
Average Child is a fiction, a descriptive convenience
like the Average Man or the Average Woman.
Theories of language development can no longer rely
on this mythical being. Any theory worth the name will
have to account for the variations that are reliably
observed in early language learning.
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TABLE 1: INDIVIDUAL DIFFERENCES IN LANGUAGE DEVELOPMENT:
SUMMARY OF CLAIMS IN THE LITERATURE.
Strand 1 Strand 2
______
Semantics
High proportion of nouns Low proportion of nouns
in first 50 words in first 50 words
Single words in early speech Formulae in early speech
Imitates object names Unselective imitation
Greater variety within lexical categories Less variety within lexical categories
Meaningful elements only Use of "dummy" words
High adjective use Low adjective use
Context-flexible use of names Context-bound use of names
Rapid vocabulary growth Slower vocabulary growth
Grammar
Telegraphic in Stage I Inflections and function words in Stage I
Refers to self and others Refers to self and others
by name in State I by pronoun in State I
Noun phrase expansion Verb phrase expansion
Morphological overgeneralization Morphological undergeneralization
Consistent application of rules Inconsistent application of rules
Novel combinations Frozen forms
Imitation is behind spontaneous speech Imitation is ahead of spontaneous
speech
Fast learner Slow learner
Pragmatics
Object-oriented Person-oriented
Declarative Imperative
Low variety in speech acts High variety in speech acts
Phonology
Word-oriented Intonation-oriented
High intelligibilty Low intelligibility
Segmental emphasis Suprasegmental emphasis
Consistent pronunciation Variable pronunciation
across word tokens across word tokens
Demographic Variables
Female Male
Firstborn Laterborn
Higher SES Lower SES

TABLE 2: INDIVIDUAL DIFFERENCES IN LANGUAGE DEVELOPMENT:
ALTERNATIVE EXPLANATIONS.
Social Explanations
Exogenous Maternal style and input
• Object vs. person focus
• Does or does not imitate the child
Social class
• Elaborated vs. restricted code
Endogenous Temperament
• Object vs. person orientation
• Reflective vs. impulsive
approach to problems
Linguistic Explanations
Within language proper • Language function vs. language form
• Open- vs. closed-class lexicon or
semantics vs. grammar
• Word order vs. morphology
Between language • Environmentally sensitive
and cognition and insensitive processes
Neurological Explanations
Interhemispheric • Right- vs. left-hemisphere emphasis
Intrahemispheric • Anterior vs. posterior emphasis
within the dominant hemisphere
Cognitive Explanations
Unidimensional • General intelligence
• Field dependence — independence
Multidimensional • Analytic vs. Gestalt/holistic processing
• Analytic vs. imitative
learning mechanisms
• Patterners vs. dramatists
• Information-sensitive vs.
frequency-sensitive
• Comprehension-driven vs.
production-driven
• Analysis for understanding vs.
analysis for reproduction

TABLE 3: EXAMPLES OF LANGUAGE PRODUCTION
BY TWO VERY EARLY TALKERS (from Thal & Bates, 1988b)
SARAH: MARYANN:
Age: 17 Months Old Age: 21 Months Old
Vocabulary 596 words Vocabulary: 627 words
Vocabulary age: 30 months Vocabulary age: > 30 months
MLU: 2.13 MLU 1.19
MLU age: 28 months MLU age: 20 months
Where cup went? Pretty.
Where chair went? Cute.
Teddy bear went? Big.
Baby doing? Round.
Dry.
Wanna walk e baby. Hungry.
Wanna put it on. Wet.
Wanna go ride it. Different.
Want mom get off. Enough.
Else.
Daddy take her. (referring to self) More.
Help with the apple. Minute.
Can't get the teddy bear. Brushing.
Teddybear the bath. Hiding.
Baby crying.
Too much carrots on the dish. Hold.
Move it around. Hold it.
Clean e bottom. Dropped it.
Bring it.
Put ne sofa.
Put in eye. Falling.
Fell.
Mommy wear hat.
Mommy smell it. Talk.
Mommy read the book. Talking.
Mommy sit down. Wash'em.
Find Becky. Shirt on.
See Becky in the morning. Teddy up.
Becky is nice. Mommy shoe.
Saw Becky and goats.