Age and Sex Biases in the Preservation of Human Skeletal Remains

Abstract

Inaccuracies introduced through biases in preservation are a major source of error in paleodemographic reconstructions. Although it is generally assumed that such biases exist, little is known about their magnitude. To investigate this problem, we studied age and sex differences in the preservation of skeletal remains from Mission La Purisima and a prehistoric cemetery (Ca-Ven-110). Comparison of mortality profiles obtained through analysis of skeletal remains and burial records from the mission indicates that biases in preservation can be very significant in poorly preserved skeletal collections. The Purisima burial records show that most of the people interred in the cemetery were either infants or elderly adults. The skeletal remains, in contrast, are predominantly those of young adults. The burial records and skeletal collection produced comparable sex ratios. These results show that age biases in preservation are much more important than sex biases. This conclusion is supported by data on the completeness of the skeletons from La Purisima and Ca-Ven-110. At both sites, the remains of young adults were better preserved than those of children or elderly adults, and the completeness of male and female skeletons was comparable.

Full text

Age and Sex Biases in the Preservation of Human Skeletal
Remains
© 1988 ALAN R. LISS, INC.
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 76:183-188 (1988)
PHILLIP L. WALKER, JOHN R. JOHNSON,
AND
PATRICIA
M.
LAMBERT
Department of Anthropology, University of California, Santa Barbara,
California 93106
KEY WORDS

Mortality profiles, Demography, Vital statistics,
American Indians, Archeology
ABSTRACT

Inaccuracies introduced through biases in preservation are a
major source of error in paleodemographic reconstructions.
Although it is gen-
erally assumed that such biases exist, little is known about their magnitude.
To investigate this problem, we studied age and sex differences in the preser-
vation of skeletal remains from Mission La Purisima and a prehistoric cemetery
(Ca-Ven-110). Comparison of mortality profiles obtained through analysis of
skeletal remains and burial records from the mission indicates that biases in
preservation can be very significant in poorly preserved skeletal collections. The
Purisima burial records show that most of the people interred in the cemetery
were either infants or elderly adults. The skeletal remains, in contrast, are
predominantly those of young adults. The burial records and skeletal collection
produced comparable sex ratios. These results show that age biases in preser-
vation are much more important than sex biases. This conclusion is supported
by data on the completeness of the skeletons from La Purisima and Ca-Ven-
110. At both sites, the remains of young adults were better preserved than those
of children or elderly adults, and the completeness of male and female skeletons
was comparable.
The use of archeological skeletal remains
to
make paleodemographic
reconstructions
rests on the assumption that the age and sex
distributions of the skeletal sample reflect
those of the burial population they are de-
rived from and that any biases introduced
through differential preservation can be rec-
ognized and accounted for (Ubelaker, 1984).
Although osteologists are generally aware
that the comparatively rapid disintegration
of the poorly calcified remains of children,
elderly adults, and females distort mortality
profiles of skeletal collections, little is known
about the magnitude of these biases. To in-
vestigate this problem, we analyzed skeletal
collections from La Purisima Mission and a
prehistoric southern California cemetery. The
skeletal collection from the Purisima Mis-
sion cemetery provides an unusual oppor-
tunity to assess the effects of differential
preservation on the demographic structure
of a burial population. Comparison of mor-
tality profiles based on the burial records
and skeletal remains from the mission cem-
etery indicates that major discrepancies have
been introduced through the rapid disinte-
gration
of
the poorly calcified remains of
children and elderly adults. Our studies of
the preservation of skeletal remains from
Ca-Ven-110 provide additional evidence for
age-related biases in preservation.
SKELETAL PRESERVATION AT MISSION LA
PURISIMA
A comparison was made of skeletal re-
mains recovered from the Purisima Mission
cemetery near Lompoc, California and bur-
ial records kept by the Franciscan priests
who operated the mission cemetery. The rec-
ords indicate that it was used as a burial
place for the mission's Indian neophytes be-
tween 1813 and 1849. After the abandon-
ment of the mission during the latter part
of the nineteenth century, the exact location
of the cemetery was forgotten.
Received November 7, 1985; accepted January 11, 1988.

18 4
P.L.
WALKER ET AL.
TABLE 1. Age distribution of individuals listed in the Purisima cemetery burial records
TABLE 2. Age and sex distributions derived from analysis of burial records and skeletal collections from
Mission La Purisima'
'
Numbers in parentheses indicate percent of total population within each age group for each source.
In 1964, Richard Humphrey (1965) did ar-
cheological work to relocate and determine
the extent of the cemetery in an area sched-
uled for development. During these exca-
vations, the remains of over 50 individuals
were discovered in the impact zone. These
burials were poorly preserved, and only 32
of the skeletons were in good enough con-
dition to remove for analysis. The poor pres-
ervation of the skeletons seems to be
explained by the sandy soil of the cemetery,
which allowed water to permeate the burials
during the rainy season and then evaporate
during the summer months. This periodic
soaking and drying has resulted in the dis-
integration of fragile bones such as verte-
brae and ribs, which are nearly absent from
the collection.
Poor preservation often made age and sex
determinations difficult. Pelvic remains were

available for only a few of the burials. In
most cases, it was necessary to use cranial
morphology and long-bone size and robus-
ticity as the basis for sex determination. Age
estimates were made based on the extent of
dental wear, cranial suture closure, and the
severity of osteoarthritis. Because of the lim-
itations of these aging criteria, it was only
possible to assign the burials to one of the
following classes: <18, 18-24, 25-45, and
>45 years of age at death.
The actual age at death of the neophytes
buried in the cemetery was determined us-
ing the
Libro de Bautismos
and
Libro de En-
tierros,
in which the mission priests recorded
all births and deaths. Although the priests
sometimes entered an estimate of a person's
age in the
Libro de Entierros
at the time of
their burial, these age estimates are often
unreliable (Cook and Borah, 1979). The age
estimates we have used were calculated by
adding the age of a person in the year of their
baptism to the difference between the year
of their baptism and the year of their death
(see Table 1). Thirty three persons for whom
age could not be determined were deleted
from our tabulations.
Not everyone listed in the
Libro de En-
tierros
was interred in the mission cemetery.
Some neophytes died and were buried while
visiting in distant villages or at other mis-
sions; these were omitted from our calcula-
tions. Conversely, some who are not listed
in the
Libro de Entierros
were buried at the
Purisima cemetery and are included in Ta-
bles 1 and 2. This latter group consists pri-
marily of 15 neophytes from Mission Santa
Ines who died at Purisima following the
Chumash Revolt of 1824. Their burials are
recorded in the
Libro de Entierros
of Santa
Ines even though they were interred at Pur-
isima.
Analysis of the burial records and skeletal
collection from the mission cemetery pro-
duced similar sex ratios. The mission re-
cords show that females accounted for 53%
(n = 536) and males for 48% (n = 484) of
the adults over 17 years of age buried in the
cemetery (Table 2). In the skeletal collection,
males and females each accounted for 50%
of the adult population. The chi-squared test
shows that the difference between the sex
ratios derived from the burial records and
skeletal collection is not statistically signif-
icant
(X
2
= 0.005; d.f. = 1; P = 0.92).
Comparison of the age distribution of the
burial population recorded in the
Libro de
PRESERVATION BIAS IN PALEODEMOGRAPHY
185
Entierros
and the skeletal collection from the
Purisima cemetery reveals major discrep-
ancies. People who were younger than 18
years of age at death accounted for 32%
(n = 470) of the people recorded as buried
in the cemetery. Only 6% (n = 2) of the in-
dividuals in the skeletal collection were
younger than 18 years of age at death. The
chi-squared test indicates that the differ-
ence between the age distribution of the peo-
ple listed in the burial register and the people
whose remains were recovered from the
cemetery is highly significant (X
2
= 43.3;
d.f.
= 3; P = 0.00001).
However, the La Purisima skeletal sample
presents certain problems. The excavated
burials represent a small portion of the cem-
etery population, and sampling error cannot
be ruled out as a possible explanation of the
discrepancy between the burial records and
the skeletal collection.
For this reason, a sec-
ond method was used to assess age and sex
biases in preservation. The completeness of
each burial was evaluated based on the pres-
ence of 14 long bones: claviculae, humeri,
radii, ulnae, femora, tibiae, fibulae. A com-
pleteness score was computed for each in-
dividual by dividing the number of these
bones present by 14. For example, a burial
with only 2 of 14 bones was assigned a score
of .14 (2/14).
Males appear to be slightly less complete
than females according to this index (males,
27%; females, 32%). However, both of the
elderly individuals (>45 years) in the Pur-
isima collection are males. If these two are
excluded from the sample, males and fe-
males are equally well preserved (males, 31%;
females, 32%). The indices by age group are
as follows: < 18 years, 0%;18-25 years, 38%;
26-45 years, 24%; >45 years, 7%. These fig-
ures indicate that the skeletons of juveniles
and elderly individuals at the Purisima cem-
etery are less well preserved than those of
young and middle aged adults. These data
are consistent with our conclusion, based on
comparison of the burial records and skel-
etal collection, that elderly people are likely
to be underrepresented in poorly preserved
skeletal collections.
SKELETAL PRESERVATION AT CA-VEN-110
To further evaluate the significance of age
and sex biases in preservation, we analyzed
a larger, better preserved collection from the
Late-Middle Period cemetery site of Ca-Ven-
110 in
Ventura
County, California. This site

18 6
P.L.
WALKER ET AL.
'The preservation index is the average proportion of 14 long bones preserved in the burials from each age group. The
calculation of the index is described in the text.
was excavated in December of 1986 by ar-
cheologists from California State University
at Northridge. Data were collected on the
presence, absence, and completeness of the
14 bones listed above as part of the osteo-
logical analysis of the burials from the site.
An estimate of how much of each bone was
preserved was calculated by dividing the ob-
served length of the preserved portion by the
expected total length of the complete bone.
The expected length of incomplete bones was
determined based on the length of the op-
posing element when this was present and
complete, and was estimated from popula-
tion averages when this information was not
available.
Missing bones were recorded as
0% present. The average completeness of an
individual's long bones was then calculated
by summing percentages for all bones and
dividing by 14. Indices of preservation were
calculated by averaging individual burial
scores for each age and sex class. To remove
biases caused by the disturbance of burials
through erosion, only those individuals with
at least 7 of 14 bones present were used in
the calculation of the indices.
The age of the Ca-Ven-110 burials was de-
termined based on public symphysis mor-
phology, epiphysis fusion, tooth eruption, and
dental wear. Equations derived through re-
gressing tooth heights against pubic ages
were used to age some of the older adults
(
Walker and Dean, 1988). Individuals were
assigned to the following age groups: 0-10,
10-20, 20-40, 40-60, >60 years. Although
older individuals are more difficult to ac-
curately age, and their absolute ages may
be somewhat in error, the age group >60
years nonetheless contains the oldest indi-
viduals in the sample. Sex determination was
based on pelvic features, cranial morphol-
ogy, and discriminate analysis of long-bone
measurements. In no instance was the latter
criterion used exclusively. Only individuals
identified as discrete burials in the field were
used in the analysis.
A
pattern of differential preservation sim-
ilar to that at Purisima is apparent in the
Ca-Ven-110 material. The indices show that
the best-preserved individuals belong to the
20-40-year-old age group, and that infants,
children, and the elderly are the least well
preserved (Table 3). The indices do not pro-
vide any clear evidence of sex differences in
preservation (Table 4).
DISCUSSION
There are nearly always more males than
females in skeletal collections from archeo-
logical sites
(
Weiss, 1972). This has been ex-
plained in part by the comparatively rapid
disintegration of lightly built female skele-
tons (Bennike, 1985). This tendency for
female skeletons to be lost from the
arche-

PRESERVATION BIAS IN PALEODEMOGRAPHY
18
7
ological record more rapidly than those of
males is thought to be especially pronounced
among postmenopausal women who expe-
rience osteoporosis after the cessation of
ovarian function (Raisz, 1982).
The close correspondence between the sex
ratio of the Purisima skeletal collection and
that recorded in the burial records shows
that sex differences in rate of skeletal dis-
integration have not significantly biased this
collection.
The burial records show that most of the
people buried in the Purisima cemetery were
either infants, children, or elderly adults. The
skeletal remains excavated from the ceme-
tery, in contrast, are predominately those of
young adults. The underrepresentation of
young children in the skeletal collection is
most likely a result of the comparatively rapid
disintegration of their incompletely calcified
bones (Gordon and Buikstra, 1981).
We have considered the possibility that a
separate cemetery, not sampled during the
archeological excavations, was maintained
for children at the Purisima cemetery, and
that this explains the underrepresentation
of children in the skeletal collection. This is
unlikely for several reasons. We have bap-
tismal records for all of the children listed
in the burial records, so there is no reason
for them not to have been buried in conse-
crated ground along with other members of
the church. Although underrepresented, the
remains of children are in fact present in the
skeletal collection from the cemetery. This
would not be the case if burials were strictly
segregated according to their age. Finally,
there is no mention of a children's cemetery
in the burial records or other historical
sources.
The underrepresentation of elderly people
in the mission skeletal collection is probably
explained at least in part by a decrease in
the resistance of their bones to disintegra-
tion due to the accelerated loss of bone cal-
cium in old age.
The Ca-Ven-110 data provides additional
evidence for age biases in preservation. In
this cemetery, as at Purisima, juvenile and
elderly individuals are less complete and more
fragmented than young and middle-aged
adults.The Ca-Ven-110 collection is also
similar to Purisima in the lack of sex differ-
ences in preservation.
It is interesting to note how closely the
age distribution of the La Purisima skele-
tons parallels age-related changes in the
skeletal mass of living individuals. Skeletal
mass increases until about the age of 20 years,
remains relatively constant in young adults,
and then begins to decrease after the age of
about 40 or 50 years
(Garn, 1970). This cor-
respondence supports the hypothesis that
age-related changes in the skeleton signifi-
cantly alter the probability that a person's
remains will be preserved in the archeolog-
ical record.
CONCLUSIONS
Comparison of mortality profiles derived
from the Mission La Purisima skeletal col-
lection and the mission burial records re-
veals major inconsistencies (Fig. 1). Infants
and elderly adults are markedly underre-
presented in the mission skeletal collection,
and are less complete in the prehistoric ma-
terial. These discrepancies can be attributed
in large part to the susceptibility of the re-
mains of people from these age classes to
disintegration
in
the ground.
Although the Purisima collection may be
an extreme example in which differential
preservation has greatly skewed the age
structure of a skeletal collection away from
that of the original burial population, most
collections of skeletal remains from ceme-
tery sites are probably biased to some extent
in favor of young adults, whose remains are
comparatively resistant to disintegration.
Evidence from the Ca-Ven-110 cemetery
supports this conclusion. If conditions in the
depositional environment remain constant,
the magnitude of this age-related preser-
Fig. 1.

Comparison of mortality profiles derived from
analysis of the burial and skeletal collections from the
Purisima cemetery.

18
8
vation bias should be roughly proportional
to the length of time a group of burials has
been in the ground. Older cemeteries would
thus be expected to contain fewer infants
and elderly people than more recent ceme-
teries, even if the age structures of the orig-
inal burial populations were identical.
Acceptance of age-specific differential
preservation as a significant source of bias
in skeletal collections provides a basis for
evaluating certain classes of hypotheses about
differences in the age structure of burial
populations exposed to similar depositional
environments. If children or elderly adults
are more common in a skeletal collection from
an ancient cemetery than from a recent one,
this is strong evidence that people from these
age classes actually were more common in
the ancient burial population. If, on the other
hand, infants or elderly people are more
common in a skeletal collection from a recent
cemetery than they are in an ancient one,
much less can be inferred about differences
in the original age structure of the two burial
populations. Such a difference would be ex-
pected due to differential preservation, even
if the age structures of the two burial pop-
ulations were identical.
Our data suggest that in poorly preserved
skeletal collections such as the one from Mis-
sion La Purisima, the underrepresentation
of immature individuals can be so great
that little evidence remains regarding the
original age structure of the burial popu-
lation.
Osteologists should, consequently,
be extremely cautious when attempting to
reconstruct the demographic structure of
prehistoric populations from collections of
poorly preserved skeletal remains.
P.L.
WALKER ET AL.
The mission burial records and skeletal
collection produced comparable sex ratios
when all age classes were pooled. Similarly,
sex differences in fragmentation were not
demonstrated at Ca-Ven-110. From this, it
appears that biases in mortality profiles re-
sulting from the relatively rapid disintegra-
tion of
gracile
female skeletons may not be
very significant.
ACKNOWLEDGMENTS
We thank Mike Imwalle for helping us
check some of the data in the Libro de En-
tierros
and the students who assisted in the
analysis of the Purisima skeletal remains.
This research was supported by National
Science Foundation grant BNS 85-07836.
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