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1Department of Psychology, University of
Pittsburgh, Pittsburgh, USA
2Department of Psychology, New York
University, New York, New York, USA
Correspondence
Joshua L. Schneider and Jana M. Iverson,
Department of Psychology, University of
Pittsburgh, Pittsburgh, USA.
Email: joshua.schneider@pitt.edu and
iversonj@bu.edu
Funding information
National Institutes of Health, Grant/Award
Numbers: F32 DC017903, R01 HD055748,
R01 HD35469, R01 HD41607, R01
HD54979
Abstract
Learning to walk expands infants’ access to the physical
environment and prompts changes in their communicative
behaviors. However, little is known about whether walking
also shapes infants’ proximity to their adult social part-
ners during everyday activities at home. Here we followed
89 infants (42 boys, 47 girls; 92% White, not Hispanic
or Latino) longitudinally and documented connections
between infant locomotion and infant-adult proximity on
two timescales: (1) across developmental time, by compar-
ing data from a session when infants could only crawl to a
later session when they could walk (M walk onset=12.15
months, range = 8-15); and (2) in real time, by testing
whether the amount of time that infants spent in motion
(regardless of their locomotor status) related to their inter-
personal distance to adults. The developmental transition
to walking corresponded to a significant, but modest,
decrease in infant-adult proximity. Infants’ moment-to-
moment locomotion, however, was strongly related to
patterns of interpersonal distance: infants who spent more
time in motion spent less time near adults and instigated
more proximity transitions, resulting in shorter and more
dispersed bouts of proximity throughout sessions. Find-
ings shed new light on how infants’ motor achievements
can reverberate across other domains of development, and
how changes in infant development that researchers often
RESEARCH ARTICLE
Infant locomotion shapes proximity to adults during
everyday play in the U.S.
Qi Chen1 | Joshua L. Schneider1 | Kelsey L. West2 |
Jana M. Iverson1
DOI: 10.1111/infa.12503
© 2022 International Congress of Infant Studies (ICIS).
Infancy. 2022;1–16. wileyonlinelibrary.com/journal/infa 1
1 | INTRODUCTION
Learning to walk reorganizes how infants engage with the physical and social environment (see
Adolph & Tamis-LeMonda,2014). For example, walking allows infants to travel greater distances
at faster speeds compared to crawling and provides them with an expansive view of their surround-
ings while they move (Adolph etal.,2012; Kretch etal.,2014). Accordingly, compared to crawlers,
walkers visit more destinations, retrieve more distal objects, and carry them to share with caregivers
and initiate play (Karasik etal., 2011; Thurman & Corbetta, 2017; Toyama,2020). The onset of
walking also co-occurs with changes in infant communication, including more frequent production
of adult-directed vocalizations, gestures, and initiation of shared attention around objects (Clearfield
etal.,2008; Clearfield, 2011; Walle,2016; West & Iverson,2021). In turn, walkers’ social initia-
tions elicit rich language and gesture input about infant action and objects from caregivers (Karasik
etal.,2014; Schneider & Iverson,2022; West & Iverson,2021).
Prior work suggests that walking has a cascading effect on infants’ social exchanges with their
caregivers, but it is unclear whether walking similarly shapes the physical arrangement of infants’
social interactions. As infants and caregivers move about their homes, they continuously increase or
decrease their distance from one another. The space between infants and caregivers likely sets the
context for their social interactions, either in seated object play near adults on living room floors
or bursts of back-and-forth exploration as infants venture out to retrieve favorite toys for sharing.
Thus, patterns of infant-caregiver proximity serve as an important contextual factor for infants’ social
interactions.
Several theoretical perspectives have stressed the importance of physical proximity for development,
particularly to support the development of attachment and emotional regulation (Ainsworth,1979;
Bowlby,1991; Brent & Resch,1987; Mahler etal.,1975). Changes in proximity provide opportuni-
ties for infants to separate from their caregivers, explore their surroundings, and then return to play.
Indeed, research shows that walking offers infants greater autonomy to venture away from caregivers
and periodically return as a means of emotional “refueling” before continuing their travels (Mahler
etal.,1975). This back-and-forth pattern of proximity may serve to regulate infants’ emotional states—
either to self-soothe infants when they are in distress, or to revitalize when arousal is low (Brent &
Resch,1987; Mahler etal.,1975).
Yet little is known about how infants’ motor skills shape patterns of proximity. Does learning to
walk co-occur with changes in infants’ interpersonal distance to adults? To address this question, we
investigated connections between infant locomotion and infant-adult proximity on two timescales: (1)
across developmental time, by comparing longitudinal data from an observation when infants could
only crawl to a later session when they could walk; and (2) in real time, by examining whether the
amount of time that infants spend in motion (regardless of their locomotor status) shapes patterns of
proximity to adults.
1.1 | Infant locomotion and proximity to social partners
Several studies have measured infant-adult proximity in the context of infants’ developing locomo-
tor skills, with mixed results. Some studies report that infants spend less time near adults after they
CHEN Et al.
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observe over months arise from infants’ moment-to-mo-
ment experiences.
begin to walk. For example, a study examining infants and caregivers during play at home found that
walking infants traveled beyond their mothers’ reach more often and stayed in different rooms longer
compared to age-matched crawlers (Biringen etal.,1995). And more generally, research shows that
the interpersonal distance between infants and adults gradually increases as infants acquire new loco-
motor skills (Rheingold & Eckerman,1970; Thurman & Corbetta,2017). It is possible that walking
infants’ increased propensity to travel to distant objects may incur a reduction in their time spent near
adults (Karasik etal.,2011; Toyama,2020). And in fact, far-off toys are enticing destinations: walkers
spend less time playing near their caregivers in a room filled with toys compared to an empty room
(Hoch etal.,2019).
In contrast, other studies find that infants spend more time near adults after they begin to walk
as they increasingly approach caregivers to seek out social interaction. Some evidence suggests that
unlike crawling, upright locomotion enables infants to approach their caregivers more often and
engage in more proximal interactions. For example, Gustafson (1984) demonstrated that merely
placing pre-walking infants upright in a mechanical walker increased the frequency with which they
locomoted to adults and consequently, the amount of time infants spent near adults during play in a
laboratory. This pattern of increased proximity was replicated in a longitudinal study, which showed
that walking infants spent more time near adults in a laboratory playroom compared to prior sessions
when infants were experienced crawlers (Clearfield,2011).
More recent work suggests that infant-adult proximity may not change when infants learn to walk.
For example, a longitudinal study measuring the interpersonal distance between infants and caregivers
during play at home did not reveal significant change in proximity across the transition from crawling
to walking (Yamamoto etal.,2020). Of course, proximity is a mutually determined behavior that is
shaped by both members of the dyad. Walking infants and their caregivers often synchronize their
movements, and the actions of one likely influence the actions of the other. Evidence suggests that
infants' and caregivers' movements are coordinated: as infants explore a laboratory playroom, caregiv-
ers often followed in-step and close the distance (Hoch etal.,2021). Consequently, developmental
change in infant locomotion may prompt a corresponding change in adult behavior. That is, if newly
walking infants are increasingly on the go, adults may modify their actions to remain nearby, resulting
in little change across time.
Finally, it is possible that infants’ real-time locomotor behaviors (i.e., how much they move in the
moment)—but not necessarily their locomotor status as a “crawler” or “walker”—shapes their proxim-
ity to adults. Locomotor development is characterized by considerable inter- and intra-individual vari-
ability (e.g., Rachwani etal.,2020). When infants spend a great deal of time in a stationary position,
regardless of whether they can crawl or walk, they may experience prolonged periods of proximity to
adults. But when infants spend more time moving, they may experience shorter intervals of proximity.
It is important to note, however, that infant locomotion need not always change their proximity to
adults. Estimates suggest that 30-60% of infants’ locomotor bouts include fewer than four steps (Cole
etal.,2016; Hoch etal.,2020; Lee etal.,2018) and frequently include steps in-place (Cole etal.,2016;
Hoch etal.,2020). Short locomotor bouts are unlikely to displace infants significantly, and therefore
are unlikely to affect patterns of proximity. Thus, the connection between infants’ moment-to-moment
locomotion and their proximity to adults remains an open question.
1.2 | Current study
Prior work presents several alternative hypotheses as to whether learning to walk influences patterns
of infant-adult proximity during daily life, when infants and caregivers can move freely through their
CHEN Et al.3
homes and choose to spend time in close proximity or at a distance from one another. To test among
these alternatives, we documented infants’ locomotion and proximity to adults longitudinally across
the transition from crawling to walking. First, we examined whether infant-adult proximity changed
across developmental time by comparing data from two at-home, naturalistic observations: the first
when infants were crawlers, and the second a month later, when all infants could walk independently.
Second, we investigated whether the frequency of infants’ in-the-moment locomotion—regardless of
their locomotor status—mapped onto patterns of proximity to adults. Finally, we documented whether
infants or adults primarily initiated changes in interpersonal distance, either bringing the dyad together
or moving apart.
2 | METHOD
2.1 | Participants
Data for the current report were drawn from two longitudinal studies that investigated motor and
language development in infancy. All infants were born at term and from uncomplicated pregnancies.
Data were collected between 2002 and 2014 in two Midwestern cities. The present study was conducted
according to guidelines laid down in the Declaration of Helsinki, with written informed consent
obtained from a parent for each infant before any assessment or data collection. All procedures involv-
ing human subjects in this study were approved by the Institutional Review Board at the University of
Pittsburgh. The sample included 75 infants with an elevated likelihood for developing autism spectrum
disorder (ASD), defined by the presence of an older sibling already diagnosed with ASD. Twenty-four
infants had no first- or second-degree relatives with ASD. A preliminary goal of this project was to
examine whether measures of infant-adult proximity varied among infants based on these sampling
characteristics. However, there were no significant differences between groups on any measure.
Ten infants received an ASD diagnosis at the conclusion of the original study (see Leezenbaum
& Iverson,2019 for additional information on diagnostic criteria). We excluded infants with an ASD
diagnosis from the current analyses given the potential host of additional factors that may influence
these infants’ patterns of proximity-seeking behavior. Thus, the final sample included data from 89
infants (47 girls, 42 boys) collapsed into a single group.Caregivers reported their infant’s race and
ethnicity: 87 infants were White (five were Hispanic or Latino) and two were Multiracial. Mothers and
fathers were similar in age (M mothers=33.84 years, SD=4.45; M fathers=36.17 years, SD=5.31)
and education (85.4% of mothers and 83.1% of fathers held a college degree or higher).
2.2 | Procedure
As part of the larger longitudinal studies, researchers visited infants and their caregivers at home
following either a biweekly (from 2-19 months) or monthly (from 5-14 months) observation schedule.
Home visits typically occurred on weekdays and during times when infants were awake and ready
to play. At each visit, infants were videorecorded during everyday activities (e.g., playing with toys,
interacting with adults, locomoting around the house) for approximately 45 minutes. Prior to filming,
infants and caregivers were given time to acclimate to the researchers’ presence, and researchers who
visited each family were consistent across sessions. After warm-up, a researcher followed the infant
with a single camera and ensured the infant’s entire body was in view at all times while also keeping
adults and the surrounding environment in frame as much as possible. Infants and caregivers spent
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the majority of their time in a few main rooms (e.g., the living room, kitchen), but frequently changed
locations. In addition to the cameraperson, two researchers were typically present. All researchers
remained at a distance during filming and took care to not interact with infants. However, infants were
free to approach any adult as we did not constrain their activities.
To examine how infant-adult proximity changed across the transition from crawling to walking,
we focused on two observations. We first identified each infant’s age at walk onset, defined as the first
day when infants were able to take five continuous, independent steps without stopping or falling. We
confirmed walk onset ages by cross-referencing caregivers’ prospective reports of their infants’ motor
skills with experimenters’ notes from each visit. On average, infants began to walk at 12.15 months
(SD=1.47, range=8-15).
After establishing walk onset, we then selected the first monthly session (to balance differences
in observation schedules) that followed each infant’s walk onset age and considered it to be the initial
walking session (e.g., if an infant began to walk at 11.5 months, the 12-month session was identified as
the walking session; M walking experience=0.16 months, SD=0.33). The session one month prior
was identified as the final crawling session (i.e., when infants could only move by crawling, cruising,
etc.; M crawling experience=2.92 months, SD=1.37). Our milestone-based design reflected indi-
vidual differences in the ages at walk onset for each infant, and thus infant ages at each session varied.
As a result, we controlled for infant age in all analyses.
2.3 | Data coding
We selected the first 10 minutes of each 45-minute observation in which infants were free to move
and not constrained by furniture (e.g., a highchair) for coding. All behaviors were coding using the
computerized coding tool Datavyu (datavyu.org). Coders were trained until overall percent agreement
reached≥90% on all coding categories for three consecutive videos. After establishing reliability, a
primary coder scored 100% of each video, and a reliability coder independently scored a randomly
selected 25% of each video to verify inter-observer reliability. Disagreements were resolved through
discussion.
2.3.1 | Infant locomotion
Each video was first coded for locomotion. All times when infants engaged in crawling (moving on
hands-and-knees), cruising (moving upright with support), and walking were identified. We coded
locomotion in bouts, or a series of steps separated by a pause in which the infant came to a complete
stop for at least 0.5 seconds (e.g., Adolph etal.,2012; Cole etal.,2016). To be included, bouts were
required to contain at least two steps or crawl cycles. A bout of locomotion began at the first frame
of video when an infant’s foot or knee began to move and ended at the first frame when the foot or
knee came to rest (Adolph etal.,2012). We calculated the total duration of infants’ locomotion time
by summing the individual durations of all bouts (regardless of type) and dividing this value by the
session duration to derive the proportion of time spent in motion. Inter-observer reliability was high
for bout identification (percent agreement=92%) and for specification of locomotion type (κ=.97).
2.3.2 | Infant-adult proximity
Given the complexity of capturing changes in interpersonal distance from video, we utilized an
interval-based coding system (see Schneider etal.,2022 for a similar application). Each 10-minute
CHEN Et al.5
observation was divided into sixty 10-second intervals. We established interval length using an iter-
ative pilot coding process in which codes were tested using a variety of interval durations. Our goal
was to select an interval that was long enough to provide an appropriate level of detail for capturing
change in infant-adult proximity over time while also maintaining coder efficiency (see Bakeman &
Gottman,1997). Based on these initial observations, a 10-second interval was chosen as the best solu-
tion for balancing these parameters.
We coded infant-adult proximity in each interval using two mutually exclusive categories. As
shown by the line drawings in Figure1, we identified whether infants were within reach of an adult
(the blue drawing) or out of reach of an adult (the yellow drawing) throughout the entire interval. We
used adults’ wingspans to determine whether infants were within or beyond reach. Specifically, if the
adult could fully stretch out their arms and touch the infant without needing to locomote or change
their posture, infants were considered to be within reach.
We only coded one of the two proximity categories if the infant remained in the same proximity
state across the full 10-second interval (i.e., the infant stayed within arm’s reach of an adult for the
entire 10 seconds). If a transition between proximity states occurred during the interval, we assigned
it to a separate proximity transitions category (the grey drawing) and noted who (infant or adult) initi-
ated the transition. Infants and adults could initiate proximity transitions by locomoting to be within
or out of reach of the other. Given that infants could approach and interact with the researchers who
were present at the session, we did not limit the coding of infant-adult proximity to just the primary
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FIGURE 1 Data on infant-adult proximity across the transition to walking. The raster plot presents individual
timelines for each infants’ patterns of proximity to adults across sixty 10-second intervals spanning a total of ten
minutes of play. The data represent both the real-time distributions of proximity states during each 10-second interval
within a session and change in patterns of proximity between sessions. Each row displays data from a single infant.
Rows are ordered from least to most time in motion within each session. Proximity states for each interval are
color-coded, and the line drawings serve as a legend. Each color represents a proximity state: blue=within reach;
yellow=out of reach; and grey=proximity transitions.
caregiver. Thus, our measures of proximity reflect the time that infants spent near vs. far away from
any adults. Inter-observer reliability was high for categorizing infant-adult proximity (coders agreed
on 91.6% of intervals, κ=0.83) and identifying the initiator of proximity transitions (agreement on
94.6% of intervals, κ=0.76).
3 | RESULTS
We documented whether infants’ locomotor status (“crawler” or “walker”) or locomotor behavior
(how much time infants spent in motion) was related to patterns of infant-adult proximity. Specifi-
cally, our primary measures included: (1) the total number of intervals in which infants were within
reach of an adult; (2) the frequency of proximity transitions (instances when the infant or adult moved
in or out of reach); and (3) the relative frequency with which infants vs. adults initiated proximity
transitions (i.e., whether the infant or adult was primarily responsible for shaping proximity). We used
generalized estimating equations (GEEs) for all analyses, which accounted for the non-independence
of repeated sessions at the crawling and walking time points (Hardin & Hilbe,2002). A Wald X
2
statistic is reported for each GEE. Partial correlations, controlling for infant age, were calculated to
follow-up on significant main effects and interactions. Infant age and sex were included as covariates
in all models. Preliminary analyses revealed no differences on any variable based on the number of
additional researchers present at home visits (M=1, range=0-3; all ps>.05).
3.1 | Infants’ time spent within versus beyond adult reach
We first analyzed how much time infants spent locomoting at each session. Overall, infants spent
similar proportions of their observation time in motion at both the crawling (M=0.11, SD=0.09)
and walking (M=0.14, SD=0.12) observations. A GEE confirmed that the proportion of time spent
in motion did not significantly change between sessions, X
2=1.06, p=.303.
To assess patterns of infant-adult proximity, we examined the distributions of time that infants
spent within vs. beyond arm’s reach of an adult. Figure1 presents raster plots depicting individual
timelines for each infant—color-coded to show when infants were near or far from adults and when
there were transitions between proximity states. The figure displays both the real-time fluctuations in
proximity between each 10-second interval within a session and change in patterns of proximity across
the crawling and walking sessions.
As shown by the blue vs. yellow bars in Figure1, infants spent far more intervals within reach
of adults (M = 38.89 intervals, SD=16.47) than they spent out of adult reach (M = 13.65 intervals,
SD=13.93). However, there was substantial individual variability in patterns of infant-adult proxim-
ity. At both the crawling and walking sessions, infants ranged from 0-60 intervals near adults. That
is, some infants were consistently within reach of an adult during the observation (see the full-length
blue bars running across each raster plot), while other infants were never within reach of an adult (the
full-length yellow bars).
Infants’ locomotor status and real-time locomotor behavior were related to patterns of infant-adult
proximity. Indeed, the GEE revealed main effects of both Locomotor Status, X
2=4.76, p=.029, and
Time in Motion, X
2=34.63, p < .001. Specifically, infants showed a modest decrease in intervals
spent near adults from the crawling (M = 40.44 intervals, SD=14.32) to the walking session (M =
37.35 intervals, SD=16.94). Within sessions, when infants spent more time locomoting, they tended
to spend fewer intervals near adults. Figure2 presents scatterplots of the relations between infants’
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time in motion and our proximity measures. As shown in Figure2a, there was a significant association
between locomotion time and proximity, such that an increase in the proportion of time that infants
spent in motion was associated with a decrease in the number of intervals within reach of adults.
Finally, the GEE also revealed a Locomotor Status x Time in Motion interaction, X
2=7.29, p=.007.
Follow-up tests showed that the negative relation between locomotion time and proximity to adults
was stronger at the crawling session compared to the walking session (rs=-.542 vs. -.386, ps<.001,
respectively; see Figure2a).
3.2 | Transitions between proximity states
We next analyzed how often intervals contained transitions between proximity states (e.g., when
an infant locomoted out of adult reach). As shown by the grey bars in Figure1, infants and adults
engaged in transitions frequently, accumulating an average of 7.46 transition intervals (SD=5.56) per
10 minutes. Again, we observed substantial variability in how often proximity transitions occurred
(range=0-28), suggesting that some infants remained near or far from adults for prolonged stretches
of time, while other infants exhibited multiple, shorter episodes of social proximity throughout the
session.
The GEE revealed only a main effect of Time in Motion, X
2=25.24, p<.001. Specifically, at both
the crawling and walking sessions, infants who spent more time locomoting also exhibited a larger
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FIGURE 2 Scatter plots showing partial correlations (controlling for infant age) between the proportion of time
that infants spent in motion and: (a) the number of intervals within reach of an adult and (b) the number of intervals
engaging in proximity transitions. Solid grey lines represent data from the crawling session; dashed black lines are
data from the walking session.
number of transition intervals (rs=.591 and .691, ps<.001, respectively; see Figure2b). Infants who
moved the least during their 10-minute observation spent prolonged stretches of intervals either near
or far from adults (see the prevalence of long, uninterrupted bars in the top halves of each raster plot in
Figure1). By contrast, infants who spent more time locomoting demonstrated a back-and-forth pattern
of proximity, in which stretches of time near adults were frequently interspersed with transitions.
There was no main effect of Locomotor Status, X
2=2.52, p=.112, and no Locomotor Status x Time
in Motion interaction, X
2=0.61, p=.436.
3.3 | Initiator of proximity transitions
Our final analyses examined whether infants or adults were primarily responsible for initiating prox-
imity transitions (e.g., by moving closer or farther away from one another). To do so, we calculated a
change score for each infant by subtracting the total number of adult-initiated transitions from the total
number of infant-initiated transitions. Thus, positive values denote that the infant was responsible for
changes in proximity more often than adults, negative values denote the reverse, and the magnitude of
the value reflects the magnitude of the difference between the infant- and adult-initiated transitions.
These data are presented in Figure3.
Overall, infants and adults generated similar numbers of proximity transitions (M infants=4.35,
SD= 4.06; M adults= 3.38, SD = 3.19). However, as shown in Figure3, the difference in each
partner’s propensity to initiate transitions was highly related to the time that infants spent in motion,
regardless of locomotor status. Infants who spent more time locomoting drove the ebb and flow of
proximity to adults and more often initiated transitions between proximity states (see the blue regions
in Figures3a-b). Conversely, when infants moved less, adults largely determined when the pair was
near vs. far (the green regions). The GEE confirmed these patterns with a main effect of Time in
Motion, X
2=16.78, p<.001, but no effect of Locomotor Status, X
2=0.01, p=.931, and no Loco-
motor Status x Time in Motion interaction, X
2=1.18, p=.278.
4 | DISCUSSION
We investigated the connection between infants’ locomotion and proximity to adults on two timescales:
over developmental time as infants transitioned from crawling to walking, and in real time as infants
moved about their homes during everyday play. Overall, the transition to walking corresponded to a
decrease in infant-adult proximity, regardless of infant age. In addition, infants’ moment-to-moment
locomotion (for crawlers and walkers alike) was strongly related to patterns of interpersonal distance.
When infants were highly mobile, they instigated bouts of proximity by approaching or departing
from adults’ reach, and their bouts were shorter and dispersed across the entire observation. When
infants were less mobile, adults instigated most bouts of proximity and bouts were less frequent, but
prolonged in duration compared to those of more mobile infants.
4.1 | Infant locomotion organizes infant-adult proximity
The onset of walking corresponded to a significant (albeit modest) reduction in infants’ time spent
near adults. There are at least two potential explanations for the decrease in social proximity. First,
walking infants have a better view of their surroundings while moving than crawlers do, which may
guide their movements differently (e.g., Kretch etal.,2014). Because walkers’ upright posture enables
CHEN Et al.9
them to see far-off locations, distant toys and spaces may entice them to venture away from caregiv-
ers for long periods of time as they engage in solo exploration. Indeed, distal toys prompt infants to
travel farther and play at a distance from their mothers (Rheingold & Eckerman,1970). Moreover,
infants walk to their caregivers less often in a playroom filled with toys relative to an empty room
(Hoch etal.,2019). In everyday life, infants’ homes are brimming with toys and household objects
(Herzberg etal.,2022), and walking infants may forgo proximal social interactions to explore other
exciting destinations.
Second, walking is a more efficient means for locomotion than crawling (e.g., Adolph etal.,2012).
Even newly walking infants travel at faster speeds, covering greater distances, than infants with
many months of crawling experience (e.g., Adolph & Tamis-LeMonda,2014). As a result, walk-
ers may be more likely to spend time “foraging” in their everyday environments; that is, moving
for movement’s sake without a specific destination in mind. Instead, locomotion is costly for crawling
infants—it requires more time and energy—and accordingly, crawlers may use locomotion more spar-
ingly and strategically to reach specific destinations, like objects and people, than do walkers (e.g.,
Cole etal.,2016; Hoch etal.,2020). Distinct movement patterns among crawlers and walkers may
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FIGURE 3 Difference scores showing whether infants (blue regions) vs. adults (green regions) initiated
proximity transitions. Panel (a) displays data for the crawling session and panel (b) shows data for the walking session.
explain why moment-to-moment locomotion was more strongly related to infant-adult proximity at
the crawling session than the walking session, as crawlers may use a greater proportion of their move-
ments to approach adults compared to walkers. Nevertheless, for all infants, the amount of time spent
moving shaped the organization of infant-adult proximity, suggesting that movement itself (rather than
how infants move) may be a more critical factor in determining the physical arrangement of social
interaction.
Here, we focused on the transition from experienced crawling to novice walking, but motor devel-
opment may shape patterns of interpersonal distance well before and after this skill is acquired. For
example, pre-locomotor infants rely almost entirely on adults to determine the spatial arrangement
of social interaction. Indeed, dyadic interactions are predominantly shaped by caregivers early in
the first year. Caregivers often place their infants in supine and supported sitting postures during
object play and position themselves to face their infants from above or directly opposite (Schneider
etal.,2022). As infants get older and acquire new motor skills (e.g., sitting, crawling), however, they
autonomously move in and out of body positions and actively shape the physical configuration of
their social interactions (e.g., choosing to position themselves close or far, towards or opposite their
caregivers). In addition, the transition from novice to experienced walker may prompt other changes
in infant-adult proximity. With each passing month of walking experience, infants become faster, more
skilled movers (Adolph etal.,2012). The decrease in infants’ time near adults observed here may be
amplified with additional walking experience, as infants move back and forth between dyadic and
independent play with greater ease.
It is important to note that the setting of existing studies on infant-adult proximity (either in infants’
homes or in novel laboratory spaces) may account for prior mixed results on the connection between
walking and interpersonal distance. Infants may behave differently in curated laboratory spaces
compared to natural environments (e.g., Tamis-LeMonda etal., 2017). For example, our findings
align with Biringen etal.(1995), who found that infants spend less time near adults after they begin
to walk—both studies took place in infants’ homes during everyday play. In contrast, laboratory-based
observations revealed that increased walking was associated with more time near caregivers (e.g.,
Clearfield,2011; Gustafson,1984). It is likely that infants’ proximity-seeking behaviors are shaped,
in part, by the familiarity of their location. Familiar settings like living rooms, bedrooms, and kitchens
may guide walking infants towards independent play, and caregivers may allow infants to explore at a
distance in the safety of their homes. But in novel settings, infants may approach caregivers to share
new toys or experiences more often, and in turn, caregivers may accompany their infants as they move
about and explore the new location.
4.2 | Developmental cascades arise from real-time behavioral cascades
Psychologists often propose that new motor skills—like learning to reach, sit, or walk—have cascading
effects on other domains of infant learning and development (e.g., Iverson,2021). Such studies typi-
cally identify associations between the timing of new motor skills and strides in social-communicative
development. For example, the onset of walking is accompanied by vocabulary growth (e.g., He
etal.,2015; Walle & Campos,2014; West etal.,2019) and alterations in caregiver communication
(e.g., Karasik etal.,2014; Schneider & Iverson,2022; West & Iverson,2021). But importantly, such
associations documented over months or years likely arise from real-time behavioral cascades. That
is, infants’ in-the-moment motor actions may play a causal role in shaping their opportunities for
social communication and interaction. Our data shed light on the importance of examining real-time
behavioral cascades that connect infant locomotion—not just their locomotor status—to aspects of
interaction; in our case, the time that infants spent near adults.
CHEN Et al.11
The real-time spatial arrangement of everyday activities may play an important role in structur-
ing the content of dyadic interaction, and in turn, the acquisition of skills across other domains. For
example, during proximal activities, close face-to-face interactions allow infants to establish mutual
gaze (e.g., Northrup & Iverson,2020), learn about social contingencies (e.g., Beebe etal.,2016; Kaye
& Fogel,1980), and jointly engage with objects and caregivers, promoting early vocabulary develop-
ment (e.g., de Barbaro etal.,2016; Suarez-Rivera etal.,2022; Yu & Smith,2012). Similarly, side-by-
side book reading allows infants to acquire pre-literacy skills (e.g., page-turning) and exposes them to
rich linguistic input benefiting language (e.g., Demir-Lira etal.,2019; Luo & Tamis-LeMonda,2017;
Muhinyi & Rowe,2019).
Activities at a distance from adults, such as rolling a ball back and forth, climbing on playground
equipment, or riding tricycles may offer infants practice refining their whole-body movements and
spatial exploration, benefiting skills like spatial cognition (e.g., Garcia etal.,2021; Oudgenoeg-Paz
etal.,2015) and the acquisition of motion verbs (e.g., West etal., 2022). Thus, different types of
play (enabled by their physical organization) may give rise to different opportunities for interaction
and infant learning. And in fact, studies have found that more frequent locomotion (and relatedly, the
back-and-forth pattern of social interaction that follows when infants are more mobile) is associated
with a larger receptive vocabulary among crawling and walking infants (Walle & Campos,2014).
4.3 | Limitations and future directions
There are several important limitations to note when interpreting our results. First, our sample included
families from predominantly White and highly educated sociocultural backgrounds from a Western
culture. Therefore, it is unclear whether our findings generalize beyond this limited demographic
sample. Infants reared in other cultural communities across the globe vary tremendously in the timing
of motor skill expression. For example, infants from cultures that engage in daily exercise and massage
routines (e.g., parts of India, Africa, and the Caribbean) begin to walk weeks or even months earlier
than Western infants do (see Karasik etal.,2010 for a relevant review). Other infants skip certain
skills altogether (e.g., crawling; Karasik & Robinson,2022). Even within the United States, socio-
demographic factors appear to influence characteristics of infants’ home environments—like how
crowded living spaces are—which may in turn account for later attainment of walking (Hospodar
etal.,2021). Differences in these sociocultural factors likely influence motor development, which in
turn may shape the association between walking and infant-adult proximity documented here. Future
work is necessary to identify how culture and context shape the connection between locomotion and
proximity.
Second, measuring interpersonal distance in the home environment poses unique challenges.
Documenting infant-adult proximity with detailed precision is difficult from video alone because
infants’ homes vary substantially in their dimensions, and thus lack a common parameter of distance.
Here we focused on times when infants were within adult reach. However, when infants were out of
adults’ reach, their precise distance from an adult could vary dramatically. For instance, the adult
could be just out of infants’ reach in a small playroom or in an entirely different room altogether.
Future research should leverage wearable technologies (e.g., TotTag; Salo etal.,2021) to precisely
quantify interpersonal distance. Further, aspects of infants’ home environments—like the number and
dimensions of rooms, or the availability and location of objects—may influence infants’ movements
and proximity to caregivers. Additional research should document how the physical characteristics of
infants’ homes shape their behaviors and proximity to adults.
Finally, although we asked caregivers to engage in their typical everyday activities, dyads
generally chose to play together during the 10-minute observation. Patterns of interpersonal
CHEN Et al.
12
distance shift over the course of daily routines as infant-caregiver dyads transition between play,
mealtimes, grooming activities, and stretches of time when caregivers are occupied by chores and
infants play independently. Interpersonal distance likely fluctuates throughout the day, and the role
of infant locomotion may be more or less influential depending on the activity context. Moreover,
infants’ time spent outside the home (e.g., in childcare settings) may offer unique opportunities
for social proximity with peers and other adults (see van Liempd etal.,2020 for a review). Future
work should measure infant-adult proximity spanning longer durations of time and across activity
contexts.
5 | CONCLUSIONS
Our findings have important theoretical and methodological implications. First, our data shed new
light on how infants’ motor achievements reverberate across domains and potentially shape the
physical context of their social interactions. Infants’ in-the-moment motor actions have immediate
consequences for their proximity to social partners, and potentially their access to social infor-
mation and ways to play and interact with their adult caregivers. As a result of real-time connec-
tions, the acquisition of new motor skills—like learning to walk—may give rise to corresponding
changes in infant-adult social interaction. Thus, infants’ social development should be considered
as an embodied process that unfolds within the context of a rapidly changing repertoire for motor
action.
Second, infancy researchers often study infant-adult interactions in laboratory spaces, with
infant-caregiver dyads placed in close proximity in a small observation room or seated at tables with
limited opportunities to move. However, infants are often “on the go” at home, and proximal social
interactions are often broken up into episodes of back-and-forth exploration as infants shift between
dyadic play and solo ventures. This temporal pattern of social proximity may meaningfully impact
how infants produce other behaviors of researchers’ interest. Finally, the connection between infants’
locomotor development and social proximity may have implications for infants with gross motor
delays, who walk later in development than their peers, and for whom the temporal and spatial organ-
ization of social interactions with caregivers may differ in important ways.
ACKNOWLEDGEMENTS
This article is based on a thesis submitted to the University of Pittsburgh in partial fulfillment of the
requirements for the degree of Bachelor of Science with honors in psychology by QC. The research
was supported by Autism Speaks and the National Institutes of Health (R01 HD41607 and R01
HD54979 to JMI), with additional support from HD 35469 and HD 055748. During preparation of
this manuscript, KLW received funding through an F32 training grant from the National Institutes
of Health (F32 DC017903). Kelsey West is now at the Center for Innovative Research in Autism,
Department of Psychology, University of Alabama. Jana Iverson is now at the Department of Physical
Therapy, Boston University. We thank Evan Rubin for help with video coding, Nancy Minshew, Diane
Williams, and Holly Gastgeb for assistance with clinical assessments, and the members of the Infant
Communication Lab at the University of Pittsburgh for their support. We also thank Carla Mazefsky
for helpful comments on earlier versions of the manuscript. Special thanks to the infants and families
who made this research possible. Portions of these data were presented at the 2021 virtual meeting of
the Society for Research on Child Development. The coding materials associated with this study are
available from the corresponding authors upon request. The authors declare they have no conflict of
interest with regard to the funding sources for this study.
CHEN Et al.13
ORCID
Joshua L. Schneider https://orcid.org/0000-0003-0635-0987
Jana M. Iverson https://orcid.org/0000-0001-9160-7825
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