Developmental Psychology Copyright 1987 by the American Psychological Assooation, Inc.
1987, VOl. 23, No. 5,655-664 0012-1649/87/$00.75
Object Permanence in 3 1/2- and 4 1/2-Month-Old Infants
Ren6e Baillargeon
University of Illinois
These experiments tested object permanence in 3 1/2- and 4 l/2-month-old infants. The method used
in the experiments was similar to that used by Baillargeon, Spelke, and Wasserman (1985). The
infants were habituated to a solid screen that rotated back and forth through a 180* arc, in the
manner of a drawbridge. Following habituation, a box was placed behind the screen and the infants
were shown two test events. In one (possible event), the screen rotated until it reached the occluded
box; in the other (impossible event), the screen rotated through a full 180″ arc, as though the box
were no longer behind it. The 4 l/2-month-olds, and the 3 V2-month-olds who were fast habituators,
looked reliably longer at the impossible than at the possible event, suggesting that they understood
that (a) the box continued to exist after it was occluded by the screen and (b) the screen could not
rotate through the space occupied by the occluded box. Control experiments conducted without the
box supported this interpretation. The results of these experiments call into serious question Piaget’s
(1954) claims about the age at which object permanence emerges and about the processes responsible
for its emergence.
Adults believe that an object cannot exist at two separate
points in time without having existed during the interval between them. Piaget (1954) held that infants do not begin to
share this belief until they reach about 9 months of age. The
main evidence for this conclusion came from observations of
young infants’ reactions to hidden objects. Piaget noticed that
prior to 9 months, infants do not search for objects they have
observed being hidden. Ifa toy is covered with a cloth, for example, they make no attempt to lift the cloth and grasp the toy,
even though they are capable of performing each of these actions. Piaget speculated that for young infants objects are not
permanent entities that exist continuously in time but transient
entities that cease to exist when they are no longer visible and
begin to exist anew when they come back into view.
Although Piaget’s (1954) observations have been confirmed
by numerous researchers (see Gratch, 1975, 1977, Harris, in
press, and Schuberth, 1983, for reviews), his interpretation of
these observations has been questioned. A number of researchers (e.g., Baillargeon, Spelke, & Wasserman, 1985; Bower, 1974)
have suggested that young infants might fail Piaget’s search
task, not because they lack object permanence, but because
This research was supported by a grant from the National Institute
of Child Health and Human Development (HD-21104).
I thank Jerry DeJong, Judy Deloache, Julia DeVos, Marcia Graber,
Stephanie Hanko-Summers, and Jerry Parrott for their careful reading
of the manuscript. I also thank Earle Heftley, Tom Kessler, and Oskar
Richter for their technical Helpance; Dawn Iacobucci and Stanley Wasserman for their help with the statistical analyses; and Marcia Graber,
Stephanie Hanko-Summers, Julie Toombs, Anna Szado, and the undergraduates working in the Infant Cognition Laboratory at the University
of Illinois for their help with the data collection. I also thank the parents
who kindly agreed to have their infants participate in the studies.
Correspondence concerning this article should be addressed to Ren6e
Baillargeon, Department of Psychology, University of Illinois, 603 E.
Daniel, Champaign, Illinois 6 ! 820.
they are generally unable to perform coordinated actions. Studies of the development of action (e.g., Piaget, 1952; Uzgiris &
Hunt, 1970) have shown that it is not until infants reach about
9 months of age that they begin to coordinate actions directed at
separate objects into means-end sequences. In these sequences,
infants apply one action to one object so as to create conditions
under which they can apply another action to another object
(e.g., pulling a cushion to get a toy placed on it or deliberately
releasing a toy so as to grasp another toy). Thus, young infants
could fail Piaget’s task simply because it requires them to coordinate separate actions on separate objects.
This interpretation suggests that young infants might show
evidence of object permanence if given tests that did not require
coordinated actions. Bower (1967, 1974; Bower, Broughton, &
Moore, 1971; Bower & Wishart, 1972) devised several such tests
and obtained results that he took to indicate that by 2 months
of age, if not sooner, infants already possess a notion of object
permanence. Bower’s tests, however, have been faulted on methodological and theoretical grounds (e.g., Baillargeon, 1986, in
press; Baillargeon et al., 1985; Gratch, 1975, 1977; Harris, in
press; Muller & Aslin, 1978).
Because of the problems associated with Bower’s tests, Baillargeon et al. (1985) sought a new means of testing object permanence in young infants. The test they devised was indirect:
It focused on infants’ understanding of the principle that a solid
object cannot move through the space occupied by another
solid object. The authors reasoned that if infants were surprised
when a visible object appeared to move through the space occupied by a hidden object, it would suggest that they took account
of the existence of the hidden object. In their study, 5 l/2-monthold infants were habituated to a screen that rotated back and
forth through a 180* arc, in the manner of a drawbridge. Following habituation, a box was placed behind the screen and the
infants were shown two test events. In one (possible event), the
screen rotated until it reached the occluded box and then returned to its initial position. In the other (impossible event),
655
656 RENI~E BAILLARGEON
Figure 1. Schematic representation of the habituation and test events shown to
the infants in the experimental and control conditions in Experiment 1.
the screen rotated until it reached the occluded box and then
continued as though the box were no longer behind it! The
screen rotated through a full 180* arc before it reversed direction and returned to its initial position, revealing the box standing intact in the same location as before. The infants looked
reliably longer at the impossible than at the possible event, suggesting that they understood that (a) the box continued to exist
after it was occluded by the screen and (b) the screen could not
rotate through the space occupied by the occluded box.
The results of Baillargeon et al. indicate that, contrary to Piaget’s claims, 5 t/2-month-old infants understand that an object
continues to exist when occluded. The first experiment reported
here attempted to extend these results by examining whether
younger infants, 4 ~/2-month-olds, expect the continued existence of occluded objects.
There are two reasons to ask whether younger infants have
object permanence. The first is purely descriptive: Before we
can propose a theory of the development of infants’ beliefs
about objects, we must establish what beliefs they hold at
different ages. The second is more theoretical: The age at which
infants are granted a notion of object permanence will undoubtedly constrain the nature of the mechanism we invoke
to explain the attainment of this notion. Piaget (1952, 1954)
attributed the emergence of object permanence to the coordination of sensorimotor schemes, which, as was mentioned earlier,
begins at about 9 months of age. The discovery by Baillargeon
et al. that 5 1/2-month-olds already possess a notion of object
permanence is clearly inconsistent with Piaget’s account. What
mechanism could explain the presence of this notion in infants
aged 5 I/2 months or less? This question will be addressed in the
General Discussion section.
Experiment 1
The method used in Experiment 1 was similar to that used
by Baillargeon et al. (1985); it is depicted in Figure 1.
There was one foreseeable difficulty with the design of Experiment 1. The infants might look longer at the impossible than
at the possible event, not because they were surprised to see the
screen rotate through the space occupied by the occluded box,
but because they found the 180 ~ screen rotation more interesting than the shorter, 112 ~ rotation shown in the possible event.
In order to check this possibility, a second group of 4 I/2-montholds was tested in a control condition identical to the experimental condition except that there was no boxbehind the screen
during the test events (see Figure 1). If the infants in the experimental condition looked longer at the impossible event because
they preferred the 180″ to the 112 ~ rotation, then the infants in
the control condition should also look longer at the 180″ event.
On the other hand, if the infants in the experimental condition
looked longer at the impossible event because they were surprised when the screen failed to stop against the occluded box,
OBJECT PERMANENCE IN 3V2- AND 4lh-MONTH-OLD INFANTS 657
then the infants in the control condition should look equally at
the 180* and the 112* events because no box was present behind
the screen, t
~lethod
Subjects
Subjeets were 24 full-term infants ranging in age from 4 months, 2
days to 5 months, 2 days (M = 4 months, 14 days). Halfofthe infants
were assigned to the experimental condition and haifto the control condition. Another 5 infants were excluded from the experiment, 3 because
of fussiness, 1 because ofdrowsiness, and 1 beeause ofequipment failure. The infants’ names in this experiment and in the sueoeeding experiments were obtained from birth announcements in a local newspaper.
Parents were contacted by letters and follow-up phone calls; they were
offered reimbursement for their travel expenses but were not eompensated for their participation.
Apparatus
The apparatus consisted of a large wooden box that w-as 120 cm high,
95 cm vade, and 74 cm deep. The infant faced an opening, 49 cm high
and 95 cm wide, in the front wail ofthe apparatus. The interior ofthe
apparatus was painted black and was decorated with narrow pink and
green stripes.
At the center ofthe apparatus was a silver cardboard screen that was
31 cm high, 28 cm vade, and 0.5 cm thick. The lower edge ofthe screen,
which was set 0.5 cm above the floor of the apparatus, was afl 9 to a
thick metal rod that was 28.5 cm long and 1 cm in diameter. This rod
was connected to a right-angie gear box that was 2 cm hig,h, 3.5 cm vade,
and 4 cm deep. A drive rod, which was 0.5 cm in diameter, was aiso
connected to the gear box. This rod was 54 cm long and protruded
through the back waU ofthe apparatus. By rotating this rod, an experimenter could rotate the screen back and forth through a 180″ arc?
A wooden box, 25 cm high, 15 cm vade, and 5 cm thick, could be
introduced into the apparatus through a hidden door in its back wall.
This box was painted yellow and was decorated with a two-dimensional
clown face. The box was placed on a platform, which was 21 cm vade
and 28 cm long, in the floor ofthe apparatus, behind the screen. This
platform was mounted on a vertical slide located underneath the apparatus. By Iowering the platform, after the screen occluded the box from
the infant’s view, an experimenter could surreptitiously remove the box
from the path ofthe screen.
The infant was tested in a brightly lit room. Four clip-on iights (each
with a 40-W lightbulb) were attached to the back and side walls ofthe
apparatus to provide additional light. Two frames, each 183 cm high
and 71 cm vade and covered with black cloth, stood at an angle on
either side of the apparatus. These frames isolated the infant from the
experimental room. At the end of each trial, a muslin-covered curtain,
65 cm high and 95 cm vade, was lowered in front ofthe opening in the
front wall ofthe apparatus.
Experimental-Condition Events
Two experimenters worked in concert to produce the events in the
experimental condition. The first operated the screen, and the second
operated the platform.
Impossible test event. To start, the screen lay fiat against the floor of
the apparatus, toward the infant. The yellow box stood clearty visible,
centered 12.5 cm behind the screen. The first experimenter rotated the
screen at the approximate rate of 45″/s until it had completed a 90*
arc, at which point she paused for 1 s. This pause ailowed the second
experimenter to lower the platform supporting the box. The first experimenter then continued to rotate the screen toward the back wall at the
saine rate of about 45″/s until it lay fiat against the floor ofthe apparatus,
covering file space previously occupied by the box. The entire process
was then repeated in reverse: The first experimenter rotated the screen
90* and paused for 1 s, allowing the second experimenter to raise the
platform; the first experimenter then lowered the screen toits original
position against the floor of the apparatus, revealing the box standing
intact in the saine position as before.
Each full cycle of movement thus lasted approximately 10 s. The box
remained occluded for about 8 ofthese 10 s: It was in view only during
the first and last seconds, when the screen was raised less than 45*. There
was a 1-s pause between successive cycles. Cycles were repeated until
the computer signaled that the triai had ended (see below). At that point,
the second experimenter lowered the cumin in front ofthe apparatus.
Possible test event. As before, the first experimenter rotated the
screen 90* at the rate of about 45″/s and then paused for 1 s, allowing
the second experimenter to lower the plafform. The first experimentœ
then continued to rotate the screen 22.5* toward the back wall (where
the screen would bave contacted the box bad the latter hOt been lowered), taking about 0.5 s to complete this movement. The first experimenter held the screen in this position for 2 s, and then the entire process was repeated in reverse: The first experimenter retumed the screen
to the 90* position, paused for 1 s (to allow the second experimenter to
raise the platform), and then lowered the screen toits initial position
against the floor of the apparatus. Each full cycle of movement thus
lasted about 9 s, with the box remaining totally occluded for about 7 of
these 9 s. 3
Habituation event. The habituation event was exactly the saine as the
impossible test event, except that the box was absent.
Control-Condition Events
180″ and 112″ test events. The t 80* and the 112* test events shown to
the infants in the control condition were identical fo the impossible and
possible test events (respectively) shown to the infants in the experimenrai condition, except that the box was absent.
Habituation event. The habituation event shown to the infants in
J The control condition was conducted without the box, rather than
with the box to the side ofthe screen as in Baillargeon et al. (1985), to
avoid a possible ambiguity. Ifthe infants looked equaily at the 180* and
the 112* events, with the box to the side, one coutd not be sure whether
(a) they had an equal preference for the two rotations or (b) they fixated
the box and ignored the screen. Baitlargeon et al. found, in their control
experiment, that the order in which the infants saw the two screen events
had a reliable effect on their looking behavior; such a finding rules out
the possibitity that the infants were merely staring at the box. Nevertheless, because of this potential confound, it seemed best to conduct the
control experiment without the box.
2 In order to help the experimenter more the screen at a constant,
steady pace, a protractor was attached to the drive rod. In addition,
the experimenter listened through headphones to a metronome clicking
once per second.
3 The 2-s pause in the possible event was introduced to make the rate
of disappearance and reappearance of the box more similar in the two
events. With the pause, the occlusion time ofthe box was 8 out of l0 s
in the impossible event and 7 out of 9 s in the possible event. Making
these two fgures highly similar helped ensure that (a) the infants could
not discriminate between the two events on the basis of rate differences
and (b) the observers could not identify the events by the rate at which
the platform was lowered and raised. Pilot data collected with the two
observers indicated that they were unable to guess which event was being shown on the basis ofthe sounds associated with the movement of
the platform.
658 RENI~E BAILLARGEON
the control condition was identical to that shown to the infants in the
experimental condition.
The platform was moved in the same manner in all of the events to
ensure that the sounds that accompanied the lowering and raising of the
platform could not contribute to differences in the infants’ looking
times between and within conditions.
Procedure
Prior to the beginning of the experiment, each infant was allowed to
manipulate the yellow box for a few seconds while the parent filled out
consent forms. During the experiment, the infant sat on the parent’s lap
in front of the apparatus. The infant’s head was approximately 65 cm
from the screen and 100 cm from the back wall. The parent was asked
not to interact with the infant while the experiment was in progress. At
the start of the test tri .Ms, the parent was instructed to close his or her
eyes.
The infant’s looking behavior was monitored by two observers who
viewed the infant through peepholes in the cloth-covered frames on either side of the apparatus. The observers could not see the experimental
events and did not know the order in which the test events were presented. Each observer held a button box linked to a MICRO]PDpo I 1 computer and depressed the button when the infant attended to the experimental events. Interobserver agreement was calculated for each trim on
the basis of the number of seconds for which the observers agreed on
the direction of the infant’s gaze out of the total number of seconds the
trim lasted. Disagreements of less than 0.1 s were ignored. Agreement
in this experiment as well as in the subsequent experiments averaged
88% (or more) per trim per infant. The looking times recorded by the
primary observer were used to determine when a trim had ended and
when the habituation criterion had been met (see below).
At the start of the experiment, each infant received a familiarization
trim to acquaint him or her with the position of the box behind the
screen. During this triM, the screen lay flat against the floor of the apparatus, with the box standing clearly visible behind it. The trim ended
when the infant (a) looked away from the display for 2 consecutive seconds after having looked at it for at least l 0 cumulative seconds, or (b)
looked at the display for 30 cumulative seconds without looking away
for 2 consecutive seconds.
Following the familiarization triM, each infant was habituated to the
habituation event described above, using an infant-control procedure
(after Horowitz, 1975). The main purpose of this habituation phase was
to familiarize the infant with the (relatively unusual) motion of the
screen. 4 Each habituation trim ended when the infant (a) looked away
from the event for 2 consecutive seconds after having looked at it for at
least 5 cumulative seconds (the duration of a hMf-cycle), or (b) looked
at the event for 60 cumulative seconds without looking away for 2 consecutive seconds. The intertriM interval was 2-3 s. Habituation trims
continued until the infant reached a criterion of habituation of a 50%
or greater decrease in looking time on three consecutive trims, relative
to the infant’s looking time on the first three trims. If the criterion was
not met within nine trims, the habituation phase was ended at that
point. Therefore, the minimum number of habituation trims an infant
could receive was six, and the maximum number was nine. Only 3 of
the infants failed to reach the habituation criterion within nine trims;
the other 21 infants took an average of 6.62 trims to satisfy the criterion.
It should be noted that, in this experiment as in the subsequent experiments, infants who failed to reach the habituation criterion within nine
trims were not terminated: At the completion of the ninth habituation
triM, the experimenters simply proceeded to the test phase.
After the habituation phase, the infants in the experimental condition
saw the impossible and the possible test events on alternate trims until
they had completed four pairs of test trims. Similarly, the infants in the
control condition saw the 180 ~ and the I 12 ~ test events on alternate trims
until they had completed four pairs of test trims. Within each condition,
hMfofthe infants saw one test event first and the other half saw the other
test event first. At the beginning ofeach test triM, the first experimenter
waited to move the screen until the computer signaled that the infant
had looked inside the apparatus for 2 cumulative seconds. This ensured
that the infants in the experimental condition had noted the presence
of the box behind the screen. The criteria used to determine the end of
each test trim were the same as for the habituation trials.
Six of the 24 infants in the experiment completed fewer than four
pairs of test trials. Five infants completed only three pairs, 3 because
of fussiness, 1 because of procedural error, and 1 because the primary
observer could not follow the direction of the infant’s gaze. The other
infant completed only two pairs, because of fussiness. All subjects (in
this experiment as well as in the subsequent experiments) were included
in the data analyses, whether or not they had completed the full complement of four pairs of test trims.
Figure 2. Looking times of the infants in the experimental and control
conditions in Experiment l during the habituation and test trims. (Note
that the habituation trims are numbered backward from the trim in
which habituation was reached.)
4 It is interesting to speculate about the role of the habituation trims
in the experiment. As stated in the text, the main rationale for including
these trims was to familiarize the infants with the (presumably unfamiliar) movement of the screen. However, it could be that such familiarization was not necessary and that the infants would have responded in the
same way had they received no habituation trims. Another possibility is
that the habituation trims served to acquaint the infants with the fact
that the screen rotated freely through empty space but stopped rotating
when it encountered a hard surface. This hypothesis predicts that the
infants would look less at the possible event (in which the screen rotated
freely until it reached the occluded box) than at the impossible event
(in which the screen continued to rotate after encountering the box).
Further research is needed to evaluate these and related alternatives.
OBJECT PERMANENCE IN 3V2- AND 4V2-MONTH-OLD INFANTS 659
Results
Figure 2 presents the mean looking times of the infants in
the experimental and control conditions during the habituation
and test phases of the experiment. It can be seen that the infants
in the experimental condition looked longer at the impossible
than at the possible event, whereas the infants in the control
condition looked equally at the 180* and the 112* events.
The infants’ looking times to the test events were analyzed by
means of a 2 • 2 X 4 X 2 mixed-model analysis of variance
(ANOVA), with Condition (experimental or control) and Order
(impossible/180* or possible/l 12* event first) as the betweensubjects factors, and with Event (impossible/180* or possible/
112*) and Test Pair (first, second, third, or fourth pair of test
trials) as the within-subjects factors. Because the design was unbalanced, the SAS GLM procedure was used to calculate the
ANOVA (SAS Institute, 1985). There was a significant main
effect of condition, F(I, 20) = 8.53, p < .01, and of event, F(I,
126) = 6.16, p < .05, and a significant Condition x Event interaction, F(1,126) = 8.50, p < .005. Planned comparisons indicated that the infants in the experimental condition looked reliably longer at the impossible (M = 29.2, SD = 20.6) than at the
possible (M = 17.7, SD = 13.1) event, F(1, 126) = 14.48, p <
.0005, whereas the infants in the control condition looked
equally at the 180″ (M = 15.1, SD = 9.3) and the 112″ (M =
16.2,SD = 12.3) events, F(1,126) = 0.12.
The analysis also revealed a significant Order x Event interaction, F(1, 126) = 4.64, p < .05. Post hoc comparisons indicated that the infants who saw the impossible/180* event first
looked reliably longer at this event (M = 24.17, SD = 16.88)
than at the possible/112 * event (M = 14.45, SD = 9.51), F(I,
126) = 10.56, p < .005, whereas the infants who saw the possible/112″ event first tended to look equally at the impossible/
180″ (M = 20.24, SD = 18.01) and the possible/112″ (M =
19.67, SD = 14.99) events, F(I, 126) = 0.03. Such order effects
are not uncommon in infancy research and are of little theoretical interest.
Discussion
The infants in the experimental condition looked reliably
longer at the impossible than at the possible event, suggesting
that they understood that (a) the box continued to exist after it
was occluded by the screen, and (b) the screen could not move
through the space occupied by the occluded box. In contrast
to the infants in the experimental condition, the infants in the
control condition tended to look equally at the 180″ and the
112″ events. This finding provides evidence that the infants in
the experimental condition looked longer at the impossible
event, not because they found the 180* screen rotation intrinsically more interesting than the 112* rotation, but because they
expected the screen to stop when it reached the occluded box
and were surprised that it failed to do so.
The results of Experiment 1 suggest that, contrary to Piaget’s
(1954) claims, infants as young as 4 t/2 months of age understand
that an object continues to exist when occluded. Experiment 2
investigated whether infants aged 3 1/2–4 months also possess a
notion of object permanence. The design of this experiment
was identical to that of Experiment 1.
Figure 3. Looking times of the infants in the experimental and control
conditions in Experiment 2 during the habituation and test trials.
Experiment 2
Method
Subjects
Subjects v~re 40 full-term infants ranging in age from 3 months, 15
days to 4 months, 3 days (M = 3 months, 24 days). More infants were
tested in Experiment 2 than in Experiment 1 because pilot data indicated that the responses of these younger infants tended to be more
variable; some infants produced consistently short looks, and other infants produced consistently long looks. Half of the infants were assigned
to the experimental condition and half to the control condition. Six
other infants were excluded from the experiment, 5 because of fussiness
and I because of drowsiness.
Apparatus, Events, and Procedure
The apparatus, events, and procedure used in this experiment were
the same as in Experiment t. Of the 40 infants in the experiment, 12
failed to reach the habituation criterion within 9 trials; the others took
an average of 7.14 trials to reach the criterion. Ten infants contributed
only three pairs of test trials to the data analyses, 7 because of fussiness,
I because he would not look at the events, 1 because of procedural error,
and 1 because of equipment failure.
Results
Figure 3 presents the mean looking times of the infants in
the experimental and control conditions during the habituation
and test phases of the experiment. The infants’ looking times
during the test phase were analyzed as in the preceding experi-
660 RENl~E BAILLARGEON
ment. The analysis revealed no significant main effects or interactions, all Fs < 2.69, ps > .05.
Fast and Slow Habituators
Examination of the infants’ looking times during the habituation and test phases of the experiment suggested that the pattern revealed by the initial analysis (statistically equal looking
times to the impossible/180* and the possible/112* events) represented the average of two distinct looking patterns. Specifically, it appeared that, in the experimental condition, the infants who reached the habituation criterion within six or seven
trials tended to look longer at the impossible than at the possible
event, whereas the infants who required eight or nine trials to
reach the criterion or who did not reach the criterion tended to
look equally at the two test events. In the control condition, in
contrast, both groups of infants tended to look equally at the
180* and the 112* events. These patterns were not unexpected,
because rate of habituation is known to relate to posthabituation performance (Bornstein & Benasich, 1986; DeLoache,
1976; McCall, 1979).
The infants were therefore classified as fast habituators (the
infants who took six or seven trials to reach the habituation
criterion) and slow habituators (the infants who required eight
or nine trials to reach the criterion or who failed to reach the
criterion within nine trials). In the experimental condition, 9
infants were classified as fast habituators and 11 as slow habituators. In the control condition, 7 infants were classified as fast
habituators and 12 as slow habituators (the remaining infant
could not satisfy the habituation criterion because he produced
very short looks on each trials; because it was unclear how
this infant should be classified, he was excluded from the next
analyses).
The looking times of the fast and slow habituators in the experimental and control conditions to the test events were analyzed by means of a 2 • 2 • 2 • 4 • 2 mixed-model ANOVA
with Habituation (fast or slow habituators), Condition (experimental or control), and Order (impossible/180* or possible/
112″) as the between-subjects factors, and with Test Pair (first,
second, third, and fourth pair of test trials) and Event (impossible/ 180* or possible/112*) as the within-subject factors. As anticipated, this analysis yielded a significant Habituation • Condition • Event interaction, F(l, 189) = 6.54, p < .05. In order
to study this interaction, four comparisons were carried out.
These indicated that in the experimental condition, the fast
habituators looked reliably longer at the impossible (M = 23.78,
SD = 18.28) than at the possible (M = 14.68, SD = 11.79)
event, F(1, 189) = 7.38, p < .01, whereas the slow habituators
looked about equally at the two events (impossible, M = 23.45,
SD = 19.05; possible, M = 27.68, SD = 20.97), F(1, 189) =
1.75, p > .05 (see Figure 4). 6 In the control condition, the fast
habituators looked about equally at the 180* (M = 17.06, SD =
15.45) and 112″ (M = 19.80, SD = 18.09) events, F(l, 189) =
0.48, as did the slow habituators (180* event, M = 20.34, SD =
16.87; 112″ event, M = 18.30, SD = 15.10), F(I, 189) = 0.41.
There were no other significant main effects or interactions (all
Fs < 3.53, ps > .05).
At the end of each habituation and test trial, the observers
rated the state of the infant. Examination of these ratings reFigure 4. Looking times of the fast and slow habituators in Experiment
2 (experimental condition) during the habituation and test trials.
vealed that during the habituation trials, the slow and fast habituators did not differ in amount of fussiness: Only four (17%)
slow and three (19%) fast habituators were judged by the observers to have been slightly or moderately fussy on two or more
trials. During the test trials, however, the slow habituators
tended to be slightly fussier than the fast habituators. Seven
(30%) slow habituators, but only one (6%) fast habituator, completed fewer than four pairs of test trials because of fussiness.
Furthermore, nine (39%) slow habituators, but only four (25%)
s Because the minimal looking time for any test trial was 5 s, an infant
had to cumulate at least 30 s of looking across three consecutive trials
in order to show a 50% decline in looking time.
6 It may seem puzzling that, although the fast and slow habituators
differed in how long they looked at the possible event (fast, M = 14.68;
slow, M = 27.68), both groups of infants looked about equally at the
impossible event (fast, M = 23.78; slow, M = 23.45). One might want to
suggest, on the basis of these data, that although both groups of infants
dishabituated to the impossible event (because it was impossible), only
the slow habituators dishabituated to the possible event (perhaps because of the novel screen rotation). However, examination of the habituation data in Figure 4 argues against this interpretation. The fast habituators’ mean looking time on their last three habituation trials (M =
14.13) was similar to their mean looking time to the possible (M=
14.68) but not to the impossible (M = 23.78) event. In contrast, the slow
habituators’ mean looking time on their last three habituation trials
(M = 22.46) was similar to their mean looking time to the impossible
(M = 23.45) and, to a lesser extent, to the possible (M = 27.68) event.
The fast and slow habituators’ equal looking times to the impossible
event would thus reflect differences in their absolute levels of looking at
the events, rather than similarities in their processing of the events.
OBJECT PERMANENCE IN 31/2 – AND 41/2-MONTH-OLD INFANTS 661
fast habituators, were rated as slightly or moderately fussy on
two or more test trials.
Why were the slow habituators fussier than the fast habituators during the test trials? One reason might be that, having
looked longer during the habituation phase, the slow habituators were more likely to become tired or bored during the test
phase. A one-way ANOVA indicated that the slow habituators
(M = 254.59, SD = 99.90) looked reliably longer overall during
the habituation trials than did the fast habituators (M = 180.94,
SD = 62.87), F(I, 35) = 6.39, p < .05. Hence, the slow habituators might have been somewhat fussier during the test trials because they were more tired, bored, or restless.
Discussion
The fast habituators in the experimental condition showed a
pronounced preference for the impossible over the possible
event, a preference akin to that observed in the 4 l/2-month-olds
in Experiment 1. In contrast, the fast habituators in the control
condition tended to look equally at the 180 ~ and the 112 ~ events.
Together, these results indicate that the fast habituators in the
experimental condition looked longer at the impossible event,
not because they found the 180 ~ rotation of the screen more
interesting than the 112 ~ rotation, but because they were surprised or puzzled to see the screen rotate through the space occupied by the occluded box. Such results suggest that at least
some infants between the ages of 3 1/2 and 4 months realize that
an object continues to exist when occluded.
The slow habituators in the experimental condition, in contrast to the fast habituators, tended to look equally at the impossible and the possible events. The marked discrepancy in the
responses of these two groups of infants will be discussed in the
General Discussion.
Experiment 3: Replication
Given the unexpected nature and potential significance of the
results obtained in the experimental condition of Experiment
2, it seemed important that they be confirmed. Experiment 3
attempted to do so with 3 1/2-month-old infants.
Method
Subjects
Subjects were 24 full-term infants ranging in age from 3 months, 6
days to 3 months, 25 days (M = 3 months, 15 days). Five additional
infants were eliminated from the experiment, 4 because of fussiness,
and 1 because the primary observer could not follow the direction of
the infant’s gaze.
Apparatus and Events
The apparatus was the same as that used in the preceding experiments, with one exception. Instead of the yellow box, a brightly colored,
three-dimensional Mr. Potato Head was used. Casual observations indicated that most infants found this toy more attractive than the box.
Because Mr. Potato Head was shorter than the box (15.5 cm as 09-
posed to 25 cm), the screen was rotated 135″, instead of 112 ~ in the
possible event. That is, after rotating the screen 90* at the usual rate of
45″/s and then pausing for 1 s, as before, the primary experimenter rorated the screen 45* toward the back wall of the apparatus, taking 1 s to
complete the movement. The first experimenter paused for 2 s and then
repeated the same actions in reverse. Each full cycle of movement thus
lasted approximately 10 s, as in the impossible event. Mr. Potato Head
was totally occluded for about 8 of the 10 s.
Procedure
The procedure was the same as that of the experimental condition in
Experiment 2, with one exception. In an attempt to abbreviate the test
phase of the experiment, no pretrials were given at the beginning of the
test trials.
Of the 24 infants in the experiment, 8 failed to reach the habituation
criterion within 9 trials; the other infants took an average of 6.94 trials
to reach the criterion. Five infants completed only three pairs of test
trials, 4 because of fussiness, and 1 because the primary observer could
not follow the direction of the infant’s gaze. Another 2 infants completed only two test pairs because of fussiness.
Results
The looking times of the infants to the impossible and possible test events were first analyzed by means of a 2 x 4 x 2
mixed-model ANOVA, with Order (impossible or possible event
first) as the between-subjects factor, and with Test Pair (first,
second, third, or fourth pair of test trials) and Event (impossible
or possible event) as the within-subjects factors. The analysis
yielded no significant main effects or interactions, all Fs < 2.05,
ps> .ll.
Fast and Slow Habituators
Of the 24 infants who participated in the experiment, 12 were
classified as fast habituators, and 12 were classified as slow habituators, using the same criteria as in Experiment 2.
Figure 5 shows the mean looking times of each group of infants during the habituation and test trials. The looking times
of the two groups to the test events were analyzed by means of
a 2 • 2 x 4 x 2 mixed-model ANOVA, with Habituation (fast
or slow habituators) and Order (impossible or possible event
first) as the between-subjects factors, and with Test Pair (first,
second, third, or fourth test pair) and Event (impossible or possible event) as the within-subjects factors. As expected, the analysis yielded a significant Habituation x Event interaction, F( l,
122) = 5.13, p < .05. Planned comparisons showed that the fast
habituators looked reliably longer at the impossible (M = 21.97,
SD = 16.33) than at the possible (M = 14.24, SD = 10.56)
event, F(l, 122) = 6.35, p < .02, whereas the slow habituators
looked at the impossible (M = 23.24, SD = 20.00) and the possible (M = 26.42, SD = 21.15) events about equally, F(1,
122) = .91.
Comparison of the fast and slow habituators indicated that
they did not differ in fussiness during the habituation trials:
Only one infant, a slow habituator, was judged to have been
fussy on two or more habituation trials. However, as in Experiment 2, the slow habituators tended to be fussier than the fast
habituators during the test trials. Five of the slow habituators,
but only one of the fast habituators, completed fewer than four
test pairs due to fussiness. In addition, seven slow habituators,
but only three fast habituators, were fussy on two or more trials.
A one-way ANOVA showed that, as in Experiment 2, the slow
662 RENI~E BAILLARGEON
Figure 5. Looking times of the fast and slow habituators in
Experiment 3 during the habituation and test trials.
habituators (M = 218.59, SD = 139.75) tended to look longer
overall during the habituation trials than did the fast habituators (M = 140.25, SD = 37.80), F(I, 21)= 3.50,p = .075.
Discussion
The results of Experiment 3 replicated those of the experimental condition in Experiment 2. The fast habituators looked
reliably longer at the impossible than at the possible event, suggesting that they were surprised or puzzled to see the screen
move through the space occupied by Mr. Potato Head.
Could the fast habituators’ preference for the impossible
event be due to their having found the 180* screen rotation intrinsically more interesting than the shorter rotation shown in
the possible event? This interpretation seems highly unlikely for
two reasons. First, the fast habituators in the control conditions
of Experiments 1 and 2 did not show an overall preference for
the 180* rotation. Second, the slow habituators in Experiment
3 looked about equally at the impossible and the possible
events. It is ditficult to imagine why the fast, but not the slow,
habituators would have found the 180″ rotation intrinsically
more interesting than the shorter, 135* rotation shown in the
possible event.
General Discussion
The 4 l/2-month-olds in Experiment 1 and the 3 V2-montholds in Experiments 2 and 3 who were fast habituators all
looked reliably longer at the impossible than at the possible
event, suggesting that they understood that (a) the object behind
the screen (i.e., box or Mr. Potato Head) continued to exist after
the screen rotated upward and occluded it and (b) the screen
could not move through the space occupied by the object. The
results of the control conditions in Experiments I and 2 provide
support for this interpretation. These results indicated that
when no object was present behind the screen, the infants did
not look longer at the 180* screen rotation.
These results call into question Piaget’s (1954) claims about
the age at which object permanence is attained, about the processes responsible for its emergence, and about the behaviors by
which it is manifested. These are discussed in turn.
Piaget maintained that it is not until infants reach about 9
months of age that they begin to view objects as permanent.
However, the results of the experiments reported here indicate
that infants as young as 3 1/2 months of age already realize that
objects continue to exist when occluded. This finding does not
mean that by 3 1/2 months, infants’ conception of occluded objects is as sophisticated as that of older infants. Further research
is necessary to determine whether young infants are able to represent not only the existence but also the physical and spatial
characteristics of occluded objects (e.g., Baillargeon, 1986, in
press; Baillargeon & Graber, in press).
Piaget also held that the emergence of object permanence depends on the coordination of sensorimotor schemes, which begins
at about 9 months of age. The present findings, like those of Baillargeon et al. (1985), are inconsistent with this explanation, because they suggest that infants possess a notion of object permanence long before they begin to perform coordinated actions.
How can we account for the presence of a notion of object
permanence in 3 l/2-month-old infants? One possibility is that
this notion is innate (e.g., Bower, 1971; Spelke, 1985). Spelke
(1985), for example, hypothesized that infants are born with
a conception of objects as spatially bounded entities that exist
continuously in time and move continuously in space, maintaining their internal unity and external boundaries. This conception, according to Spelke, provides infants with a basis for
recognizing that objects continue to exist when occluded. A second possibility is that infants are born, not with a substantive
belief in the permanence of objects, but with a learning mechanism that is capable of arriving at this notion given a limited
set of pertinent observations. 7 These observations could arise
from infants’ examination of the displacements and interactions of objects (Mandler, 1986) as well as from infants’ actions
upon objects. Although infants do not show mature reaching
for objects until about 4 months of age (e.g., Granrud, 1986;
von Hofsten, 1980), infants less than 4 months often perform
arm extensions in the presence of objects (e.g., Bruner & Koslowski, 1972; Field, 1976; Provine & Westerman, 1979). Infants
might notice, when performing these arm extensions, that their
hands sometimes occlude and sometimes are occluded by objects (Harris, 1983). The same point can be made about infants’
manipulations of objects. White (1969) reported that beginning
at about 3 months of age, objects that are placed in one hand
are often brought to the midline to be simultaneously viewed
and explored tactually by the other hand. Infants might notice
71 have left open the question of whether the learning mechanism is
constrained in terms of the types of observations it can detect or the
nature of the generalizations it can derive.
OBJECT PERMANENCE IN 3V2- AND 41/2-MONTH-OLD INFANTS 663
in the course of these manipulations that their hands occlude
(parts of) the objects.
The results of the present experiments are not sufficient to
determine which (if either) of the two hypotheses mentioned
above better explains the presence of object permanence in
3 ‘/2-month-old infants. What these results do indicate, however,
is that whatever explanation is proposed cannot depend on perceptual or motor abilities more sophisticated than those available after the third month of life.
Piaget viewed the search for hidden objects as the hallmark of
object permanence. Yet the present results indicate that infants
possess object permanence long before they begin to engage in
search activities. How can we explain this discrepancy? Why
do infants’ actions lag so far behind their understanding? One
possibility, alluded to in the introduction, is that young infants
may fail to search because they are generally unable to perform
sequences of actions in which one action is applied to one object
in order to create conditions under which another action can be
applied to another object (e.g., pulling a cushion to get an object
placed on it). Why infants should have difficulty with these
types of action sequences remains somewhat of a mystery. Piaget’s (1952) remarkable observations of the development of action in infancy make it clear that 3- and 4-month-old infants
can perform means–end sequences in which one action is applied to one object (e.g., pulling a chain) in order to produce a
result involving another object (e.g., shaking a toy attached to
the other end of the chain). Further research is needed to compare the cognitive and motor requirements of the means-end
sequences observed at 3–4 and at 7-8 months to identify the
source of the latter’s difficulty.
A final issue raised by the results of the present experiments
concerns the differences between the performances of the fast
and slow habituators in Experiments 2 (experimental condition) and 3. Recall that the fast habituators looked longer at the
impossible than at the possible event, whereas the slow habituators looked about equally at the two events. At least two explanations could be offered for the discrepancy between these two
groups, one appealing to transient and the other to more stable
differences between the fast and slow habituators.
The first possibility is that the slow habituators were less engaged by the experimental events than the fast habituators. Perhaps the slow habituators were less alert, or more distressed by
their novel surroundings. In any case, they were less involved
with the events: (a) they required more trials to reach the habituation criterion, if they reached it at all, and (b) they were more
likely to become fussy during the test trials. In other words, because the task of interpreting the impossible and possible events
was a difficult one for the young infants in Experiments 2 and
3, only the infants whose attention was fully engaged were able
to grasp the underlying structure of the events (i.e., realized that
the screen rotated through the space occupied by the occluded
box). The infants whose attention wandered–because they
were fussy, bored, hungry, or ill at ease in their unfamiliar surroundings-were apparently unable to do so.
Two predictions follow from this first interpretation. One is
that whether a given 3 l/2-month-old looks longer at the impossible event in any one test session should depend on his or her
attentional state at the time of the session. The other is that, as
infants grow older and the task of understanding the impossible
and possible events becomes less difficult, attentional states
should have less impact on their performances. Partial support
for this prediction comes from the experimental condition of
Experiment 1. Of the 12 infants in this condition, 8 were fast
habituators (6-7 trials to criterion), and 4 were slow habituators
(8-9 trials to criterion). Six of the 8 fast habituators (75%) and
3 of the 4 slow habituators (75%) looked at the impossible event
for at least 9 s longer than at the possible event. For the younger
infants in Experiments 2 (experimental condition) and 3 combined, the comparable figures were 17 of 21 (81%) fast habituators and 7 of 23 (31%) slow habituators.
The second interpretation for the discrepancy in the responses of the fast and slow habituators is that it reflects, not
transient fluctuations in alertness and mental involvement, but
stable, meaningful differences between the two groups of infants. Suppose that the fast habituators were generally more
efficient than the slow habituators at processing information
about the physical world–at detecting, discriminating, representing, and categorizing regularities about objects and events.
One consequence might be that at the time of testing, the fast
habituators had already formed strong expectations about the
permanence and the solidity of objects, whereas the slow habituators had just begun developing these expectations. This
would explain why the fast habituators showed marked and
consistent attention to the impossible event and why the slow
habituators did not.
Note that the hypothesized difference between fast and slow
habituators need not be cognitive in origin: It could also be motivational (e.g., fast habituators might be more motivated or
more persistent in their examination of the physical world), social (e.g., fast habituators might have caretakers who consistently direct their attention toward objects), or physiological
(e.g., fast habituators might have better state control) (cf.
Bornstein & Benasich, 1986; Bornstein & Sigman, 1986).
Longitudinal data will be needed to decide which (if either)
of the two interpretations put forth is accurate. If one finds that
the same infants show a preference for the impossible event
when they habituate quickly and fail to show this preference
when they habituate more slowly, one might be warranted to
conclude that the infants possess a notion of object permanence
but only manifest this notion when they are sufficiently alert
and calm to attend to the events. A simpler test of object permanence, one necessitating less sustained attention, might be less
subject to the vagaries of infants’ attentional states.
The results of the experiments reported in this article indicate
that by 3 1/2 months of age, infants already possess expectations
about the behavior of objects in time and space. Specifically, infants assume that objects continue to exist when occluded and that
objects cannot move through the space occupied by other objects.
It is likely that further investigations of young infants’ physical
knowledge will bring to light further competence. As the picture
of infants’ physical world becomes more complex, the task of describing how they attain, represent, and use their physical knowledge will undoubtedly open new avenues into the central issue of
the origins of human cognition.
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Received July 18, 1986
Revision received February 14, 1987
Accepted February 17, 1987 9

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