Journal of Comparative Psychology
2004, Vol. 118, No. 2, 140 –148
Copyright 2004 by the American Psychological Association
0735-7036/04/$12.00 DOI: 10.1037/0735-7036.118.2.140
Recognizing Impossible Object Relations: Intuitions About Support
in Chimpanzees (Pan troglodytes)
Trix Cacchione
Horst Krist
University of Zürich
University of Greifswald
Using looking-time measures, the authors examined untrained chimpanzees’ (Pan troglodytes) ability to
distinguish between adequate and inadequate support. In 3 experiments, the chimpanzees’ sensitivity to
different support relations between 2 objects was assessed. In each experiment, the chimpanzees saw a
possible and an impossible test event, presented as digital video clips. Looking times in the 3 experiments
suggest that chimpanzees use amount of contact between 2 objects, but not type of contact, to distinguish
between adequate and inadequate support relations. These results indicate that chimpanzees have some
intuition about support phenomena but their sensitivity to relational object properties may differ from that
of human infants (Homo sapiens) in this domain.
relations between two objects? In the present experiments we
approached this question from a comparative perspective, relating
it to recent research on human infants’ intuitions about support
(Baillargeon, Needham, & DeVos, 1992; Baillargeon, Raschke, &
Needham, 1995; Dan, Omori, & Tomiyasu, 2000; Huettel &
Needham, 2000; Needham & Baillargeon, 1993a, 1993b). This
work has provided evidence that human infants exhibit amazing
competencies in distinguishing adequate (stable) and inadequate
(instable) support relations. To achieve optimal comparability, we
adapted the expectancy violation method used in this research for
our chimpanzee experiments.
The basic rationale underlying the expectancy violation method
is to infer, from a looking preference (usually indicated by looking
times) for an “impossible” event over a similar possible event, that
the impossible event violated the infant’s expectation. Whether (or
which) looking preferences are perceptually or representationally
mediated is currently under debate (see Discussion). Nonetheless,
the expectancy violation method appears to be among the most
sensitive procedures available for uncovering nonverbal forms of
various kinds of knowledge. In other words, early competencies
that cannot be revealed by this method are unlikely to be diagnosable by other behavioral methods.
With research on nonhuman primates, the use of looking-time
measures also has important advantages. Most important, lookingtime measures allow researchers to assess cognitive capacities
spontaneously, without training or food incentives that may distort
the data. Recently, several researchers have successfully used
variants of the expectancy violation methodology with nonhuman
primates to investigate physical knowledge of tamarins and rhesus
macaques (Macaca mulatta) (Hauser & Carey, 1998, 2003;
Hauser, MacNeilage, & Ware, 1996; Munakata, Santos, O’Reilly,
Hauser, & Spelke, 2000; Santos & Hauser, 2002; Uller, Carey, Xu,
& Hauser, 1997; Uller, Hauser, & Carey, 2001) and the theory of
mind in chimpanzees (Uller, 2003).
Concerning apes’ perception and understanding of object relations, however, investigators have thus far relied exclusively on
assessing problem-solving behavior. Most of the pertinent studies
Not only are chimpanzees phylogenetically closely related to
humans but they also share a similar environment, governed by the
same physical principles. But do chimpanzees also perceive physical objects and their interactions the same way that humans do?
One of the first to address the question of a potential evolutionary
development of mental capacities was Wolfgang Köhler (1957) in
his famous studies on insightful problem solving by nonhuman
primates. Would chimpanzees have the insight to stack one box on
top of another to gain access to an otherwise inaccessible banana?
Köhler found that apes seem to understand that they can diminish
the distance between themselves and the target object by stacking
one box on top of another. It was in the building process itself that
the chimpanzees met the limit of their capacities. Köhler concluded that the difficulty of the task was to “add one box to the
other, so that it stays there firmly . . . . [The] total impression of all
observations made repeatedly on the animals leads to the conclusion that there is practically no statics to be noted in the chimpanzee” (pp. 128 –130).
Apparently, chimpanzees meet with great difficulties when they
try to implement their insightful strategy to place one box on top
of another, climb onto the boxes, and reach for the food. But is
Köhler’s (1957) negative conclusion concerning the “naive statics”
of chimpanzees really warranted? Is it true that chimpanzees are
unable to distinguish between adequate and inadequate support
Trix Cacchione, Department of Psychology, University of Zürich, Zurich, Switzerland; Horst Krist, Department of Psychology, University of
Greifswald, Greifswald, Germany.
We thank David Bjorklund, Marc Hauser, and Friedrich Wilkening for
their insightful comments; Ernst Federer and the Walter Zoo staff for their
cooperation and assistance in conducting the research; and Henri Gossweiler and Walter Schmid for their help with the apparatus and stimulus
design.
Correspondence concerning this article should be addressed to Trix
Cacchione, Allgemeine und Entwicklungspsychologie, Psychologisches
Institut, Universität Zürich, Attenhoferstrasse 9, CH-8032 Zürich, Switzerland. E-mail: tcac@gmx.ch
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RECOGNIZING IMPOSSIBLE OBJECT RELATIONS
have involved numerous trials, in which the apes were motivated
by the prospect of obtaining real food. A number of other researchers have investigated chimpanzees’ ability to solve the so-called
classical support problem using such methodology. This problem
was originally used by Piaget (1952) in his seminal work on
sensorimotor intelligence in infants. Piaget found that, at approximately 12 months old, infants could obtain a target object out of
reach by pulling a support. Spinozzi and Potı́ (1993) confronted 2
chimpanzees with Piaget’s support problem. In a choice task, the
animals were required to assess which of two pieces of cloth would
give access to a food reward. One of the 2 chimpanzees successfully differentiated between object-on-cloth and object-off-cloth
conditions.
Hauser, Kralik, and Botto-Mahan (1999) conducted similar experiments with cotton-top tamarins (Saguinus oedipus). In the first
task (on problem), tamarins had to choose between food (a pellet)
located on a cloth versus off a cloth. Tamarins performed well on
this task, consistently selecting an on-cloth over an off-cloth food
pellet. In the second task (connected problem), the tamarins had to
choose between a food pellet resting on a regular cloth and a
second food pellet resting on a cloth that consisted of two pieces
separated by a visible gap. The tamarins correctly ignored irrelevant feature differences (e.g., color, size, and shape of cloth or
food), but they failed to consider some relevant feature differences
(e.g., position, shape, and orientation of the gap). Using traditional
operant procedures (a classification task), Kralik and Hauser
(2002) tried to determine the limits of the tamarins’ ability to
perceive and respond to the abstract relational concept of connectedness. Their results suggest that the tamarins had not acquired a
completely abstract relational concept of connectedness. The tamarins used not only connectedness to some degree but also nonrelational information to solve a picture-classification task.
Although most researchers have interpreted the classical support
problem as a test for understanding means– end relations, Povinelli
(2000) addressed the support problem as a special case of a weak
physical connection between two objects (afforded by the weight
of the object resting on a support). Povinelli argued that if the
Köhlerian position1 is correct, then use of the basic version of the
classic support task cannot help one determine if apes can distinguish between relevant and irrelevant types of contact. After
conducting a series of experiments with choice tasks, Povinelli
concluded that chimpanzees solve the support problem on the basis
of specific perceptual features related to the spatial arrangement of
goal object and support. These perceptual judgments, Povinelli
said, have nothing to do with an underlying conceptual representation of the support phenomenon as an instance of physical
connection.
Irrespective of the validity of Povinelli’s (2000) conclusion, the
studies (Hauser, Kralik, & Botto-Mahan, 1999; Kralik & Hauser,
2002; Spinozzi & Potı́, 1993) on nonhuman primates’ consideration of the connectedness relation clearly indicate that (up to a
certain degree) these animals can use the existence or nonexistence of visible contact between objects to discriminate between different spatial arrangements of objects. These studies are
of particular interest for the present research because sensitivity to
(a) contact (vs. no contact), (b) type of contact, and (c) amount of
contact between an object and a supporting platform has been
shown to emerge in this order with human infants. On the basis of
their examination of looking-time data from experiments in which
141
the expectancy violation method was used (Baillargeon et al.,
1992; Baillargeon, Raschke, & Needham, 1995; Needham & Baillargeon, 1993a, 1993b), Baillargeon, Kotovsky, and Needham
(1995) postulated the following developmental sequence (see also
Baillargeon, 1995, 2002): (a) At age 3 months, human infants
acquire an initial concept of support, (contact or no contact) in
which they expect that two objects form a stable relation if and
only if there is any kind of contact between them; (b) at age 4.5
months, infants show the first differentiation of this global support
notion and also comprehend the type of contact between two
objects— only if one object is placed on top of another object is it
assumed to remain stable; and (c) by age 6.5 months, infants are
aware that the amount of contact between two objects must also be
considered.
The present chimpanzee studies were modeled after this developmental sequence. In three consecutive experiments, designed to
be as comparable to the corresponding infant studies (Baillargeon
et al., 1992; Baillargeon, Raschke, & Needham, 1995; Needham &
Baillargeon, 1993b) as possible, we investigated chimpanzees’
sensitivity to the visible features of contact or no contact (Experiment 1), type of contact (Experiment 2), and amount of contact
(Experiment 3). This sequence allowed us to assess how chimpanzees’ level of competence relates to the developmental progression
found with human infants. In particular, we wanted to establish
whether chimpanzees would exhibit longer looking times for an
impossible no-contact event than for a possible contact event but
would not otherwise distinguish between adequate and inadequate
support relations. Such a result would support Köhler’s (1957) as
well as Povinelli’s (2000) negative conclusions about the naive
statics of chimpanzees. Additional looking preferences for an
impossible type of contact and/or an impossible amount of contact
in Experiments 2 and 3, respectively, would provide a first piece of
evidence that chimpanzees’ intuitions about support are subtler
than previously thought.
Experiment 1
Experiment 1 examined whether chimpanzees are sensitive to
the basic contact relation between an object and a potential support. In all three experiments we used a new expectancy violation
procedure, which offered the possibility to test untrained animals
of a wide age range (from 1 to 44 years).
Method
Participants. Experiment 1 was conducted on a group of 10 chimpanzees (Pan troglodytes) ranging in age from 1.5 to 44.1 years (7 females and
3 males). All chimpanzees were housed at the Walter Zoo in St. Gallen,
Switzerland, and lived in a single social group. There were substantial
differences in their rearing histories: 4 were born in the Walter Zoo, 2 were
born in other Swiss zoos, 1 was found as an orphan in the wild, and 3 were
former performers in a circus (1 of these 3 was raised by humans). None
of them had ever participated in an experimental investigation. One chimpanzee had to be excluded from the sample because he did not attend to the
display adequately.
1
Köhler (1957) wrote, “It appears doubtful whether the conception of
‘connexion’ in our practical human sense signifies more for the chimpanzee than visual contact in a higher or lower degree” (p. 33).
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The living area of the apes covered 1,250 m2 and included a covered
dome with natural floor, two outside areas with trees and water, and a night
house with a conventional floor that could be divided into separate sleeping
chambers. In the dome an artificial termites’ nest, ropes for climbing and
swinging, and a few human artifacts (toys and blankets) were provided to
keep the chimpanzees busy.
Testing environment and apparatus. Testing was done in the mornings
and early afternoons. The separable sleeping chambers (3.4 m ⫻ 2.17 m ⫻
3.4 m) of the night house and one of the dome compartments served as
testing environments. We used two notebooks to record looking times and
to present three events as digital video clips: (a) a familiarization event, (b)
a possible test event, and (c) an impossible test event. Each event was
presented in a continuous loop until a certain criterion was met (see below).
Figure 1 illustrates the experimental setting. The first computer was used
to present the video clips, which were embedded in a Microsoft PowerPoint
presentation and displayed on an external monitor (33 cm ⫻ 24 cm). On the
second computer, a program designed for infant-habituation studies was
installed. It was used to record looking times. The experimenter (serving as
the primary observer) pressed and released the Control key of the second
computer to record the ape’s looking at the monitor and his looking away,
respectively. Analogously, the second caretaker (serving as the secondary
observer) recorded looking times by pressing and releasing the left button
of a mouse connected to the same computer. The presentation of each video
clip was controlled by automatically triggering click events on the first
computer, thereby starting and stopping the clip depending on the ape’s
looking behavior as registered by the experimenter on the first computer.
This coupling was achieved by connecting a modified mouse to both
computers. The modified mouse sent regular mouse inputs to the first
computer based on electronic inputs from the second computer (via one of
its serial ports). The mouse clicks were triggered by an optoelectronic
switch.
With this setup, we were able to use a novel method of an animalcontrolled stimulus presentation. The presentation of each event loop was
continued only as long as the ape kept looking at the monitor, as recorded
by the primary observer. As soon as the ape looked away, the video clip
presentation was stopped (frozen) until the ape resumed looking at the
monitor or until the computer program signaled the end of the trial. This
procedure was chosen to make sure that the ape would see all the information necessary to comprehend the possibility or impossibility of the
physical phenomenon presented. The caretakers could not see the video
clips during testing and were blind to the experimental condition on which
the chimpanzee was being tested.
Design. All participants were assigned to one familiarization event and
two test trials. One test trial demonstrated a physically possible event, and
Figure 1. Schematic representation of the experimental setting. C1 ⫽
computer 1; C2 ⫽ computer 2.
the other demonstrated a physically impossible event. The order of the test
trials was counterbalanced across participants.
In contrast to human infants, untrained chimpanzees could not be expected to attend to the stimuli over many trials. For this reason, only one
familiarization trial was administered to introduce all the materials that
were used in the subsequent test events (cf. Hauser et al., 1996).2 To
prevent the perceptual novelty of the display from distorting the looking
times in the direction of the expected effect, we designed the impossible
test event, as much as was feasible, to be perceptually more similar to the
familiarization event than to the possible test event. We believed that if the
chimpanzees simply preferred perceptual novelty, they would tend to look
longer at the possible test event.
Stimuli. To make sure that all the video clips had an identical time
structure, they were recorded using a metronome beating at 54 beats per
minute. Each video clip lasted 11 beats or 12.2 s. The critical moment in
the impossible test event (the moment showing the physical impossibility)
was reached after 7 beats or 8 s.3 Figure 2 shows the possible and
impossible test events. The impression of physical impossibility was
achieved by using a Perspex base over which the banana was pushed and
white lighting to make the Perspex base invisible in the video clip.
The video clip demonstrating the possible test event showed a gloved
hand entering the scene from the left side (2 beats), pushing a banana from
the left to the right side of a platform (3 beats), then releasing it and
retreating for a short distance (2 beats), then taking the banana again (1
beat) and pulling it back to the starting position (3 beats). In the impossible
test event, the participants saw the same sequence, with the exception that
beginning in the middle of the platform, the gloved hand pushed the banana
entirely off the right side of the platform so that the banana stayed
suspended in midair. The familiarization video clip showed the same
sequence, with the exception that the gloved hand pushed the banana from
the middle to the right side of the platform.
Procedure. Because none of the chimpanzees had prior experience
with experimental testing, a strict experimental control could not be maintained in every detail. Instead, the chimpanzees’ emotional well-being and
balance had priority. This made two major adjustments necessary. First, the
chimpanzees were allowed to “select” the place of the testing, within
certain limits (see further discussion below). Second, the infants (younger
than 3 years) were allowed to stay with their mothers during the testing. In
addition, some irregular behavior (e.g., movement between trials, individual delays between trials) was tolerated. Participants that did not watch the
display for the necessary 8 s or showed very disturbing behaviors (e.g.,
standing headfirst or begging for food), however, were excluded.
All participants were tested individually, except for infants with their
mothers. Each session lasted a maximum of 15 min. So the chimpanzees
would not be distressed in any way, they were not forced to go into the
testing chamber. The first caretaker waited for 1 chimpanzee to isolate
herself or himself from the rest of the group in the night house and then
closed the connecting doors. If the chimpanzee went into a sleeping
chamber by herself or himself, the sliding doors were closed. If not, the
first caretaker lured the chimpanzee into the nearest sleeping chamber, if
necessary by tempting the ape with some food as a reward. Participants that
did not separate themselves easily from the group and never entered the
night house alone were tested in a side room of the dome.
2
Hauser et al. (1996) were confronted with a similar problem while
studying wild rhesus monkeys. They decided to administer only one test
trial to each monkey in a between-subjects design. Because we did not have
a great number of participants available, we used a within-subjects design
with one familiarization trial.
3
Strictly measured, the critical moment was reached after 7 beats or
7.7 s. However, because the video clips were embedded in a Microsoft
PowerPoint presentation, a short delay at the start of the clip made it
necessary for us to fix the criterion at 8 s.
RECOGNIZING IMPOSSIBLE OBJECT RELATIONS
Figure 2.
143
Possible (left) and impossible (right) test events in Experiment 1.
Once the chimpanzee had entered the testing place, the first caretaker
called the ape by his or her name, pointed with the index finger at the
monitor, and encouraged the chimpanzee to watch the display by saying,
“Look, look!” As soon as the chimpanzee watched the monitor, the first
caretaker withdrew her hand and sat quietly next to the monitor. At the
same time, the familiarization trial was started, and the experimenter and
the second caretaker recorded the looking times online, as described above.
Each trial was finished after the ape had looked at the display for at least
8 s cumulatively (the duration necessary to see all the important features of
the video clip) and then looked away for 2 s continuously, or after the ape
had looked at the display for 120 s (whichever came first; in the present
study, however, no participant looked for 120 s). If the ape left his or her
position in front of the monitor before he or she had watched the video clip
for 8 s cumulatively (and therefore had not yet seen the critical features of
the event), the first caretaker repeated the starting procedure, called the ape,
and encouraged her or him to watch.
The same procedure was used for the subsequent test trials. At the end
of the session, all of the chimpanzees got a food reward, regardless of their
performance and whether or not they completed the experimental trials.
Analysis. Looking times in the test trials were subjected to a 2 ⫻ 2
analysis of variance (ANOVA) with repeated measures on the factor type
of event (physically possible or impossible) and the between-subjects
factor trial order (possible test event first, or impossible test event first).
For exploratory purposes, we also tested for possible age effects by
calculating the correlation between age (in months) and the individual
differences in looking times for the impossible and possible test events.
showed that all participants looked longer at the impossible test
event than at the possible test event. There was a significant effect
for type of event, F(1, 7) ⫽ 7.96, p ⬍ .05, but no effect for trial
order. Thus, the participants looked longer at the physically impossible test event in both experimental conditions, irrespective of
the order of the presented events. None of the interactions was
significant. This effect was independent of age (r ⫽ ⫺.13, p ⫽
.75).
Experiment 2
In Experiment 2 we examined whether chimpanzees are sensitive to type of contact and if they detect the difference between
horizontal and vertical contact.
Method
Participants. Experiment 2 was conducted on the same group of 10
chimpanzees, now ranging in age from 1.6 to 44.3 years (7 females and 3
males). Experiments 1 and 2 were spaced between 14 and 70 days apart,
Results
Interobserver agreement ranged from 86.7% to 99.3% for the
familiarization and test trials. Agreement was defined as the proportion of total trial time in which both observers indicated either
that the participant was looking at the display or that the participant was not looking at the display. All statistical analyses were
done on the primary observer’s data.
Figure 3 shows the mean looking times for the familiarization
and test events of Experiment 1. The apes tended to look longer at
the familiarization event (M ⫽ 10.89 s, SD ⫽ 2.53 s) than at the
possible test event (M ⫽ 9.39 s, SD ⫽ 1.48 s), and they looked
longer at the impossible test event (M ⫽ 16.01 s, SD ⫽ 7.91 s) than
at the possible test event. Examination of the individual data
Figure 3. Mean looking times in Experiment 1 (n ⫽ 9).
CACCHIONE AND KRIST
144
depending on the cooperation of each individual. Two animals had to be
excluded from the sample because of behavioral inadequacies.
Testing environment, apparatus, and design. Testing environment, apparatus, and design were identical to those of Experiment 1.
Stimuli. Video clips were recorded using a metronome beating at 54
beats per minute. The video clips lasted 11 beats or 12.2 s. The critical
moment in the impossible test event (the moment demonstrating the
physical impossibility) was reached after 7 beats or 8 s. Figure 4 illustrates
the possible and impossible test events. The impression of physical impossibility was achieved by using magnets to fix the apple at the side of the
platform.
In the possible test event, the participants saw a gloved hand holding an
apple entering the scene from the right side (2 beats), then depositing the
apple against the right side of a platform on top of a smaller box (3 beats)
and retreating for a short distance (2 beats), then taking the apple again (1
beat) and bringing it back to the starting position (3 beats). The impossible
test event was identical to the possible test event, with the exception that
the apple was not positioned on top of the small box but well above it. The
familiarization video clip was identical to the impossible test event, with
the exception that the gloved hand retained its grasp on the apple and never
released it.
Procedure. The same procedure was used as in Experiment 1, and the
same analyses were carried out.
Results
Agreement between the two observers ranged from 84.31% to
98.0% per trial. All statistical analyses were conducted on the
primary observer’s data.
Figure 5 depicts the mean looking times for the familiarization
and test events of Experiment 2. At first glance, the same tendencies as those shown in Experiment 1 seem to be noticeable. On
average, the participants looked slightly longer at the familiarization event (M ⫽ 11.91 s, SD ⫽ 4.20 s) than at the possible test
event (M ⫽ 11.55 s, SD ⫽ 4.80 s), and they looked longer at the
impossible test event (M ⫽ 12.57 s, SD ⫽ 4.58 s) than at the
possible test event. But examination of the individual data revealed
an entirely different picture. In contrast to the first experiment, in
Experiment 2 half of the participants looked longer at the possible
test event, and half of them looked longer at the impossible test
Figure 4.
Figure 5. Mean looking times in Experiment 2 (n ⫽ 8).
event. The ANOVA yielded no significant effects in Experiment 2,
and the difference in looking times for the possible and impossible
test event was independent of age (r ⫽ .44, p ⫽ .27).
Experiment 3
In Experiment 3 we investigated whether chimpanzees are sensitive to the amount of contact (between object and platform) that
is needed for the object to maintain its position.
Method
Participants. In Experiment 3 the same group of 10 chimpanzees
participated, now ranging in age from 1.9 to 44.4 years (7 females and 3
males). Experiments 2 and 3 were spaced between 42 and 98 days apart,
depending on the cooperation of each individual. Three animals had to be
excluded from the sample because of behavioral inadequacies.
Possible (left) and impossible (right) test events in Experiment 2.
RECOGNIZING IMPOSSIBLE OBJECT RELATIONS
Testing environment, apparatus, and design. Testing environment, apparatus, and design were the same as in Experiments 1 and 2.
Stimuli. Video clips were recorded using a metronome beating at 54
beats per minute. The video clips lasted 11 beats or 12.2 s. The critical
moment in the impossible test event (the moment demonstrating the
physical impossibility) was reached after 7 beats or 8 s. Figure 6 shows the
possible and impossible test events. As in Experiment 1, we achieved the
impression of physical impossibility by using a Perspex base and white
lighting to make the base seem invisible.
In the possible test event, the chimpanzee saw a gloved hand entering the
scene from the left side (2 beats), pushing a banana from the left to the right
side of a platform so that 70% of the banana’s bottom surface remained on
the platform (3 beats), then releasing the banana and retreating for a short
distance (2 beats), then taking the banana again (1 beat) and pulling it back
to the starting position (3 beats). In the impossible test event, the hand
pushed the banana from the left to the right side of the platform so that only
15% of its bottom surface remained on the platform. Again, the familiarization event was identical to the impossible test event, except that a second
platform was placed against the right side of the first platform so that the
banana always remained fully supported.
Procedure. The same procedure was used as in Experiments 1 and 2,
and the same analyses were carried out.
145
Figure 7. Mean looking times in Experiment 3 (n ⫽ 7).
Experiments 1 and 2, the difference in looking times was statistically independent of age (r ⫽ ⫺.33, p ⫽ .48).
Results
Interobserver agreement ranged from 84.4% to 99.6% per trial.
Figure 7 summarizes the mean looking times for the familiarization and test events of Experiment 3. On average, the participants looked longer at the familiarization event (M ⫽ 10.68 s,
SD ⫽ 1.67 s) than at the possible test event (M ⫽ 9.26 s, SD ⫽
0.68 s), and they looked longer at the impossible test event (M ⫽
11.42 s, SD ⫽ 1.54 s) than at the possible test event. Examination
of the individual data showed that all participants tended to look
longer at the impossible test event than at the possible test event.
The ANOVA revealed a significant main effect for type of event,
F(1, 5) ⫽ 9.95, p ⬍ .05, but no effect for trial order. Again, all
interactions were not significant. Thus, the chimpanzees looked
reliably longer at the impossible test event than at the possible test
event, irrespective of the order of the presented events. As in
Figure 6.
Discussion
In Experiments 1 and 3, all chimpanzees looked longer at the
impossible than at the possible test event, and their mean looking
times were reliably longer for the former than for the latter event
in both experiments. For Experiment 1, these results might suggest
that the chimpanzees assumed the banana could not remain stable
without support and believed that the banana was adequately
supported only when resting on the platform but not when sustained in midair. Hence, they expected the banana to fall in the
impossible test event and were surprised to see that it did not. For
Experiment 3, the results might suggest that the apes realized that
the banana was adequately supported when 70% but not when 15%
of its bottom surface rested on the platform, and therefore expected
Possible (left) and impossible (right) test events in Experiment 3.
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CACCHIONE AND KRIST
the banana to fall in the impossible test event and were surprised
when it did not. By contrast, in Experiment 2, the animals tended
to look equally long at the possible event (object on top of a
platform) as at the impossible event (object is placed against a
platform). Thus, chimpanzees appear to know that an object needs
sufficient support (amount of contact) to prevent it from falling,
whereas they do not consider whether an object is placed on top of
or against a platform (type of contact).
Admittedly, this is a rich interpretation (Haith, 1998) of our
findings; yet, it is equivalent to the interpretation that Baillargeon,
Needham, and colleagues gave for their corresponding results
obtained with human infants (Baillargeon et al., 1992; Baillargeon,
Raschke, & Needham, 1995; Needham & Baillargeon, 1993a,
1993b). Baillargeon, Needham, and colleagues interpreted looking
longer at physically impossible events as an indication of conceptual knowledge if certain preconditions are met. Concerning support events, the idea is that conceptual knowledge leads infants to
expect inadequately supported objects to fall and to show surprise
(indicated by increased looking time) when their expectation is
violated. Recently, the appropriate interpretation of the perceptual
and cognitive capacities revealed by the expectancy violation
method sparked a heated debate (Bogartz, Shinskey, & Schilling,
2000; Bogartz, Shinskey, & Speaker, 1997; Cashon & Cohen,
2000; Haith, 1998; Munakata, 2000; Rivera, Wakeley, & Langer,
1999; Schilling, 2000; Thelen & Smith, 1994; for a rebuttal, see
Baillargeon, 2000, 2002). An alternative interpretation of looking
preferences in expectancy violation experiments is that perceptual
experience alone leads infants to notice that the impossible event
is unusual. Looking longer at such events would thus indicate
infants’ preference for perceptual novelty or anomaly rather than
indicating expectancy violations in any deeper epistemological
sense (cf. Mandler, 1998).
In the present context, it seems neither necessary nor appropriate
to take sides in this ongoing controversy. There is general agreement among researchers that looking-time preferences for impossible over possible events can occur for a variety of reasons, which
can only be narrowed down empirically. One ingenious approach
to rule out low-level accounts has been reported by Needham and
Baillargeon (1993b). Their results suggest that 3-month-old infants
can take advantage of prior information to make sense of an
impossible test event. Infants that were first shown a second hand
holding the back of a box no longer exhibited a preference for an
impossible no-contact event, although the supporting hand was no
longer visible (for further examples, see Baillargeon, 1994).
Whatever interpretation of infants’ physical intuitions will eventually prevail, our present findings indicate that chimpanzees exhibit intuitions concerning support relations similar to those of 6 to
7-month-old human infants. This is a new finding indicating that
chimpanzees’ naive statics may be subtler than previously thought.
At the same time, the results of Experiment 2 suggest that there
might also be fundamental differences between chimpanzees and
human infants. These two main aspects of the present findings
need to be addressed in further research; the following discussion
of these aspects should be considered preliminary.
The observation that in Experiment 1, the chimpanzees exhibited preferential looking for the no-contact event is in line with
previous results suggesting that nonhuman primates discriminate
between adequate and inadequate support on the basis of the
qualitative feature of contact or no contact (Hauser et al., 1999;
Povinelli, 2000; Spinozzi & Potı́, 1993). In Experiment 3, the
chimpanzees even distinguished between an adequate and inadequate amount of contact. In conjunction with Köhler’s (1957)
negative findings in this regard, this result may be viewed as
pointing to a dissociation between performance displayed in action
or problem-solving tasks and performance displayed in a
preferential-looking context. Santos and Hauser (2002) found that
such a dissociation, which is characteristic of human infants, also
exists in rhesus macaques. It is therefore possible that the same
holds true for chimpanzees in certain respects.
How can it be explained that the same chimpanzees were
sensitive to amount of contact but not type of contact in Experiments 3 and 2, respectively, whereas infants appear to take into
account type of contact before they consider amount of contact?
Do chimpanzees rely on other perceptual features than do human
infants, either generally or in the particular context of support?
Although there are anatomical and physiological homologies between human and nonhuman vision, many questions remain concerning possible differences between visual representations of
humans and nonhuman primates. Nonetheless, previous research
(Povinelli, 2000; Thompson & Oden, 2000; Tomasello & Call,
1997) has indicated that chimpanzees use surface features, such as
shape, size, color, and texture, to individuate and categorize objects. Further, it can be assumed that chimpanzees are also sensitive to the spatial arrangements of the objects perceived. Thompson and Oden (2000) reported that apes use abstract relational
properties (including spatial information, such as “inside of” or
“on top of”) to categorize objects; they even appear to do so more
readily than do monkeys. Thus, it seems safe to conclude that
chimpanzees are capable of perceptually discriminating the events
displayed in our experiments and that the perception of these
events does not differ fundamentally between chimpanzees and
human infants.
Yet, if chimpanzees are capable of perceptually discriminating
between the two test events in Experiment 2, as we assume, why
did they not show a looking preference for the impossible event?
To be sure, preference implies discrimination but not vice versa. It
could thus be that chimpanzees do not “understand” that an object
can be supported only from below, although they perfectly discriminate between an object placed against or on top of another
object.
At this point, it is interesting to note that Köhler (1957) tried to
make sense of certain errors his chimpanzees made (e.g., pressing
a box against the wall and trying to climb onto it or pulling out a
box on which they were standing). He wondered whether chimpanzees were ignorant of the fundamental dependence of our
statics on the generally firm orientation of above and below, the
vertical and horizontal planes. This account appears to be weakened by recent findings showing that nonhuman primates display
a strong gravity bias in search tasks (Hauser, Williams, Kralik, &
Moskovitz, 2001; Hood, Hauser, Anderson, & Santos, 1999).
However, these results show merely that nonhuman primates are
sensitive to the vertical orientation of gravity in the context of
searching for invisibly displaced objects. It is a different issue
whether they are also sensitive to the orientation of gravity in a
support context (i.e., when solving support problems or watching
support phenomena).
The most parsimonious interpretation of the present results is
that chimpanzees are sensitive only to amount of contact but not
RECOGNIZING IMPOSSIBLE OBJECT RELATIONS
type of contact when they are watching one object being released
in the vicinity of a potential support. There are, however, a number
of alternative explanations for the negative result of Experiment 2.
Most obvious is the explanation that the video clip for Experiment
2 was the only clip showing an apple instead of a banana. That the
chimpanzees simply displayed a preference for the banana over the
apple can be ruled out, however, because this possibility cannot
explain why they looked longer at the impossible than at the
possible test event in the case of the banana but not the apple. Nor
can it explain why they tended to look longer at the familiarization
and possible test events in Experiment 2 than in Experiments 1 and
3 (see Figures 3, 5, and 7). More plausible is an alternative
explanation concerning the production of the video clip. To produce the impossible test event, we had to use magnets in Experiment 2 only. The chimpanzees may have perceived the short jolt
that was visible when the hand attached the apple to the side of the
platform. They may have interpreted this short jolt, caused by the
magnetic forces between apple and platform, to mean that the
apple was sticking to the platform; therefore, they did not find the
event unusual or impossible. A replication of the second experiment without the use of magnets could help to answer this
question.
To summarize, our results suggest that untrained chimpanzees
have some spontaneous expectations (or anticipations) about the
interaction of objects in the context of support. Their expectations
appear to be based on the perceived contact relation between two
objects, both qualitatively (contact vs. no contact) and quantitatively (sufficient vs. insufficient amount of contact) but not on
information about the objects’ absolute orientation in space. Additional research is required to better understand the present findings and to clarify whether chimpanzees form relational categories
similar to those of human infants or if their intuitions about support
are different from those of human infants.
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Received January 15, 2003
Revision received August 1, 2003
Accepted August 7, 2003 䡲