Elsevier

Cognition

Volume 67, Issue 3, 15 July 1998, Pages 311-351
Cognition

The development of calibration-based reasoning about collision events in young infants

https://doi.org/10.1016/S0010-0277(98)00036-5Get rights and content

Abstract

Previous research indicates that, when shown a collision between a moving and a stationary object, 11-month-old infants believe that the size of the moving object affects how far the stationary object is displaced. The present experiments examined whether 6.5- and 5.5-month-old infants hold the same belief. The infants sat in front of a horizontal track; to the left of the track was an inclined ramp. A wheeled toy bug rested on the track at the bottom of the ramp. The infants were habituated to an event in which a medium-size cylinder rolled down the ramp and hit the bug, propelling it to the middle of the track. Next, the infants saw two test events in which novel cylinders propelled the bug to the end of the track. The two novel cylinders were identical to the habituation cylinder in material but not in size: one was larger (large-cylinder event) and one was smaller (small-cylinder event) than the habituation cylinder. The 6.5-month-old infants, and the 5.5-month-old female infants, looked reliably longer at the small- than at the large-cylinder event. These and control results indicated that the infants (a) believed that the size of the cylinder affected the length of the bug's trajectory and (b) used the habituation event to calibrate their predictions about the test events. Unlike the other infants, the 5.5-month-old male infants tended to look equally at the small- and large-cylinder events. Further results indicated that this negative finding was not due to the infants' (a) failure to remember how far the bug rolled in the habituation event or (b) inability to use the habituation event to calibrate predictions about novel test events. Together, the present results suggest the following conclusions. First, when shown a collision between a moving and a stationary object, infants aged 5.5–6.5 months (a) believe that there is a proportional relation between the size of the moving object and the distance traveled by the stationary object and (b) can engage in calibration-based reasoning about this size/distance relation. Second, female infants precede males by a few weeks in this development, for reasons that may be related to sex differences in the maturation of depth perception.

Introduction

Traditionally, researchers assumed that infants understand very little of the physical events that take place around them (e.g. Piaget, 1952; Piaget, 1954). With the advent of new methodologies, however, investigators began to realize that even young infants possess intuitions about the physical world (e.g. Leslie, 1984a; Baillargeon et al., 1985; Baillargeon, 1986; Hood and Willatts, 1986; Baillargeon and Graber, 1987; Leslie and Keeble, 1987). Most of these initial investigations were relatively narrow in their focus: they sought competencies where none had been expected or demonstrated before, and their perspectives tended to be static rather than developmental in nature. Experiments were designed to establish whether young infants possessed specific competencies, with little attention to how these competencies might develop over time.

Since the beginning of the 1990s, however, the exploration of infants' physical world has undergone a dramatic change. Issues of development are now at the core of many research enterprises (e.g. Spelke et al., 1992; Baillargeon, 1995; Oakes and Cohen, 1995; Xu and Carey, 1996; Needham et al., 1997; Wilcox and Baillargeon, 1998). Although these investigators vary widely in the facets of physical knowledge that they study, all agree that the careful consideration of developmental findings – of the complex patterns of successes and failures that emerge across ages and across tasks – is one of the most powerful tools available to developmental scientists for shedding light on infants' approach to learning about the physical world.

Largely as a result of this new developmental focus, several accounts have been proposed in recent years that attempt to capture and explain important regularities in the development of infants' physical knowledge (e.g. Karmiloff-Smith, 1992; Spelke, 1994; Thelen and Smith, 1994; Baillargeon, 1995; Leslie, 1995; Mandler, 1997). Our own account (e.g. Baillargeon, 1994a; Baillargeon, 1995; Baillargeon, 1998, Baillargeon et al., 1995; Kotovsky and Baillargeon, 1998b; holds that infants are born with a specialized learning mechanism that facilitates their acquisition of physical knowledge. The mechanism is thought to be responsible for at least two, closely-intertwined, learning processes. One is the formation of object and event categories. Object categories refer to the distinct types of objects that exist in the world. We believe that infants' early object categories include: animate objects (objects such as people who possess certain facial features, can express emotions, are capable of a wide range of self-motions, and so on); inanimate self-moving objects (objects such as cars that lack many of the properties of animate objects but are capable of at least limited self-motion); and inanimate inert objects (objects such as cups that move only when acted upon). Event categories correspond to distinct ways in which objects behave or interact. We suspect that infants' early event categories include: collision events (events in which an object approaches and hits another object); arrested-motion events (events in which an object approaches and hits a broad surface such as a wall or floor); occlusion events (events in which an object becomes occluded by another, closer object or surface); and support events (events in which an object becomes supported by another object or surface).

The second process that is controlled by infants' learning mechanism is the identification, for each event category, of an initial concept and variables. We believe that, when learning about an event category, infants first form an initial concept centered on a simple, all-or-none distinction. With further experience, infants identify variables that elaborate and refine this initial concept, resulting in increasingly accurate predictions and interpretations over time. To illustrate this developmental pattern, consider the results of experiments on infants' knowledge about support events (e.g. Baillargeon et al., 1992; Needham and Baillargeon, 1993; R. Baillargeon, H. Raschke, and A. Needham, unpublished data; see Baillargeon, 1995and Baillargeon et al., 1995, for reviews). In these experiments, infants aged 3–6.5 months were presented with simple support problems involving a box and a platform; the box was released in one of several positions relative to the platform (e.g. off the platform, on top of the platform, against the side of the platform, and so on), and the infants judged whether the box should remain stable when released. The results indicated that, by 3 months of age, infants have formed an initial concept centered on a contact/no-contact distinction: they expect the box to fall if released off the platform and to remain stable otherwise. At this stage, any contact with the platform is deemed sufficient to ensure the box's stability. At least two variables are identified between 3 and 6.5 months of age. First, infants become aware that the type of contact between the box and the platform must be taken into account when judging the box's stability. Infants initially assume that the box will remain stable if released either on the top or against the side of the platform. However, by 4 to 5.5 months of age (females precede males by a few weeks in this development), infants distinguish between these two types of contact and recognize that only the first can lead to stability. Second, infants begin to appreciate that the amount of contact between the box and the platform affects the box's stability. Initially, infants believe that the box will be stable even if only a small portion (e.g. the left 15%) of its bottom surface rests on the platform. By 6.5 months of age, however, infants expect the box to fall unless a large portion of its bottom surface is supported.

What is the nature of the learning mechanism that directs infants' formation of event categories and identification of initial concepts and variables? One way to shed light on this question is to investigate distinct event categories and trace their respective developmental courses. We believe that examination of the sequence of variables that emerges for each event category – what variables are identified, when they are identified, and how they come to be identified – can yield important insights about the strengths and limitations of infants' learning mechanism.

In this context, we have begun a series of experiments on the development of infants' knowledge about collision events between inert objects (e.g. Kotovsky, 1992; Kotovsky and Baillargeon, 1994; Kotovsky and Baillargeon, 1998a; Kotovsky and Baillargeon, 1998b. The goal of this research program is (a) to confirm that collision events, like support events, lend themselves to a developmental description involving an initial concept and variables and (b) to specify the sequence of variables that infants identify in the course of learning about collision events. Is it the case that infants initially expect any collision between a moving and a stationary object to result in a displacement? Do infants then go on to identify variables that enable them to predict more and more accurately whether a stationary object is likely to be displaced when hit, and how far or how fast it is likely to be displaced? The present research was conducted as a part of this general enterprise and focused on 5.5- and 6.5-month-old infants' reasoning about collision events. Before describing these experiments, however, we first review prior findings from our laboratory and elsewhere.

At the time that we began our research, there were already several reports in the developmental literature of experiments in which infants were presented with collision events (e.g. Leslie, 1982; Leslie, 1984b; Baillargeon, 1986; Leslie and Keeble, 1987; Baillargeon and DeVos, 1991; Spelke et al., 1992; Cohen and Oakes, 1993; Oakes, 1994). However, these investigations generally focused on issues very different from those explored here, for two reasons. First, we were primarily interested in infants' reasoning about inert objects and most of the experiments involved self-moving objects: infants watched filmed or computer-generated events depicting the successive motions of two self-moving objects and judged whether the second object's motion was spontaneous or was caused by the first object's motion (e.g. Leslie, 1982; Leslie, 1984b; Leslie and Keeble, 1987; Cohen and Oakes, 1993; Oakes, 1994). In Section 7, we attempt to integrate the results of these experiments with those of the present research.

Second, the few collision experiments that made use of inert objects were not directly concerned with infants' expectations about collision events; rather, the experiments sought to determine whether infants realize that an object cannot move through the space occupied by another, occluded object (e.g. Baillargeon, 1986; Baillargeon and DeVos, 1991; Spelke et al., 1992). For example, in one series of experiments, 8-, 6.5-, and 4-month-old infants sat in front of a small screen; to the left of the screen was an inclined ramp (Baillargeon, 1986; Baillargeon and DeVos, 1991). The infants were habituated to the following event: first, the screen was raised (to reveal that there was no object behind it) and then lowered; next, a toy car rolled down the ramp, passed behind the screen, and finally exited the apparatus to the right. Following habituation, the infants saw two test events identical to the habituation event except that an object, such as a large toy mouse, now stood behind the screen; this mouse was revealed when the screen was raised. In one event (off-track event), the mouse was placed either in back or in front of the car's tracks; in the other event (on-track event), the mouse stood on top of the car's tracks, blocking its path. The 8- and 6.5-month-old infants, and the 4-month-old female infants (females succeeded at this task a few weeks before males did), looked reliably longer at the on-track than at the off-track event, suggesting that they were surprised1 to see the car roll past the screen when the mouse stood in its path. These and control results provided evidence that the infants realized that the car could not roll through the space occupied by the hidden mouse; what the results did not reveal, however, was what the infants believed should have been the outcome of the collision between the car and the mouse. Did the infants expect that (a) the car would simply stop against the mouse, (b) the car would stop and the mouse would move down the track, or (c) the car and the mouse would move together down the track? Such questions led us to undertake experiments focusing directly on infants' expectations about collision events.

Our first, preliminary experiment (Kotovsky, 1992cited in Kotovsky and Baillargeon, 1994) examined whether 5.5-month-old infants expect a stationary object to be displaced when hit by a moving object. The infants sat in front of a long horizontal track; to the left of the track was an inclined ramp. The infants were first habituated to a large cylinder that rolled down the ramp; two small stoppers prevented the cylinder from rolling past the ramp onto the track. Following habituation, a large wheeled toy bug was placed on the track, and the infants saw two test events. In both events, the cylinder rolled down the ramp as before. In one event (no-collision event), the bug was placed 10 cm from the ramp; therefore, no collision took place between the cylinder and bug, which remained stationary on the track. In the other event (collision event), the bug was positioned directly at the bottom of the ramp, between the two stoppers, so that a collision did take place between the cylinder and bug; nevertheless, the bug remained stationary, as in the no-collision event. The infants looked reliably longer at the collision than at the no-collision event. These and control results indicated that the infants expected the bug to be displaced when hit and were surprised in the collision event when this expectation was violated.

The results of this initial experiment indicated that, by 5.5 months of age, infants expect a stationary object to be displaced when hit by a moving object. Our next experiment (Kotovsky and Baillargeon, 1994) examined whether 11-month-old infants understand that how far a stationary object is displaced in a collision event depends on the moving object's size (we refer to the moving object's size rather than mass because our data are insufficient to judge which variable the infants used to form their predictions). The apparatus was identical to that in the preceding experiment. During the familiarization and test events, a cylinder rolled down the ramp and hit the bug, causing it to roll down the track (see Fig. 1). Three cylinders of identical material but different size and color were used in the events: there was a small orange cylinder, a medium blue cylinder, and a large yellow cylinder. The question of interest was whether the infants would expect that, all other things being equal, the larger the cylinder the farther the bug should travel down the track.

When we began designing this experiment, it immediately became clear that even adults were very poor at predicting how far the bug should roll when hit by any one cylinder: an informal survey with naive subjects yielded estimates ranging from the bug rolling a few centimeters to its crashing full speed through the far wall of the apparatus. At the same time, it was apparent that adults could form strong and consistent predictions about the length of the bug's displacement when given a calibration point for their predictions. Thus, after being shown that the bug rolled to the middle of the track when hit by the medium cylinder, adult subjects expected the bug to roll farther with the large but not the small cylinder. Similarly, after being shown that the bug rolled to the end of the track with the medium cylinder, adult subjects (a) expected the bug to do the same with the large cylinder and (b) found it acceptable that the bug should do the same with the small cylinder (they assumed that the track was too short to reveal effects of cylinder size). Adults thus readily used the information they were given about the medium cylinder to calibrate their predictions about the large and small cylinders.

Like our adult subjects, our 11-month-old subjects were given one of two calibration points. The infants in the midpoint condition watched a familiarization event in which the medium cylinder rolled down the ramp and hit the bug, propelling it to the middle of the track. The infants in the endpoint condition saw the same event except that the bug rolled to the end of the track. Next, the infants saw one of two test events: the medium cylinder was replaced by the large (large-cylinder event) or the small (small-cylinder event) cylinder, and both cylinders caused the bug to roll to the end of the track.

The results mirrored those obtained with the adult subjects. The infants in the midpoint condition looked reliably longer at the small- than at the large-cylinder event, whereas the infants in the endpoint condition tended to look equally at the two events. These results indicated that the infants (a) believed that the cylinder's size should affect the length of the bug's displacement and (b) used the familiarization event to calibrate their predictions about the test events. After watching the bug roll to the middle of the track when hit by the medium cylinder, the infants were surprised to see the bug roll to the end of the track with the small but not the large cylinder. In contrast, after watching the bug roll to the end of the track when hit by the medium cylinder, the infants were not surprised to see the bug do the same with either the small or the large cylinder.

These results suggested three conclusions. First, 11-month-old infants recognize that, in a collision between a moving and a stationary object, the larger the moving object, the greater the distance traveled by the stationary object. After watching the medium cylinder propel the bug to the middle of the track, the infants expected the bug to travel farther with the large but not the small cylinder. Second, by 11 months of age, infants can engage in calibration-based reasoning about the size/distance relation examined here. Depending on the specific calibration point they were given during the familiarization trials, the infants had different expectations as to how far the bug should travel when hit by the small cylinder. Thus, the infants were surprised to see the small cylinder propel the bug to the end of the track after seeing the medium cylinder propel the bug to the middle (midpoint condition) but not the end (endpoint condition) of the track. Finally, when engaging in calibration-based reasoning about the size/distance relation under examination, 11-month-old infants take into account factors such as whether an object slows to a gentle stop or whether it comes to an abrupt stop against an obstacle. The infants in the endpoint condition apparently had no expectation that the bug should travel less far with the small than with the medium cylinder, presumably because they (a) noticed during the familiarization trials that the bug stopped only when it hit the right wall of the apparatus and (b) realized, under these conditions, that the small cylinder could propel the bug the same distance as the medium cylinder.

The present research attempted to extend these findings to younger infants: 6.5-month-olds were tested in Experiment 1 and 5.5-month-olds in Experiment 2.

Section snippets

Experiment 1

Experiment 1 addressed two questions. First, did 6.5-month-old infants believe that, in a collision between a moving and a stationary object, the size of the moving object affects the length of the stationary object's displacement? Second, could infants this age engage in calibration-based reasoning about this size/distance relation?

The infants were tested with a procedure similar to that of our initial experiment (Kotovsky and Baillargeon, 1994), with a few exceptions. The 11-month-olds in

Subjects

The subjects were 27 healthy, full-term infants ranging in age from 4 months, 29 days to 6 months, 6 days (M=5 months, 17 days). There were 13 males (M=5 months, 18 days) and 14 females (M=5 months, 16 days). An additional 11 infants were tested but eliminated, six because of apparatus failure, three because they looked the maximum number of seconds allowed on five or more test trials, one because of fussiness, and one because of procedural problems.

Apparatus, events, and procedure

The apparatus, events, and procedure in

Experiment 2A

The most likely interpretation for the results obtained with the 5.5-month-old female infants in Experiment 2 was that these infants, like the 6.5-month-old male and female infants in Experiment 1, (a) believed that the size of the cylinder affected the length of the bug's trajectory and (b) were able to engage in calibration-based reasoning about this size/distance relation. However, another interpretation of the results of Experiment 2 was that the female infants found the small cylinder

Experiment 2B

The results obtained in the midpoint condition with the 6.5-month-old male and female infants in Experiment 1 and the 5.5-month-old female infants in Experiment 2 were remarkably consistent: the infants all looked reliably longer at the small- than at the large-cylinder test event. In marked contrast, the 5.5-month-old male infants in Experiment 2 tended to look equally at the two test events. How should this negative finding be interpreted? At least three explanations were possible. One was

Subjects

The subjects were 16 healthy, full-term male infants ranging in age from 4 months, 29 days to 5 months, 27 days (M=5 months, 14 days). An additional seven infants were tested but eliminated, two because of apparatus failure, two because of fussiness, two because of procedural error, and one because he looked the maximum number of seconds allowed on all test trials.

Apparatus, events and procedure

The apparatus, events, and procedure in Experiment 2C were identical to those of the midpoint condition in Experiments 1 and 2, with

Conclusion

As was discussed in Section 1, the general goal of our research program is to trace the development of infants' knowledge about collision events, and to specify the innate and experiential factors that contribute to this development. Our prior research (Kotovsky, 1992; Kotovsky and Baillargeon, 1994) indicated that (a) by 5.5 months of age, infants expect a stationary object to be displaced when hit by a moving object and (b) by 11 months of age, infants realize that how far the stationary

Acknowledgements

This research was supported by a grant from the National Institute of Child Health and Human Development (HD-21104) to the second author. We thank Jerry DeJong, Judy DeLoache, Cindy Fisher, Gavin Huntley-Fenner, Lisa Kaufman, and Greg Murphy for helpful comments; Alison Hauser and Anne Hillstrom for their help with the data analyses; and Andrea Aguiar, Laura Brueckner, Beth Cullum, Myra Gillespie, Lisa Kaufman, Valerie Kolstad, Amy Needham, Helen Raschke, Teresa Wilcox, and the undergraduate

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