Elsevier

Cognitive Psychology

Volume 58, Issue 4, June 2009, Pages 441-486
Cognitive Psychology

Young infants’ reasoning about physical events involving inert and self-propelled objects

https://doi.org/10.1016/j.cogpsych.2008.11.001Get rights and content

Abstract

The present research examined whether 5- to 6.5-month-old infants would hold different expectations about various physical events involving a box after receiving evidence that it was either inert or self-propelled. Infants were surprised if the inert but not the self-propelled box: reversed direction spontaneously (Experiment 1); remained stationary when hit or pulled (Experiments 3 and 3A); remained stable when released in midair or with inadequate support from a platform (Experiment 4); or disappeared when briefly hidden by one of two adjacent screens (the second screen provided the self-propelled box with an alternative hiding place; Experiment 5). On the other hand, infants were surprised if the inert or the self-propelled box appeared to pass through an obstacle (Experiment 2) or disappeared when briefly hidden by a single screen (Experiment 5). The present results indicate that infants as young as 5 months of age distinguish between inert and self-propelled objects and hold different expectations for physical events involving these objects, even when incidental differences between the objects are controlled. These findings are consistent with the proposal by Gelman, R. (1990). First principles organize attention to and learning about relevant data: Number and the animate–inanimate distinction as examples. Cognitive Science, 14, 79–106, Leslie, A. M. (1994). ToMM, ToBY, and Agency: Core architecture and domain specificity. In L. A. Hirschfeld & S. A. Gelman (Eds.), Mapping the mind: Domain specificity in cognition and culture (pp. 119–148). New York: Cambridge University Press, and others that infants endow self-propelled objects with an internal source of energy. Possible links between infants’ concepts of self-propelled object, agent, and animal are also discussed.

Introduction

Investigations of early physical reasoning over the past 20 years have revealed that, by 6 months of age, infants already possess rich expectations about physical events (e.g., Aguiar and Baillargeon, 2002, Baillargeon and DeVos, 1991, Baillargeon et al., 1985, Durand and Lécuyer, 2002, Goubet and Clifton, 1998, Hespos and Baillargeon, 2001b, Hespos and Baillargeon, 2008, Hofstadter and Reznick, 1996, Hood and Willatts, 1986, Kochukhova and Gredebäck, 2007, Kotovsky and Baillargeon, 1998, Lécuyer and Durand, 1998, Leslie, 1984a, Leslie and Keeble, 1987, Luo and Baillargeon, 2005b, Luo et al., 2003, Needham and Baillargeon, 1993, Ruffman et al., 2005, Sitskoorn and Smitsman, 1995, Spelke et al., 1992, Spelke et al., 1995, von Hofsten et al., 2007, Wang et al., 2004, Wang et al., 2005, Wilcox, 1999, Wilcox et al., 1996). Some of these experiments used inert objects (e.g., inert balls, boxes, cylinders, toy bugs, toy cars or toy lions), whereas others used self-propelled objects (e.g., self-propelled balls, boxes, cylinders, toy bears, toy carrots or toy mice). The choice of inert or self-propelled objects was typically made for reasons of methodological convenience and had little effect on the results. For example, investigations focusing on occlusion events found that infants aged 3.5 months and older expect an object, whether inert or self-propelled, (1) to continue to exist when behind an occluder; (2) to follow a continuous, unobstructed path when behind an occluder; and (3) to remain partly visible when taller or wider than the occluder (e.g., Baillargeon, 1986, Baillargeon, 1987, Baillargeon and DeVos, 1991, Baillargeon and Graber, 1987, Hespos and Baillargeon, 2001a, Hespos and Baillargeon, 2006, Hofstadter and Reznick, 1996, Kochukhova and Gredebäck, 2007, Luo and Baillargeon, 2005b, Luo et al., 2003, Ruffman et al., 2005, Spelke et al., 1992, von Hofsten et al., 2007, Wang et al., 2004, Wilcox, 1999, Wilcox et al., 1996; for a possible exception we return to later on, see Kuhlmeier, Bloom, & Wynn, 2004).

On the basis of such results, we might be tempted to conclude that young infants’ expectations about physical events are framed in terms of a single category of objects, namely, physical objects. Such a conclusion would be premature, however, for the following reasons. First, many investigations of infants’ physical reasoning to date have focused on physical events where even adults would hold similar expectations for inert and self-propelled objects. Second, when experiments have involved physical events where different expectations could conceivably have arisen for inert and self-propelled objects, no such comparison was performed (or indeed required), because it fell outside of the investigators’ expressed research agenda.

An important exception to this last generalization comes from work by Kosugi, Saxe, Woodward, and their colleagues (e.g., Kosugi and Fujita, 2002, Kosugi et al., 2003, Saxe et al., 2005, Saxe et al., 2007, Spelke et al., 1995, Woodward et al., 1993). This research examined whether infants aged 7 months and older recognize that (1) a self-propelled object can initiate its own motion, whereas an inert object cannot, and (2) an inert object can be set into motion only through contact with (and the application of force by) another object. In one experiment, for example, 7-month-old infants were assigned to an inert or a self-propelled condition (Woodward et al., 1993; see also Spelke, Phillips, et al., 1995). The infants in the inert condition were habituated to a videotaped event involving two large (human-sized) wheeled blocks that differed in height, width, shape, pattern, and color. To start, one block moved into view on the left side of the television monitor and disappeared behind the left edge of a large occluder at the center of the monitor; the second block was partly visible at the right edge of the occluder. After an appropriate interval, the second block moved to the right and disappeared on the right side of the monitor. The entire event sequence was then repeated in reverse. Following habituation, the occluder was removed, and the infants saw two test events in which the blocks moved as before; the only difference between the events had to do with what happened during the previously occluded portion of the blocks’ trajectories. In one event (contact event), the moving block collided with the stationary block and set it into motion; in the other event (no-contact event), the moving block stopped short of the stationary block, which then set off on its own. The infants in the self-propelled condition saw identical events except that the two blocks were replaced with a man and a woman who walked along the same path as the blocks.

The infants in the inert condition looked reliably longer at the no-contact than at the contact event, whereas those in the self-propelled condition looked about equally at the two events. These and control results suggested three conclusions. First, because there was no clear indication that the blocks were self-propelled during the habituation trials (it was unclear what caused them to roll into view on either side of the television monitor), the infants categorized them as inert; infants thus appear to hold the default assumption that a novel object is inert unless given unambiguous evidence that it is self-propelled (see also Luo & Baillargeon, 2005a). Second, the infants understood that inert objects can be set into motion only through contact with (and the application of force by) other objects, and thus they inferred that one block must be colliding with the other block behind the occluder. Third, the infants realized that humans are self-propelled objects, which can move at will.1

The preceding results suggest that by 7 months of age infants hold different expectations for at least some physical events involving inert and self-propelled objects. Where might these different expectations come from? One hypothesis, put forth by Gelman, 1990, Gelman and Spelke, 1981, Gelman et al., 1995, Subrahmanyam et al., 2002 and Leslie, 1984a, Leslie, 1994, Leslie, 1995, Leslie and Keeble, 1987, is that part of the skeletal causal framework infants bring to bear when interpreting physical events is a fundamental distinction between inert and self-propelled objects. When infants watch a novel object begin to move or change direction, their physical-reasoning system attempts to determine whether the change in the object’s motion state is caused by forces internal or external to the object. According to Leslie (1994), “the more an object changes motion state by itself and not as a result of external impact, the more evidence it provides, the more likely it is, that it is [self-propelled]” (p. 133). An object that is judged to be self-propelled is endowed with an internal source of energy. A self-propelled object can use its internal energy directly to control its own motion and indirectly (through the application of force) to control the motion of other objects. We refer to this hypothesis as the internal-energy hypothesis, for ease of communication.

If the internal-energy hypothesis is correct, then young infants may hold different expectations for physical events involving inert and self-propelled objects whenever they believe that an application of internal energy can bring about a different outcome. Thus, infants may be surprised to see an inert but not a self-propelled object remain stationary when hit, if they reason that the self-propelled object can use its internal energy to resist efforts to move it. In contrast, infants may be surprised to see an inert or a self-propelled object disappear into thin air, if they realize that no application of internal energy could result in the disappearance of the self-propelled object.2

The preceding reasoning led us to undertake an extensive series of experiments to systematically compare 5- to 6.5-month-old infants’ responses to various physical events involving an inert or a self-propelled object. To control for extraneous factors, the inert and the self-propelled object used in the experiments was the same small box. During familiarization, half the infants were given evidence that the box was self-propelled (e.g., the box initiated its own motion in plain view); the other infants were given no such evidence and so presumably categorized the box as inert. Whether self-propelled or not, the box always moved in exactly the same manner: its motion was actually controlled by a mechanical device, to ensure uniformity across trials and conditions. During test, the infants saw various physical events involving the box. The experiments tested whether infants (1) would view the outcomes of some events as surprising when they categorized the box as inert but not as self-propelled, because in the latter case they could infer that the box had used its internal energy to bring about the observed outcomes; and (2) would view the outcomes of other events as surprising whether they categorized the box as inert or as self-propelled, because they realized that no application of internal energy could explain the observed outcomes.

We speculated that evidence that infants hold different expectations for some but not other physical events involving inert and self-propelled objects, under these controlled conditions, would be important for three reasons. First, it would provide strong evidence that infants’ expectations about physical events are framed in terms of not one but two categories of objects: inert and self-propelled objects. Second, it would support the internal-energy hypothesis proposed by Gelman and Leslie (e.g., Gelman, 1990, Gelman et al., 1995, Leslie, 1994, Leslie, 1995). Finally, it would lead to interesting questions about possible links between the concept of self-propelled object in the present research (see also Baillargeon, Wu, Yuan, Li, & Luo, in press), the concept of agent in the psychological-reasoning literature (e.g., Csibra, 2008, Johnson et al., 2007, Johnson et al., 2008, Luo and Baillargeon, 2005a), and the concept of animal in the conceptual-development literature (e.g., Carey, 1985; Gottfried & Gelman, 2005; Mandler, in press, Subrahmanyam et al., 2002). We return to these issues in the Section 7.

Section snippets

Experiment 1: can an inert or a self-propelled object spontaneously reverse direction?

We have seen that infants hold different expectations for the onset of inert and self-propelled objects’ (horizontal) displacements: they realize that a self-propelled object can initiate its own motion, whereas an inert object cannot (e.g., Kosugi and Fujita, 2002, Kosugi et al., 2003, Saxe et al., 2005, Saxe et al., 2007, Spelke et al., 1995, Woodward et al., 1993). Do infants also hold different expectations for the path inert and self-propelled objects are likely to follow once in motion?

Experiment 2: can an inert or a self-propelled object pass through an obstacle?

The results of Experiment 1 suggested that 5-month-old infants are surprised when an inert but not a self-propelled object spontaneously reverses direction. However, an alternative interpretation of the results was that the infants were confused by the self-propelled box and hence held no specific expectation about its behavior, resulting in equal looking times at the near- and far-wall test events. This interpretation was unlikely: as was mentioned in the last section, numerous experiments

Experiment 3: can an inert or a self-propelled object remain stationary when hit or pulled?

We have seen that young infants appreciate that a self-propelled object can use its internal energy to reverse its motion. In Experiment 3, we asked whether young infants also believe that a self-propelled object can use its internal energy to resist efforts to move it and hence to remain stationary when hit or pulled.

The point of departure for this experiment came from investigations of infants’ responses to collision events. Prior research with inert objects (e.g., Baillargeon, 1995, Kotovsky

Experiment 4: can an inert or a self-propelled object remain stable in midair or with inadequate external support?

The preceding experiments suggest that, by 5–6 months of age, infants assume that a self-propelled object can use its internal energy to control its motion in several different ways: it can initiate its motion, it can alter the direction of its motion, and it can resist efforts to set it into motion. Experiment 4 asked whether 6.5-month-old infants also believe that a self-propelled object can use its internal energy to remain stable when released without adequate external support.

The point of

Experiment 5: can an inert or a self-propelled object disappear when behind an occluder?

The results of Experiments 1 through 4 indicate that 5- to 6.5-month-old infants hold different expectations for some but not other physical events involving inert and self-propelled objects. If infants categorize a novel object as inert, they are surprised if it spontaneously reverses its motion, remains stationary when hit or pulled, or remains stable when released in midair; if infants categorize the object as self-propelled, they find none of these outcomes surprising. On the other hand,

General discussion

The present research built on previous findings that infants distinguish between inert and self-propelled objects and recognize that self-propelled but not inert objects can initiate their own motion (e.g., Kosugi and Fujita, 2002, Kosugi et al., 2003, Kotovsky and Baillargeon, 2000, Saxe et al., 2005, Saxe et al., 2007, Spelke et al., 1995, Woodward et al., 1993). Our research extended these findings in two ways: it examined infants’ responses to several different physical events involving an

Acknowledgments

This research was supported by research funds from the University of Missouri to the first author and by a grant from the National Institute of Child Health and Human Development (HD-21104) to the third author. We thank Jerry DeJong, Cindy Fisher, and Kristy vanMarle for helpful discussions Susan Carey, Rochel Gelman, and Alan Leslie for helpful suggestions; the staff of the University of Illinois Infant Cognition Laboratory for their help with the data collection; and the parents and infants

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