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Copyright 2001 by the American Psychological Association, Inc. 0012-1649/01/$5.00 DOI: 10.1037//O012-16220.127.116.119
Is Early Differentiation of Human Behavior a Precursor to the 1-Year-Old's Understanding of Intentional Action? Comment on Legerstee, Barna, and DiAdamo (2000) Gyorgy Gergely Hungarian Academy of Sciences In a recent issue of Developmental Psychology, M. Legerstee, J. Barna, and C. DiAdamo (2000) reported a study showing that 6-month-olds expect people to talk to persons rather than to inanimate objects and to manipulate inanimates rather than persons. They interpreted this ability as a "precursor" to later understanding of intentionality. The present article takes issue with the authors' 2 different levels of interpretation that contradict each other and raise problems in their own right. It is suggested that M. Legerstee et al.'s finding is most parsimoniously explained by associative learning and may not constitute a precursor to later understanding of intentionality in any well-defined sense of the term. The present article argues for the importance of differentiating between associative and inferential processes and reviews evidence that the understanding of goal-directed action around 9 months of age involves principle-based inferences.
The Stronger Interpretation: The Mentalistic View
Legerstee, Barna, and DiAdamo (2000) reported a welldesigned habituation study with proper controls and clear-cut results indicating that 6-month-old infants expect people to talk to persons rather than to inanimate objects and to physically manipulate inanimate objects rather than persons. In the habituation phase, infants observed a human model either talk to or manipulate something behind an occluder. In the test phase, the occluder was removed and the infants were presented with either a human person or an inanimate object behind the occluder. In the talking condition, infants dishabituated more when they saw the inanimate object, whereas in the manipulation condition, they looked longer when the revealed object was a person. I find the results of the Legerstee et al.'s study convincing and uncontroversial and believe that it certainly contributes to our understanding of how early in life infants develop differential expectations about the stimulus conditions in which different types of human actions typically take place. The different levels of interpretation, however, that the authors proposed for their result raise a number of problematic issues that I discuss in this article.
At certain points in their article, Legerstee et al. (2000) clearly seemed to claim that their 6-month-olds' performance implies an already mentalistic understanding of intentional actions of others: We argue that the infant's differential responsiveness to people and objects during the test events .. . does suggest that they construe intentional actions such as talking and reaching in terms of desires. . . . These behaviors,. . . like thoughts and beliefs,... are mental or internal experiences about something, (p. 632)
Clearly, however, there is nothing in the data that would warrant such a "rich" interpretation, a point that seems to be conceded in other places in their article by the authors themselves (see The Weaker Interpretation section below). First, it seems clear that there is a much more straightforward and parsimonious explanation for the findings in terms of simple associative learning that, to me at least, certainly seems the most plausible interpretation of their result. By 6 months of age infants have had ample experience with the differential stimulus circumstances in which human actions such as talking and physical manipulation typically occur; they are able to form statistical associations that underlie their demonstrated expectation that talking is likely to take place in the distal presence of a person rather than with an inanimate object and that manipulating is more likely to involve inanimate objects than humans. Second, most researchers in the field have been cautious to avoid interpreting evidence for early differential responsiveness to humans versus inanimate objects during the first 6 - 8 months of life as involving understanding intentionality or mental states in others. I use as an influential example Tomasello's (1999) theory of the emergence of the intentional stance at 9 months; Legerstee et al. seem to endorse and build heavily on Tomasello's approach. Tomasello (1999, p. 74) drew a critical distinction between (a) the understanding demonstrated by 5-6-month-olds of animate beings as capable of self-propulsion and of behaving in certain ways rather than in others to make things happen in the world and
The general claim that Legerstee et al. (2000) made is that their study contributes significantly to our understanding of how infants come to comprehend intentional actions of others at the end of the first year of life (e.g., Tomasello, 1999). In arguing for this claim, they developed two rather different interpretations for their results that (a) contradict each other and (b) raise a number of problematic questions in their own right. I refer to the two accounts as the "stronger" and the "weaker" interpretations and discuss each in turn.
Correspondence concerning this article should be addressed to Gyorgy Gergely, Department of Developmental Psychology, Institute for Psychology, Hungarian Academy of Sciences, 1394 Budapest, POB 398, Hungary. Electronic mail may be sent to [email protected] 579
(b) the 9-month-old's understanding of others as intentional agents capable of making behavioral and perceptual choices that are rational in pursuing their goals. Discussing demonstrations such as those by Leslie (1984) and Woodward (1998) indicating that 5-6-month-olds show surprise when they observe people's hands doing things that they normally do not do, Tomasello (1999) said that understanding others as animate beings—that is, as beings that make things happen—is not the same thing as understanding others as intentional agents with an interrelated functioning of goal, attention, and behavioral strategy . . . . [The latter] awaits developments in which the infant differentiates goals from behavioral means . . . [which] .. . provides for something of the directedness, or even "aboutness," dimension of intentionality that is missing when infants only understand that others have the power to make things happen in some global way. (p. 74) I would add that one of the most crucial differences between the 9-month-old's understanding of intentional action and the 5-6month-old's understanding of animacy or personhood is the fact that whereas the latter can be explained in terms of an increase in associative knowledge based on movement cues (such as selfpropulsion or biomechanical movement), featural cues (facial features), and types of contingencies that characterize animates and/or humans in contrast to inanimate objects, the newly emerging competences of the 9-month-old go beyond purely associative mechanisms and involve inferences and reasoning that are theoryor principle-based. For example, my colleagues and I have demonstrated in a series of habituation studies (Csibra, Gergely, Bir6, Koos, & Brockbank, 1999; Gergely, Nadasdy, Csibra, & Bir6, 1995) that 9- and 12-month-olds (but not 6-month-olds) can infer a novel but rational goal approach that they expect an agent to follow when a previously present obstacle separating the agent from the goal is removed. As our controls make it clear, this inference to a novel action is not based on previous associations but stems from the infant's emerging naive theory of rational action, which assumes that agents pursue their goals in the most efficient manner available to them given the constraints of reality (Csibra & Gergely, 1998). Note also that the presence of such a principle-based inferential understanding still does not necessarily imply a mentalistic construal of intentional action in 9-month-olds. As I have argued elsewhere (Csibra & Gergely, 1998; Gergely & Csibra, 1997), the 9-month-old's naive theory of rational action can be conceived of as a nonmentalistic, purely teleological interpretive system that makes reference to representations of the relevant states of current reality (such as the presence of the obstacle) and future reality (the goal state) without attributing such representations to the actor's mind as causal intentional mental states such as beliefs and desires (cf. Gergely & Csibra, 2000). In general, there is converging evidence (see Keil, Smith, Simons, & Levin, 1998; Sloman, 1996) suggesting that human infants and children develop their representations of the physical and the social world by two qualitatively different types of processes—namely, associative and theory-driven inferential processes—whereas nonhuman organisms, including primates, may be restricted to associative mechanisms only (see Povinelli & Eddy, 1996; Tomasello, 1999). A certain degree of differentiated knowledge about aspects of human action may be acquired very
early by infants through accumulation of statistically based associations of observable contingencies. However, given the relevant experience, such knowledge may be similarly acquired by nonhuman primates as well who, nevertheless, unlike humans, seem unable to achieve a mentalistic understanding of intentionality (Tomasello, 1999). Therefore, one should be extremely cautious about the danger of overinterpreting evidence of early differential sensitivity to persons versus inanimates in terms of theory of mind, as such evidence may reveal no more than association-based knowledge that does not involve understanding abstract mental states. In fact, in my view, when one aims to investigate the origins of understanding intentional action in humans, one should purposefully create experimental demonstrations that necessitate the postulation of inferential capacities that go beyond the power of a purely associationist account. The Weaker Interpretation: The Precursor View Elsewhere in their article, Legerstee et al. (2000) proposed a much "leaner," nonmentalistic interpretation for their result that, at least at first sight, seems amenable to the simpler associationist account that I was advocating previously. For example, they wrote that "these findings suggest that infants expect the actions of people to be functionally related to objects in ways that are continuous with more mature understandings that involve intentions" (Legerstee et al., 2000, p. 633). Although what is actually meant by actions of people being "functionally related" to objects remains undefined, from the above formulation it seems clear that it does not yet involve the "more mature understandings . . . [of] . . . intentions" (p. 633) characteristic of older infants. This is reinforced by their further statement that "this capacity does not indicate that infants understand the mental aspects of people's focus of attention" (p. 633). The first problem here, of course, is that one cannot have it both ways: This weaker interpretation is clearly incompatible with Legerstee et al.'s stronger mentalistic interpretation described just a page earlier in their article (see above). The second problem is that if this weaker account is interpreted as referring to no more than infants' associating of certain types of human actions (say, talking) with certain types of objects (persons) rather than with others (inanimates), then the answer to their question—"What do these results reveal about the infants' developing understanding of intentional behavior?" (Legerstee et al., 2000, p. 632)—should be a modest "Well, not that much." Although I would certainly agree with this conclusion, they clearly do not, as their reading of their weaker interpretation turns out to be richer than the associationist account that I suggested. This becomes apparent when they consider and then explicitly reject such an associationist interpretation of their result: "[T]his [associationist learning] account... is not sufficient, because it is not a precursor to infants' understanding of others as intentional beings" (p. 632). Note, however, that the associationist interpretation is proposed here to explain the results of their study and not the development of understanding intentionality or its precursors for that matter. It is not clear what they mean by the term precursor here. In fact, insofar as the associationist interpretation can account for Legerstee et al.'s finding in a parsimonious and plausible manner (which I maintain it does), their result may not represent a demonstration of an important precursor competence to theory of mind development at all.
PRECURSORS TO INTENTIONALITY: COMMENT ON LEGERSTEE ET AL. However, Legerstee et al. (2000) maintained that "the development of expectations about the effective behaviors of people is important because they are precursors to an understanding of intentionality" (p. 631). The obvious question, then, is in what sense do Legerstee et al. consider the 6-month-old's ability to differentially respond to people versus objects to be a precursor to the 1-year-old's understanding of the intentional actions of others? Legerstee et al. (2000) argued that "the development of a theory of mind . . . is a function of infants' prior abilities to differentiate between people and objects" (p. 633). They claimed that "during the first 5 months of life, infants understand others as well as themselves to be agents that make things happen, and they associate goal-directed behavior with them [italics added] and not with inanimate objects" (p. 632). It is unclear to me on what supporting evidence (I know of none) they base this rather striking claim that during the first 5 months of life, infants already understand the behavior of human agents (but not of inanimate objects) as goaldirected actions. (In their article, Legerstee et al. certainly do not provide any empirical basis for this claim.) But irrespective of the empirical origins of this idea (if any), at least it makes it clearer what they may have in mind when they consider early differentiation of human action to be a precursor to theory of mind development: They believe that early goal attribution is intimately linked to the ability to categorize objects as persons. In this regard, Legerstee et al. (2000) referred to Meltzoff s (1995) seminal study in which 18-month-olds (but certainly not young infants under 5 months of age!), when presented with failed attempts to achieve a goal performed by a human model, reenacted (and therefore inferred and attributed to an incomplete action) the intended goal they had never actually seen realized. As Legerstee et al. emphasized, the infants engage in such imitation only when the model is human: They do not imitate the corresponding behavior of an inanimate robot. Therefore, if my reading of Legerstee et al.'s view is correct, they consider the distinction between persons and inanimate objects to be a crucial precursor of later theory of mind development insofar as they believe that goals are attributed to human behavior only, from as early on as infants start to attribute goals to actions. In contrast to this view, my colleagues and I have demonstrated in several habituation studies (Csibra, Gergely, Brockbank, Biro, & Koos, 1998; Gergely & Csibra, 1998) that infants as young as 12 months of age can infer a goal for an incomplete action even when the actor is not human but rather a computer-animated, twodimensional disc with no facial features or biomechanical movement characteristics. This finding suggests, therefore, that early goal attribution is not restricted to human persons. Why is it then that the 18-month-old participants in Meltzoff s (1995) study did not imitate the behavior of the inanimate robot that could have suggested goal-directedness? Elsewhere (e.g., Csibra et al., 1999) we have argued that one reason for this may have to do with the fact that imitation (rather than goal attribution) is restricted to human action only. In fact, Legerstee (1991) herself provided evidence suggesting that when presented with analogous actions performed by humans rather than inanimate objects, infants imitate only the human model. Another possibility is that whereas at 12 months of age, the infant's goal attribution is not restricted yet to humans, by the time the infant is 18 months of age, she or he may have formed a statistical association based on experience that links humans and animate objects more strongly than inanimate objects
to an expectation of goal-directed rational behavior. At any rate, our habituation studies clearly raise the possibility that goal attribution may not initially depend on categorizing the actor as a human person, as suggested by Legerstee et al. (2000). In fact, several theorists of modularist persuasion (BaronCohen, 1994; Leslie, 1994; Premack, 1990) have argued that the attribution of intentionality is triggered by more abstract movement cues such as self-propulsion, which identifies a larger class of objects than human persons as intentional. However, Legerstee et al. (2000) rejected "the idea that intentionality is activated by certain patterns of motion, such as self-propelled motion," claiming that "animates cannot be distinguished from inanimates simply on the basis of motion because the cause of motion . . . is ambiguous" (p. 632). To illustrate what they may mean by this, Legerstee et al. quoted an example from Gelman, Durgin, and Kaufman (1995) about a round object in the desert with needles that one may at first think is a cactus until it begins to move, at which point one realizes that it is an echidna. The use of this example is rather puzzling, however. If anything, it clearly illustrates the disambiguating power of self-propulsion as a cue indicating animacy. So Legerstee et al.'s (2000) conclusion that "hence, there are attributes other than movements that are important in identifying animates" (p. 632), while perhaps true, certainly does not seem to follow from this example. The more substantial point that I would like to make, however, is that irrespective of whether self-propulsion is a necessary cue for animacy judgments in infancy, there is evidence that it is neither sufficient nor necessary for goal attribution. Thus, modifying the habituation displays used in an earlier study (Gergely et al., 1995), my colleagues and I have demonstrated (Csibra et al., 1999) that 9- and 12-month-olds can attribute a goal to the behavior of an abstract computer-animated figure and can infer its novel action in a changed situation even if the behavior of the figure does not exhibit any sort of movement cue, such as self-propulsion or irregular movement, that may indicate animacy or agency. (The circle "flew in" from the side of the screen and, following an inert parabolic trajectory, passed over a barrier to contact and stop next to a target circle at the other side of the screen.) More recently, we (Csibra & Gergely, 2000) replicated our earlier result (Gergely et al., 1995) in a modified situation in which the circle "flying over" the obstacle to the goal object was visibly directly launched by another circle running into it. Together, these studies suggest that what drives goal attribution is the perception of "justifiable adjustment of movement" in relation to situational constraints on goal approach (such as the presence of an obstacle) rather than the presence of movement or featural cues indicating personhood or animacy (see Csibra et al., 1999). Conclusion Although Legerstee et al. (2000) are probably right in claiming that some level of differentiation between people and objects is present in infants by 6 months of age, such knowledge may be based on purely associative learning processes and may not be directly related as a significant precursor to the 9-month-old's emerging understanding of intentional action. In fact, in spite of the compelling strength of one's intuitions suggesting an inherent link between personhood (or animacy) and rational goal-directed action, the currently available evidence indicates that in its initial
stage, teleological understanding of action is not yet linked to objects categorized as human (or animate; see Csibra & Gergely, 1998; Gergely & Csibra, 1997). Thus, adult intuitions about an inherent relation between agency and intentionality may not reflect a "prewired" structural connection but possibly an emergent association based on learning about the robust statistical relationship that exists between physical agency and rational goal-directed behavior in the world.
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Received August 24, 2000 Revision received December II, 2000 Accepted December 29, 2000
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