Significance We provide an experimental demonstration that young infants possess abstract biological expectations about animals. Our findings represent a major breakthrough in the study of the foundations of human knowledge. In four experiments, 8-mo-old infants expected novel objects they categorized as animals to have filled insides. Thus, infants detected a violation when objects that were self-propelled and agentive were revealed to be hollow, or when an object that was self-propelled and furry rattled when shaken, as though mostly hollow. We describe possible characterizations of infants’ expectations about animals’ insides, including a characterization that emphasizes human predator–prey adaptations. We also discuss how infants’ expectation that animals have insides lays a foundation for the development of more advanced biological knowledge.

IntroductionConsider the following scene: A man wearing a backpack is pacing leisurely back and forth in a large airport room. As he strolls, he occasionally crosses his arms, twirls the dangling straps of his backpack, or stuffs his hands in his pant pockets. At one point, he sits down, takes off his backpack, and removes from it a bag filled with assorted gummy bears; as he peers intently inside the bag, he selects and eats, one at a time, five red gummy bears.As adults, we would have no difficulty interpreting the man's actions. We might view his initial actions-pacing, crossing his arms, and so on-as intentional (as opposed to accidental) though not as directed toward any particular goal. In contrast, we might perceive his subsequent actions-removing the bag of gummy bears from his backpack and eating five red ones in succession-as both intentional and goal-directed. In analyzing these actions, we might build an explanation that attributes to the man a causally coherent set of motivational and epistemic mental states: He wants to eat gummy bears, he is particularly fond of red ones, and so when he spies one in the bag, he removes it and eats it.Next, consider a new scene: As the man is happily chewing on red gummy bears, he notices a second man approaching who is carrying two heavy suitcases. At this point, our scene might unfold according to different scenarios. In one, the first man greets the second man, offers to carry one of the suitcases, and holds out his bag of gummy bears. In another scenario, the first man continues to watch the second man but makes no move to approach him. In yet another scenario, the first man sticks out a leg to trip the second man, causing him to fall heavily.As adults, we would interpret and evaluate the first man's actions in the three scenarios very differently. In the first scenario, we might infer that the two men have a social group in common: They might be friends, coworkers, or relatives, for example. In the second scenario, we 3 would conclude that the two men are strangers. In both the first and second scenarios, we would view the first man's behavior as acceptable: Offering help and sharing food are expected prosocial behaviors in interactions with ingroup, but not outgroup, individuals. In contrast, the first man's behavior in the third scenario would seem to us beyond the pale: Unprovoked harmful actions, even against outgroup individuals, are generally viewed as unacceptable. We would categorize the first man as an antisocial lout, and we might file away distinctive characteristics as possible markers of a social group to be avoided in the future.Our discussion of the two scenes above illustrates the rich analyses that adults spontaneously engage in when watching others act. What are the developmental origins of these interpretations? Over the past 25 years, there has been a great deal of research on social cognition in infancy. This research can be roughly organized into two sets of questions that map neatly onto the two scenes above. First, when watching an agen...

The psychological capacity to recognize that others may hold and act on false beliefs has been proposed to reflect an evolved, species-typical adaptation for social reasoning in humans; however, controversy surrounds the developmental timing and universality of this trait. Cross-cultural studies using elicited-response tasks indicate that the age at which children begin to understand false beliefs ranges from 4 to 7 years across societies, whereas studies using spontaneous-response tasks with Western children indicate that false-belief understanding emerges much earlier, consistent with the hypothesis that false-belief understanding is a psychological adaptation that is universally present in early childhood. To evaluate this hypothesis, we used three spontaneous-response tasks that have revealed early falsebelief understanding in the West to test young children in three traditional, non-Western societies: Salar (China), Shuar/Colono (Ecuador) and Yasawan (Fiji). Results were comparable with those from the West, supporting the hypothesis that false-belief understanding reflects an adaptation that is universally present early in development.

Much of the research on object individuation in infancy has used a task in which two different objects emerge in alternation from behind a large screen, which is then removed to reveal either one or two objects. In their seminal work, Xu and Carey (1996) found that it is typically not until the end of the first year that infants detect a violation when a single object is revealed. Since then, a large number of investigations have modified the standard task in various ways and found that young infants succeed with some but not with other modifications, yielding a complex and unwieldy picture. In this article, we argue that this confusing picture can be better understood by bringing to bear insights from a related subfield of infancy research, physical reasoning. By considering how infants reason about object information within and across physical events, we can make sense of apparently inconsistent findings from different object-individuation tasks. In turn, object-individuation findings deepen our understanding of how physical reasoning develops in infancy. Integrating the insights from physical-reasoning and object-individuation investigations thus enriches both subfields and brings about a clearer account of how infants represent objects and events.

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Comparison of infant findings from the physical-reasoning and object-individuation literatures reveals a contradictory picture. On the one hand, physical-reasoning results indicate that young infants can use featural information to guide their actions on objects and to detect interaction violations (when objects interact in ways that are not physically possible) as well as change violations (when objects spontaneously undergo featural changes that are not physically possible). On the other hand, object-individuation results indicate that young infants typically cannot use featural information to detect individuation violations (when the number of objects revealed at the end of an event is less than the number of objects introduced during the event). In this article, we attempt to reconcile these two bodies of research. In a new model of early individuation, we propose that two systems help infants individuate objects in physical events: the object-file and physical-reasoning systems. Under certain conditions, disagreements between the systems result in catastrophic individuation failures, leading infants to hold no expectation at all about how many objects are present. We report experiments with 9- to 11-month-old infants (N = 216) that tested predictions from the model. After two objects emerged in alternation from behind a screen, infants detected no violation when the screen was lowered to reveal no object. Similarly, after two objects emerged in alternation from inside a box, which was then shaken, infants detected no violation when the box remained silent, as though empty. We end with new directions, suggested by our model, for research on early object representations.

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