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.

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.

Two experiments examined whether 4-month-olds (n = 120) who were induced to assign two objects to different categories would then be able to take advantage of these contrastive categorical encodings to individuate and track the objects. In each experiment, infants first watched functional demonstrations of two tools, a masher and tongs (Experiment 1) or a marker and a knife (Experiment 2). Next, half the infants saw the two tools brought out alternately from behind a screen, which was then lowered to reveal only one of the tools (different-objects condition); the other infants saw similar events except that the same tool was shown on either side of the screen (same-object condition). In both experiments, infants in the different-objects condition looked reliably longer than those in the same-object condition, and this effect was eliminated if the demonstrations involved similar but non-functional actions. Together, these results indicate that infants (a) were led by the functional demonstrations they observed to assign the two tools to distinct categories, (b) recruited these categorical encodings to individuate and track the tools, and hence (c) detected a violation in the different-objects condition when the screen was lowered to reveal only one tool. Categorical information thus plays a privileged role in individuation and identity tracking from a very young age.

Anthropological and psychological research on direct third-party punishment suggests that adults expect the leaders of social groups to intervene in within-group transgressions. Here, we explored the developmental roots of this expectation. In violation-of-expectation experiments, we asked whether 17-mo-old infants (n = 120) would expect a leader to intervene when observing a within-group fairness transgression but would hold no particular expectation for intervention when a nonleader observed the same transgression. Infants watched a group of 3 bear puppets who served as the protagonist, wrongdoer, and victim. The protagonist brought in 2 toys for the other bears to share, but the wrongdoer seized both toys, leaving none for the victim. The protagonist then either took 1 toy away from the wrongdoer and gave it to the victim (intervention event) or approached each bear in turn without redistributing a toy (nonintervention event). Across conditions, the protagonist was either a leader (leader condition) or a nonleader equal in rank to the other bears (nonleader condition); across experiments, leadership was marked by either behavioral or physical cues. In both experiments, infants in the leader condition looked significantly longer if shown the nonintervention as opposed to the intervention event, suggesting that they expected the leader to intervene and rectify the wrongdoer’s transgression. In contrast, infants in the nonleader condition looked equally at the events, suggesting that they held no particular expectation for intervention from the nonleader. By the second year of life, infants thus already ascribe unique responsibilities to leaders, including that of righting wrongs.

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Do children construe leaders as individuals whose position of power entails primarily more responsibility or more entitlement, compared with nonleaders? To address this question, 5‐year‐old children (n = 128) heard a story involving a hierarchical dyad (a leader and a nonleader) and an egalitarian dyad (two nonleaders), and then assessed protagonists’ relative contributions to a collaborative endeavor (Experiments 1 and 2) or relative withdrawals from a common resource pool earned jointly (Experiment 3). Children expected a leader to contribute more toward a joint goal than its nonleader partner, and to withdraw an equal share (not more) from a common pool. Children thus gave evidence that they construed leaders as more responsible, rather than more entitled, relative to nonleaders.

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