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Individuation of agents based on psychological properties in 10 month-old infants.

What makes agents fundamentally different from each other from the viewpoint of a 10-month-old infant? While infants at this age can already individuate human-like objects from non-humanlike ones and self-propelled agents from inert objects, little is known of when and how they start individuating within the domain of agents. What is clear from previous studies is that differences in surface and dynamic features are not sufficient. We hypothesized that mental properties-in this case the agents' preferences-can serve as an individuating property. In our study, we familiarized infants with two animated agents who had different preferences. The agents sequentially and repeatedly emerged from behind an occluder, and then each agent approached one of two target objects before returning behind the occluder. After familiarization, the occluder was lifted, revealing either one agent or two agents. While infants successfully individuated the agents in the preference-demonstration condition, they failed to do so in the exposure-only condition in which perceptually similar surface and dynamic features of the agents were presented but without indicating preferences. Our study thus provides evidence that mental properties can help individuate agents, grounding the claim that infants understand agents as mental entities at their core.