Intuitions about personal identity are rooted in essentialist thinking across development.

What aspects of a person determine whether they are the same person they were in the past? This is one of the fundamental questions of research on personal identity. To date, this literature has focused on identifying the psychological states (e.g., moral beliefs, memories) that people rely on when making identity judgments. But the notion of personal identity depends on more than just psychological states. Most people also believe that the physical matter that makes up an individual is an important criterion for judging identity; changes to the physical stuff in a person's body, even if they are not accompanied by any psychological changes, are judged to change who the person is at some level. Here, we investigate the sources of these beliefs and propose that they stem from the broader cognitive tendency to assume that unseen physical essences make things what they are-psychological essentialism. Four studies provided support for this claim. In Studies 1 and 2, exposing participants to essentialist reasoning led to stronger endorsement of physical continuity as a criterion for personal identity. Similarly, individual differences in participants' essentialist thinking predicted the extent of their reliance on physical continuity (Study 3), and this relationship was observed even among 6- to 9-year-old children (Study 4). These studies advance theory on the psychology of personal identity by identifying a reason why people assign a central role to physical composition when judging identity.