Theory of mind—our capacity to understand what other people think and feel—grows significantly in the preschool and early school years. This is recognised as a crucial period for social development yet 30 years of research has not yet answered these basic questions: Do children with relatively advanced theories of mind also excel at navigating the everyday world of friendship and peer group dynamics at school? And conversely, do developmental delays in acquiring a theory of mind explain some of the peer problems that children with autism or deafness face? The proposed longitudinal project aims to be the first to supply a comprehensive and causally convincing answer to these core questions.