Our ability to reason about the thoughts of other people is the bedrock of our daily lives. Adults must think about others' thoughts constantly, from simple conversations to bluffing in a poker game. Even one-year-old babies learning their first words rely upon sophisticated inferences about the intentions of the speaker. Thinking about thoughts is ubiquitous, but it presents a serious cognitive and computational challenge. Prior research by Dr. Rebecca Saxe of MIT suggests that typical human adults achieve this feat partly by using a dedicated neural mechanism -- a group of brain regions that are recruited for thinking about thoughts. This CAREER award will pursue the implications of these results: How do specialized brain regions for thinking about thoughts develop in childhood? What role do these regions play in the social cognitive deficits observed in Autism? How is the development of these brain regions affected by changes in childhood experiences, such as delayed access to language? The proposed studies will investigate these questions by using behavioral tasks to measure development in children's ability to think about others' thoughts, and functional neuroimaging to measure concomitant development in the brain. The experiments will compare task performance and neural responses in typically developing children, children diagnosed with Autism, and children who experienced delayed exposure to language (e.g., deaf children of non-signing parents). The research proposed here will generate new insights both within developmental cognitive neuroscience, concerning the structure and function of a fundamental domain of human cognition, and across disciplinary boundaries, addressing key questions in developmental psychology and neuroscience. The project may also help guide the translation of this basic research into interventions for children who are socially at risk. Additionally, the project will support the development of an elementary school neuroscience curriculum to bring the generated knowledge about children's developing brains back to the children themselves.