Neural sensitivity to others' belief states in infancy predicts later theory of mind reasoning in childhood.

While pre-verbal infants may be sensitive to others' mental states, they are not able to accurately answer questions about them until several years later, an ability referred to as having a theory of mind. Here we ask whether infant social-cognitive sensitivity is subserved by the same brain mechanisms as those that support theory of mind in childhood. To do so, we explored the relationship between functional sensitivity of the right temporal-parietal junction to mental state processing in infancy, a region known to underlie theory of mind in older children, and explicit theory of mind reasoning in the same group several years later. In a small initial sample (N = 33), we find evidence of a longitudinal brain-behavioral link from infancy to childhood, providing preliminary support for a common mechanism for theory of mind across development. However, the brain metric that was predictive of individual differences was not the response to conditions that required tracking the beliefs, but instead, the response to a control condition where belief tracking was not obligatory to predict others' behavior. In hindsight, the ambiguity of this control condition may have best distinguished between infants who had different propensities to engage in belief tracking, suggesting a potential role for active experience in infancy contributing to individual differences in later theory of mind development in childhood. Given the exploratory nature of the study, other alternative explanations for these results must also be considered.