We tested a model of prosocial development, which predicted that prosocial action might decline, not increase, throughout childhood, becoming increasingly selective, individual, gender-related, and linked to emotional dysregulation. Sixty-six focal children at 18, 24, or 30 months of age were observed at home with familiar peers and then again 6 months later. Episodes of peer interaction were analysed for instances of sharing. The predicted decline in sharing with age was qualified by cohort differences and many associations with gender. Most children shared less as they grew older, but the oldest girls slightly increased their rate of sharing over time. As peer relationships developed, girls were more likely to share with other girls; boys were more likely to show reciprocity in sharing. Individual differences in sharing were moderately stable over time and linked to another form of prosocial action, sensitivity to the peer's distress. As predicted, children who shared at higher rates were rated more negatively by their mothers. The positive relationship between prosocial action and mothers' ratings of negative personality traits was especially strong for boys.