We investigate the development of two types of prosocial behavior, trust and reciprocity, as defined using a game-theoretical task that allows investigation of real-time social interaction, among 4 age groups from 9 to 25 years. By manipulating the possible outcome alternatives, we could distinguish among important determinants of trust and reciprocity that are related to the risk and benefit of trusting. The results demonstrate age-related changes in sensitivity to outcome for others from late childhood until late adolescence, with different developmental trajectories for trust and reciprocity and differential sensitivity to risk and benefit for self and others. (Contains 2 tables and 4 figures.)