AbstractThe transition from childhood to adolescence is marked by increasingly sophisticated social cognitive abilities that areparalleled by significant functional maturation of the brain. However, the role of social and neurobiological development infacilitating age differences in prosocial behavior remains unclear. Using a cross-sectional sample of children andadolescents (n = 51; 8–16 years), we examined the age-related correlates of prosocial behavior. Youth made costly andnon-costly prosocial decisions to anonymous peers during a functional magnetic resonance imaging scan. Among asubsample of youth who made prosocial decisions (n = 35), we found quadratic age differences in neural activation thatpeaked in early adolescence relative to childhood and older adolescence. In particular, early adolescents showed heightenedrecruitment of the posterior superior temporal sulcus (pSTS), temporal pole and inferior frontal gyrus (IFG) when engagingin costly prosocial behavior at the expense of gaining a reward, whereas they evoked heightened pSTS and dorsolateralprefrontal cortex/IFG activation when engaging in costly vs non-costly forms of prosocial behavior. Given that we did notfind age differences in prosocial behavior, this suggests that early adolescents show unique patterns of brain activation toinform similar levels of prosocial behavior.Key words: prosocial behavior; pSTS; fMRI; childhood; adolescence