This study presents evidence on the role of social environment for the formation of prosociality. We show that socioeconomic status (SES) as well as intensity of mother-child interaction and mothers? prosocial attitudes are related to elementary school children?s prosociality. We also present evidence on a randomly assigned variation of the social environment, providing children with a mentor for 1 year. Our data reveal a significant and persistent increase in prosociality in the treatment relative to the control group. Moreover, enriching the social environment closes the prosociality gap between low- and high-SES children. A mediation analysis suggests that prosociality develops in response to prosocial role models and intense social interactions.