The present study addresses the influence that group norms exert on individual aggressive and prosocial behavior. The study hypothesis is that for early adolescents who change their peer group affiliations, the characteristics of the group they are leaving (departing-group influence) are not as influential as those of the group that they are joining (attracting-group influence). From a larger sample of fifth and sixth graders who were followed over a one-year period, 198 early adolescents were identified as those who changed peer group affiliations. Peer nominations on aggression, prosociality, social preference and popularity, and social network information were collected. Results confirmed that there were significant attracting- but not departing-group influences on aggression and prosociality. Expected associations between aggression, prosocial behavior, and social status were confirmed. The discussion is framed around a social-ecological perspective that emphasizes the short-term adaptive nature of aggressive behavior in some peer groups and the need for considering social mobility when assessing group influence on individual behavior.