We use nationally representative data to calculate correlations in achievement and delinquency between genetically differentiated siblings within a family, between peers as defined by adolescents’ “best friend” nominations, between schoolmates living in the same neighborhood, and between grademates within a school. We find the largest correlations between siblings, especially identical twins. Grademate and neighbor correlations are small. Peer-based correlations are considerably larger than grademate and neighbor correlations but not larger than most sibling correlations. The data suggest that family-based factors are several times more powerful than neighborhood and school contexts in affecting adolescents’ achievement and behavior.