Based on a sample of 406 seven-year-old twins, this study examined whether exposure to friends' social or physical aggression, respectively, moderates the effect of heritability on children's own social and physical aggression. Univariate analyses showed that children's own social and physical aggression were significantly explained by genetic factors, whereas friends' social and physical aggression represented "true" environmental factors that were unrelated to children's genetic dispositions. Multivariate analyses further suggested a possible gene-environment interaction in the link between friends' and children's physical aggression but not in the link between friends' and children's social aggression. Instead, friends' social aggression was directly related to children's social aggression, in addition to genetic effects on this behavior. Theoretical and practical implications of these results are discussed.