Do peer group norms moderate the expression of genetic risk for aggression?

• We examine genetic influences on aggression as a function of peer group norms.
• Peer group norms vary in their acceptance of physical or relational aggression.
• Genetic expression of physical aggression is stronger when norms are favourable.
• Relational aggression is strongly influenced by the environment regardless of norms.

PurposeUsing a genetically informed design based on 192 Monozygotic and Dizygotic twin pairs assessed in kindergarten, this study examined whether the expression of genetic risk for physical aggression or for relational aggression varies depending on the peer group’s injunctive behaviour norms.MethodsPhysical aggression and relational aggression, as well as injunctive peer group norms in regard to these behaviours, were measured via peer nominations in the twins’ kindergarten classes.ResultsPeer groups varied considerably in terms of the level of acceptability of both physical and relational aggression. Bivariate Cholesky modeling revealed a significant gene-environment interaction, indicating that a strong genetic disposition for physical aggression was much more likely to be expressed when peer group injunctive norms were favourable to such behaviour. In contrast, genetic factors essentially played no role in explaining inter-individual differences when peer group norms discouraged physical aggression. Relational aggression was generally less explained by genetic influences and more by environmental influences regardless of peer group norms, but environmental influences became even more important when peer group norms favoured such behaviour.ConclusionsThese findings speak to the importance of the peer group in shaping aggression already in young children by either condoning or penalizing such behaviour.