Gene–Environment Interplay Beyond Interactions: Psychosocial Environments Mediate Genetic Effects Underlying Externalizing-Behavior Trajectories

We investigate gene-environment interplay for the development of externalizing (EXT) behaviors in Add Health. Latent trajectories of substance use (SUB) and antisocial behaviors (ASB) spanning ages 13 to 41 were extracted in a prior study (Sasia et al., 2025). Adolescent psychosocial environments (i.e., supportive parenting, delinquent peer affiliation, school connectedness, and neighborhood connectedness) were modeled to moderate and mediate the effects of EXT polygenic scores (PGS) in predicting SUB and ASB trajectories in European (n = 5,025; main analyses), African (n = 1,728), Asian (n = 382), and Hispanic (n = 773) genetic similarity groups. There was no evidence of moderation effects. However, PGS effects operated indirectly through psychosocial environments, notably via delinquent peer affiliation. Despite the prominence of genetic effects underlying EXT, peer influences may play an especially important role in the ways in which genetic effects influence EXT trajectories. Clinical and theoretical implications are discussed.