Association and moderation analyses for the prospective sample were conducted using the SURVEYREG procedure in SAS, which accounts for the nesting of individuals within families (90 and 84 families were represented in the adolescent and young adult subsamples, respectively). Covariates included gender and age at assessment. For the association analyses, we examined the variance (R2) in the externalizing disorder composite and impulsivity-related traits explained by polygenic scores in the adolescent and young adult prospective subsamples. We examined these associations in adolescents and young adults separately because of measurement differences and in view of the numerous social and legal changes during the transition to adulthood that may change gene-behavior associations. We then tested for gene-environment interaction with parental monitoring and peer substance use in the adolescent subsample (i.e., the subsample for which these environmental measures were available and likely to be developmentally relevant). It has been suggested that variants having a nominal association with an outcome are likely to be enriched for gene-by-environment interaction (Thomas, 2010). Accordingly, we focused our gene-by-environment analyses in adolescents on an externalizing disorder composite because we
