Finally, polygenic scores had differential associations with the externalizing disorder composite in the context of different environments, with stronger effects for peer substance use. Genetic differences were more pronounced under conditions of low parental monitoring or high peer substance versus conditions of high parental monitoring or low peer substance use. This parallels findings from twin studies showing that genetic influences for externalizing behavior increase under conditions of low parental monitoring and high peer deviance (Button et al., 2007; Harden et al., 2008; Hicks et al., 2009). Such moderation effects likely reflect conditions or processes that may limit the expression of an individual's genetic predispositions toward externalizing (Shanahan & Hofer, 2005). We also note that there was evidence for gene-environment correlation, which is common for psychiatric disorders (Kendler & Baker, 2007). Although gene-environment correlations can produce spurious gene-environment interactions, that concern is mitigated by previous twin analyses in this area that yield evidence for gene-environment interaction after accounting for gene-environment correlation (Button et al., 2007; Hicks et al., 2009) and by our supplementary analyses where we partialed for gene-environment correlation.
