CHRM2 has previously been associated with externalizing symptoms in a sample ascertained through alcohol-dependent individuals (Luo et al., 2005; Wang et al., 2004). We tested for association between this gene and prospectively measured externalizing behavior in a longitudinal, community-based sample of adolescents. No SNPs yielded evidence for a main effect of CHRM2; however, we found significant evidence of gene-environment interaction in the form of an interaction between genotype and parental monitoring. Specifically, the association between the genotype and externalizing behavior was accentuated under conditions of lower parental monitoring. This finding is in line with our previous twin work, which indicated that genetic influences on substance use became more important as parental monitoring decreased (Dick, Viken, et al., 2007). Also, we have previously found evidence for an interactive effect of another gene, GABRA2, and parental monitoring on externalizing behavior in the CDP (Dick et al., 2009). However, the data suggested a crossover effect for all CHRM2 SNPs with significant interaction effects: The genotype associated with the highest levels of externalizing behavior at the low end of parental monitoring was associated with
