All analyses were conducted separately for the adolescent sample (individuals between the ages of 12 and 17) and the young adults (18–26 years of age). This division was based on significant developmental changes across this age range that are particularly relevant for substance use outcomes. Most youth leave home after age 18, and this represents a considerable leap in independence, with an associated reduction in parental supervision and monitoring. Although alcohol use remains illegal for individuals aged 18–20, the emotional and logistical shifts associated with reaching adulthood generally offer enhanced opportunity and incentive for engaging in substance use. Previously, independent studies have found that genetic associations between specific genes and alcohol use outcomes are not evident until young adulthood (Dick et al., 2006b; Guo et al., 2007), likely because of the greater importance of environmental factors that influence adolescent experimentation with alcohol (Rose et al., 2001). In contrast, association with conduct problems has been reported earlier in adolescence for genes associated with alcohol dependence in adulthood, suggesting the possibility of heterotypic continuity of genetic effects, that is, the same gene
