However, twin studies can provide opportunities for identifying important environmental influences operating both as main effects and as GXE interactions. When information about the environment is explicitly measured, this can be incorporated into twin models to test whether the importance of genetic and environmental effects varies as a function of the measured environment (Dick, Rose, Viken, Kaprio, & Koskenvuo, 2001b; Purcell, 2002). One of the earliest illustrations of gene-environment interaction in the area of substance use research demonstrated that genetic influences on alcohol use were greater among unmarried women, whereas having a marriage-like relationship reduced the impact of genetic influences on drinking (Heath, Jardine, & Martin, 1989). Religiosity has also been shown to moderate genetic influences on alcohol use among females, with genetic factors playing a larger role among individuals without a religious upbringing (Koopmans, Slutske, van Baal, & Boomsma, 1999). Genetic influences on adolescent substance use are also enhanced in environments with lower parental monitoring (Dick et al., 2007c) and in the presence of substance-using friends (Dick et al., 2007b). Similar effects have been demonstrated for more general externalizing
