We have also found that parental monitoring can dramatically alter the importance of genetic and environmental influences on adolescent substance use, with genetic effects assuming far greater importance under conditions of lower parental monitoring (Dick, Viken, et al., 2007). Further, we have found that genetic influences on adolescent alcohol use assume greater importance when the adolescent has more peers who also report substance use (Dick, Pagan, et al., 2007), a finding that has been replicated in other independent samples (Harden, Hill, Turkheimer, & Emery, 2008). Similar effects have been demonstrated for more general externalizing behavior: genetic influences on antisocial behavior were higher in the presence of delinquent peers (Button et al., 2007). In studies conducted in other large twin cohorts, 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).
