Third, our findings are congruent with prior studies in suggesting that when significant gene×environment interaction effects are found, they show genetic effects on alcohol use to increase when social constraints are minimized (e.g. low parental monitoring, low prosocial behaviors and low parental bonding), or when the environment permits easy access to alcohol and/or encourages its use (e.g. high alcohol availability or high peer deviance) (Kendler, 2001; Shanahan & Hofer, 2005). For example, previous twin studies have demonstrated that genetic influences on adolescent substance use and externalizing behavior are magnified under conditions of lower parental monitoring (Dick et al. 2007b) and higher peer substance use/deviance (Button et al. 2007; Dick et al. 2007a). More recently, this has also been demonstrated with respect to specific genes, showing stronger genetic effects in the presence of lower parental monitoring (Dick et al. 2009) and higher peer deviance (Latendresse et al., unpublished observations).
