development of AD. For instance, genetic effects on drinking has been shown to be greater in urban versus rural residential settings (Dick et al., 2001; Rose et al., 2001), possibly because of differences in the level of social control between rural and urban environments (i.e., social control or structural constraints may be greater in rural environments, limiting the manifestation of genetically determined behaviors). Findings from the COGA study also suggest that variation within an AD susceptibility gene (GABRA2; rs279871) is related to a person’s marital status. Individuals with the high risk GABRA2 variant were less likely to be married partly because of their elevated risk for antisocial personality disorder; marital status also moderated the effect of other variants within GABRA2 on AD (Dick et al., 2006a). In addition to the evidence for gene x environment interaction, gene-environmental correlations are also relevant to alcohol. For example, several studies (Cleveland et al., 2005; Fowler et al., 2007; Harden et al., 2008) have shown that a person’s genes influence their exposure to 1) alcohol, and 2) their exposure to peers who use alcohol. In their study of 862 twin pairs, Fowler et al. (2007) found significant correlations (>0.60) between genetic influences on friends
