Thus far, our discussion has focused on the importance of genetic and environmental factors, independent of each other. However, there has been increasing interest in and recognition of the importance of gene-environment interplay in the etiology of substance use and other psychiatric disorders.88 There are two related mechanisms by which genes work in concert with environmental exposures – gene-environment correlation and gene-environment interaction. GE correlation refers to genetic predisposition that influences our likelihood of being exposed to a certain environment. For instance, heritable influences have been found to influence deviant peer affiliations suggesting that one’s own vulnerability to substance use is partly responsible for exposure to deviant peer groups. On the other hand, gene-environment interaction refers to moderation of genetic predisposition as a consequence of environmental exposure – for example, studies of adolescent Finnish twins indicated that in less stable neighborhoods there was greater evidence of genetic influence.89 Conversely, in more supervised and restricted environments, there was less opportunity to express genetic predispositions and greater influence of environmental effects.89,90 These analyses suggest that less restrictive environments provide greater opportunities for
