There is accumulating evidence from twin studies that the importance of genetic influences on substance use and SUD varies as a function of environmental exposure (this is logically equivalent to saying that the importance of environmental influences varies as a function of genotype). The development of substance use problems is, in some sense, by definition a gene by environment interaction, as access to the substance is an ‘environmental’ condition that is necessary for the development of problems associated with use of the substance. Accordingly, it is not surprising that environmental conditions that affect access to the substance moderate the degree to which genetic influences act on risk for SUD. For example, genetic influences on adolescent smoking were attenuated after legislation took effect to prohibit smoking in public places14. Similarly, genetic influences on adolescent alcohol use are greater in neighborhoods with higher alcohol sales15, presumably through easier access for those with a predisposition. In fact, many of the environments that reduce genetic influences on substance use seem to operate through social control. Environments that are more controlling and provide greater social
