Gene-environment correlation for SUD (perhaps more accurately termed “genetic controls of exposure to the environment”12) largely arises because genetic risk factors for SUD influence individuals to select themselves into environments that convey a high risk for substance use and abuse. The high-risk environment is here mediational—sitting in the causal pathway between genes and the end-phenotype of SUD. This process is well illustrated by the social construct of ‘peer deviance’, defined as the degree to which close friends engage in antisocial behaviors, ranging from underage smoking, cutting school and cheating on tests to use of hard drugs, stealing and selling drugs. High levels of peer deviance in adolescence strongly predict subsequent SUDs. However, individuals do not associate with their peers at random. Rather, people actively select individuals to socialize with who have similar proclivities. Although an ‘environmental’ measure, peer deviance is substantially influenced by genetic factors, and these genetic factors increase in importance during development13. So, part of the pathway to SUD looks like this: genes → temperament → peer deviance → SUD.
