obtained a family environmental estimate of .15 for CUD in women). Why might family environmental factors contribute more strongly to cannabis-related problems than other substance use disorders? This may be partly due to availability and access, which has been shown to moderate the family environmental influences on substance involvement. For instance, the heritability estimates for alcohol consumption are increased and the shared environmental estimates are decreased in urban compared with rural areas, which may be attributable to the higher number of alcohol outlets in these regions (Rose, Dick, Viken, & Kaprio, 2001). With regard to tobacco consumption, it has been shown that the genetic influences on smoking declined significantly following legislation prohibiting smoking in public places (Boardman, Blalock, & Pampel, 2010). Gillespie and colleagues (2009) demonstrated that nearly all of the shared environmental influence on cannabis abuse was explained by perceived availability of cannabis, and that the influence of availability on abuse was mediated through cannabis initiation. Given that cannabis is an illicit drug, shared environmental estimates may be increased because individuals have less access to cannabis than licit substances such as tobacco or alcohol.
