Rates of illicit drug use are higher among individuals with substance-using peers and those who live in areas where drugs are easily obtained (18–27). However, genetically informative studies to date have not examined the association between such environmental risk factors and drug initiation. Contrary to the belief that it is an entirely `environmental' risk factor, variation in cannabis availability can be explained by genetic and environmental factors. Recently, Gillespie and colleagues (28) found that between the ages 8 and 25 there is an increase in the additive genetic and decline in the shared environmental effects in self-report measures of cannabis availability. To what extent these same genetic and environmental influences can account for the liability to initiate cannabis and then progress to abuse is unknown.
