It is unknown if the above prevalence differences are confounded by legal, cultural and economic differences which may impact differentially on the availability of licit and illicit substances. These same differences may also alter the observed association between availability, initiation and abuse. For instance, if the variance in measures of cannabis availability is likely to be more restricted in places such as the Netherlands, then other aspects of the shared environment, or genetic risks, may prove to be better risk indicators of initiating use and progression to abuse.
