Since that initial publication, 4 studies (45–48) have systematically replicated this association. In particular, in a Dutch sample, Lynskey and colleagues (47) found that rates of party and hard drug use were considerably elevated (18 vs. 4%) in the cannabis-using twin relative to their co-twin and even after adjustment for confounders, the residual association between cannabis use and use of hard/party drugs persisted. The authors suggested that, as this association remained in a sample for whom cannabis was legal, the previously observed associations between cannabis and other drug use could not be explained by the legal status of cannabis. Similarly, utilizing all twins (not just those from discordant pairs), Agrawal and colleagues (48) found that this correlation could be partially ascribed to person-specific factors that influenced both early-onset of cannabis use and the subsequent use of hard drugs. In particular, Cleveland & Wiebe attribute this effect to the natural developmental course of substance use, which is genetically influenced (49).
