findings. However, given the low rates of comorbid drug involvement in community- and general-population samples, samples in the tens of thousands will be necessary to obtain sufficient variability across alcohol, tobacco, cannabis, and in particular illicit substances. It should also be noted that the strength of the associations among substances and the evidence for the common phenotypic factors defined by drug dependence and problem use items, may depend on the prevalence of symptoms in our sample. Larger samples of substance users might show different patterns of comorbidity. However, large studies among drug users in community- and general population samples have provided substantial evidence in support of a general vulnerability to drug dependence (18, 43). As such, one would expect to identify at least a common factor (under similar genetic influence [as evidenced in the current study]) that accounts for the majority of the phenotypic covariance and possibly one or more additional factors that reflect unique variance shared by other substances (22).
