Furthermore, studies suggest that the “common liability” (i.e., each substance has its own set of genetic and environmental liabilities that are shared with other substances) and “alternative forms” (i.e., comorbidity across substances arises because each substance is an alternate manifestation of a common underlying liability for deviant behaviors) models best explain the observed comorbidity for dependence across different substances (19-22). Like Figure 1, these models assume that dependence upon multiple substances is attributable to correlated latent liabilities or a single latent continuous liability, respectively. Recently, a comparison of three multivariate genetic models indicated that a model which attributes the covariance among different substances to a single latent trait parsimoniously describes alcohol, tobacco, and cannabis dependence in a community-based sample (21). The identified factor, which was referred to as “Substance Dependence Vulnerability” was highly heritable (64%) across genders and has been demonstrated to be stable over time (23). Likewise, evidence for a general tendency to misuse substances has also given rise to dimensional measures of comorbid drug problems, in particular, dependence vulnerability (DV), which is a heritable (h2 = 0.40) summary measure that reflects the average number of DSM-IV dependence symptoms across substance used (24).
