across dependence on multiple substances and have found them to be in excess of 0.5. However, despite the phenotypic indications of a latent risk, we are unaware of any twin studies that have explicitly tested a model indicating a generalized risk for comorbid DSM-IV drug dependence symptoms during young adulthood (i.e., the period during which the highlest levels of drug use are seen in large epidemiological surveys (SAMHSA, 2008)). Given the common neurological and environmental mechanisms that influence susceptibility to repeated drug exposure, we hypothesized that lifetime assessments of DSM-IV drug dependence symptoms during young adulthood would indicate a heritable common mechanism.
