The present study adds to our understanding of the genetic etiology of comorbid substance dependence in young adulthood by comparing two multivariate biometrical models to provide an estimation of the genetic, shared environmental, and non-shared environmental influences on vulnerability to comorbid drug dependence. The identification of intermediate phenotypes that describe susceptibility to different substances has become exceedingly important given the difficulties in identifying gene variants that explain some of the genetic vulnerability for substance-related disorders.
