To understand the etiology of comorbid drug dependence we derived a series of multivariate models that were based upon the results of the univariate models (i.e., (i) no evidence for gender-specific genetic effects, (ii) slight gender differences in the magnitude of genetic and environmental effects, and (iii) no evidence for non-additive genetic effects. We compared an Independent Pathway model that allowed for gender differences in the magnitude of genetic (A) and environmental (C and E) effects (χ2 = 4327.63, df = 3712, AIC = −3096.37) to a Common Pathway model that allowed for gender differences in the magnitude of genetic and environmental effects, as well as the magnitudes of the factor loadings on the latent trait (χ2 = 4341.47, df = 3720, AIC = −3098.53). This model comparison indicated that of the two models, the Common Pathway model provided a more parsimonious description of data (Δχ2 = 13.84, Δdf = 8, P = 0.09, ΔAIC = −2.16).
