Ac, Cc, and Ec) and one that is specific to each one of them (As, Cs, and Es). In the second model, a common pathway model, we assume that the common variance components (Ac, Cc, and Ec) are mediated by a latent phenotype represented by a common factor (called “vulnerability to drug abuse” in Fig. 1). The common pathway model is nested within the independent pathway model. This latent phenotype mediates the common genetic and environmental effects, as the paths from Ac, Cc, and Ec run via the latent phenotype. Although our sample size is considerable, the models include relatively rare phenotypes and have limited statistical power. We therefore follow the recommendations based on simulations which show that in such situations, parameter estimates from the full model are typically more accurate than those from submodels even if the latter provide a better model fit (Sullivan and Eaves, 2002). Models were fit in the OpenMx software (Boker et al., 2011).
