There was evidence that the latent structure of the age of substance use initiation differed across men and women, and this was largely accounted for by differences in the strength of the relation between alcohol and cannabis initiation and the latent factor. Further, multivariate model-fitting supported retaining a full model among men, but dropping common sources of genetic influence and specific sources of shared environmental influence among women. Thus, these data suggest differences in the contribution of familial influences to first use of multiple substances in males and females. There have been no prior multivariate studies of the timing of first substance use that employed both men and women; therefore, current findings regarding sex differences in initiation require replication. Concerning disorder, results differ somewhat from a previous study of tobacco, alcohol, and cannabis dependence (Palmer et al., 2012), in which the best-fitting common pathway model differed across men and women due to differences in substance-specific estimates. As mentioned above, however, this study differed with respect to phenotype definition and sample characteristics. Also, strong inferences regarding sex differences in CPM structure
