The current study is the first to investigate the multivariate latent structure of the timing of substance use initiation. A large degree of genetic and environmental variation was substance-specific, which is to be expected given the modest correlations among the age of initiation measures (men: rs=0.25–0.34, women: rs=0.25–0.29). The variation common to all three substances, however, was mediated through a single phenotype. With regard to substance use disorder, there was a significant decrement in fit when the common pathway model was compared to the independent pathway model, but the CPM still fit the data very well. Previous multivariate analyses of substance use in adolescent and adult samples (e.g., Han et al., 1999; Hettema, Corey, & Kendler, 1999; Karkowski, Prescott, & Kendler, 2000) have found support for a common pathway structure. Further, two prior analyses of nicotine, alcohol, and cannabis dependence (Palmer et al., 2012; Xian et al., 2008) found support for a common pathway model. Differences in phenotype definition and sample characteristics exist between these studies and the current report, which may explain the slight difference in our findings.
