Table 6 describes the standardized parameter loadings from the full Common Pathway Model and the best-fitting Common Pathway Model. Across genders, individual differences in comorbid drug dependence symptom counts were attributable to a single latent trait that accounted for roughly 25%–36% of the variability in a substance with the remaining variance attributable to substance-specific genetic and environmental effects. The best-fitting Common Pathway model consisted of (i) the same magnitude of the loadings from the observed traits to SDV across males and females, (ii) the same magnitude of genetic (A) and non-shared environmental (E) effects on SDV across males and females (shared environmental effects (C) were dropped), (iii) AE substance-specific effects for alcohol and tobacco in males and CE substance-specific effects on alcohol and tobacco in females, and (iv) E-only cannabis-specific effects in males and AE cannabis-specific effects in females. Based on the best-fitting Common Pathway Model, the heritability of SDV was approximately 65% in males and females (i.e., adhering to path tracing rules (i.e., forwards from one variable to another; passing through each variable only once in a chain of variables; and, tracing through no more than one two-way arrow in each chain of paths; h2SDV = 0.81 × 0.81)).
