Based on the best-fitting Common Pathway Model, genetic and environmental effects on each substance originated from common effects that influence SDV, and for certain phenotypes, substance-specific mechanisms. Genetic effects on SDV accounted for 48% of the genetic effects on alcohol dependence symptoms, 39% of the genetic effects on tobacco dependence symptoms, and 72% of the genetic effects on cannabis dependence symptoms in males. In females, genetic effects on SDV accounted for 100% of the genetic effects on alcohol and tobacco dependence symptoms (i.e., there are no substance specific genetic influences that are not accounted for by SDV) and roughly 50% of the genetic effects on cannabis dependence symptoms. While these estimates are quite large, it is important to interpret the overall magnitude of these effects in the context of the twin correlations, specifically how they are affected by our ability to the covariance among dependence on different substances to SDV, especially in the case of alcohol in females. More specifically, the pattern of the cross-twin-cross-trait correlations and the univariate models (Table 4) indicated limited evidence for additive genetic effects on
