A series of bivariate models were tested in order to partition the covariation between cannabis abuse/dependence and cannabis withdrawal into that due to A, C, and E. Table 4 summarizes the model fitting steps. Based on the results from the univariate models where we did not find evidence for sex-limitation, we fitted a general ACE model (model 1) with the A, C, and E parameters equated between sexes. In this model, the phenotypic correlation between withdrawal and abuse/dependence was estimated to be very high (r=0.92, p<0.001). Accordingly, the genetic correlation between the traits was also very high and significant (r=0.99, p<0.001), and so was the unshared environmental correlation (r=0.84, p<0.001). The shared environmental correlation was not significant (r=1.0, p=0.55), due to the weak influence of the shared environment on both traits. Figure 1 shows the parameter estimates in the bivariate model (transformed into a correlated factors model (Loehlin 1996)), including the proportions of variance in withdrawal and abuse/dependence accounted for by genetic effects (heritability; h2) and shared and unshared environmental influences, along with the genetic, shared environmental, and unshared environmental correlations between the two traits.
