Inspection of Table 2 reveals the following: (1) The within-trait MZ twin correlations were larger than the DZ twin correlations (shaded cells), indicating genetic influences on all phenotypes studied; (2) the within-trait DZ correlations were closer in magnitude to the MZ correlations for initiation than disorder, providing greater evidence for shared environmental contributions to initiation; (3) most of the cross-trait MZ correlations were larger than the DZ correlations, implicating genetic factors in the covariation between the age of substance use initiation and substance use disorder; and (4) the pattern of MZ and DZ correlations differed somewhat across males and females, providing some evidence for quantitative sex differences. These observations were rigorously tested in the biometric model-fitting.
