Table 3 presents the pattern of twin correlations within and between substances. In several instances, the MZ twin correlations exceeded DZ twin correlations, which suggested moderate additive genetic influences on variation in dependence symptoms in young adulthood. Notably, in some cases, the MZ twin correlation was more than twice the DZ twin correlation, which suggested the role of non-additive genetic factors; however, formal tests of these parameters yielded results that were consistent with the prior literature (i.e., limited non-additive genetic effects). Shared environmental effects were also evident in instances where the DZ twin correlation was more than one half the MZ twin correlation, such as the cross substance correlation between alcohol and cannabis in females (rMZ=0.29, rDZ=0.22).
