For both sexes, MZ and DZ twins did not differ significantly with respect to the prevalence of withdrawal or cannabis abuse/dependence. Also, we found no sex differences between male and female MZ nor male and female same sex DZ twin pair correlations for both variables, suggesting genetic and environmental effects of similar magnitude in males and females. However, the twin pair correlation for opposite-sex DZ twin pairs for abuse/dependence was significantly lower than that for the same-sex DZ pairs (Δχ21=4.94, P=0.03), suggesting that there may be qualitative sex differences in sources of familial aggregation in abuse/dependence. In a univariate twin model, though, we were unable to detect significant sex differences in the source of genetic or shared environmental influences, so we do not deal with this further.
