Table 1 shows that, for both sexes, the probandwise concordance rate and tetrachoric twin-pair correlations for withdrawal and cannabis abuse/dependence were higher in MZ versus DZ twins, suggesting a role for genetic influences on both phenotypes. However, the twin pair correlations were only significantly higher for MZ females than DZ same sex females for abuse/dependence (Δχ21=4.13, P=0.04), while for withdrawal for females and, for both variables, for males the differences in twin pair correlations did not reach significance (all P>0.10).
