The tetrachoric correlations between and within relative pairs for the three forms of drug abuse in MZ twins, DZ twins and near-aged siblings are seen in, respectively, Tables 2a–2c. Four patterns of findings are noteworthy. First, within individuals, registration for cannabis, stimulant and sedative abuse are highly intercorrelated with most correlations ranging from +0.70 to +0.90. Second, due to the rarity of our phenotypes and the resulting low numbers of concordant pairs, we could obtain only two stable cross-twin correlations in the DZ twins for cannabis abuse, and between cannabis abuse in twin1 and stimulant abuse in twin2. However, both these values (+0.53 and +0.46, respectively) were very similar to the parallel findings for the near-aged siblings (+0.52 and +0.46, respectively). Third, the correlations between twins for the same individual substance and for different substances were substantially higher in MZ twins than those seen in DZ twins and siblings suggesting an important role for genetic factors. Fourth, in the MZ and near-aged siblings, the cross-twin correlations within substances were consistently higher than across substances although the cross-substance correlations were substantial.
