The parameter estimates for the best-fitting common pathway model are seen in Fig. 1. The estimated total contributions of additive genetic, shared and unique environmental effects on the three forms of DA, and the proportion of each that is common versus substance class specific in its effect is seen in Table 3. Six results are particularly worthy of comment. First, all three forms of DA loaded quite strongly on the common factor, with the loading for stimulant abuse (+0.94) being slightly higher than that seen for sedatives (+0.90) and cannabis abuse (+0.89). Second, the latent liability to DA was highly heritable (a2 = 0.67) but also appreciably influenced by shared environmental effects (c2 = 0.22). Third, estimates for the total heritability for the three forms of substance abuse were similar and high, ranging from 64% for stimulant and sedative abuse to 70% for cannabis abuse. Fourth, estimates for total shared environmental contributions to the three classes of DA were more modest and quite similar: 20% for stimulant abuse and 18% for both cannabis and sedative abuse. Fifth, substance specific gene
