Evidence for the heritability of the use of and dependence on other illicit drugs has also been investigated in the California Indians. Heritability for initiating use was found to be high for marijuana (0.59), opiates (0.58), PCP (0.51), sedatives (0.49), and stimulants (0.38). Heritability was only modest for initiation to cocaine (0.14), hallucinogens (0.26), and solvents (0.13). The heritabilities of drug dependence diagnoses and symptoms were also estimated in that population and found to be significant for marijuana dependence with antisocial traits [23], stimulant dependence with craving [24], and heavy tobacco usage [25]. Taken together, these studies suggest that both initiation of drug use and transition to dependence have a significant genetic component. These findings are largely consistent with findings from general population twin studies suggesting substantial genetic influences, in the same numerical ranges, in the liability for the initiation of illicit drug use as well as the transition to dependence [see 26].
