Twin data that compares co-occurance frequencies in monozygotic vs dizygotic twin pairs provides evidence for shared heritability for some of these phenotypes. For other phenotypes, the magnitude of genetic influences and the frequency of co-occurance with substance dependence each indicate the likelihood of pleiotropic influences of some of the same allelic variants on both phenotypes. Finally, we also present here the idea that “transitive” genetic approaches may also identify evidence for generalization of effects of some pleiotropic influences. If substance dependence and intermediate heritable phenotypes share genetic overlap and co-occur, then a third heritable phenotype that is documented to co-occur with the intermediate phenotype might also share substantial heritability with addiction vulnerability.
