Support for the idea that vulnerability to addictions is a complex trait with strong genetic influences that are largely shared by abusers of different legal and illegal addictive substances [45–48] comes from classical genetic studies. Family studies document that first degree relatives (eg sibs) of addicts display greater risk for developing substance dependence than more distant relatives [45, 49]. Adoption studies find greater similarities between levels of substance abuse between adoptees vs biological relatives than adoptees vs members of the adoptive families [45]. In twin studies, differences in concordance between genetically identical and fraternal twins also support heritability for vulnerability to addictions [47, 50–56]. Twin data allows quantitation of the amount, about half, of addiction vulnerability that is heritable. Twin data also supports the idea that the environmental influences on addiction vulnerability that are not shared among members of twin pairs are much larger than those that are shared by members of twin pairs (eg e2 >> c2 in virtually every such study). Most environmental influences on human addiction vulnerability are thus likely to come from outside of the immediate family environment (Figure 1).
