The substantial role of genetics in addictive disorders is now well established (for a review, see Goldman, Oroszi, & Ducci, 2005). Additive genetic factors are estimated to have a heritability of 50–60% (Goldman et al., 2005), meaning that approximately half of the statistical probability of developing an addictive disorder is attributable to genetic influences. Moreover, the recent revolutionary advances in human molecular genetics and high-throughput bioinformatics have offered an unprecedented opportunity to potentially identify the specific genetic polymorphisms responsible for conferring genetic risk. Paradoxically, however, from early candidate gene association studies examining one variant in a single gene to more recent genome-wide association studies using 1M or more polymorphisms across genome1, the results have been disappointing because the genetic associations observed in early studies have often not been replicated and, where present, the effect sizes have been very small (Goldman et al, 2005; Treutlein & Rietschel, 2011). In other words, in spite of the established high levels of heritability, the specific genetic variants that confer risk for (or protection against) addictive disorders remain largely obscure. This disconnect between moderate heritability and very small individual genetic associations applies to most psychiatric disorders (Turkheimer, 2011).
