Human genetic studies of alcohol are organized into two broad categories, quantitative genetic (i.e., family and twin studies) and molecular genetic studies. Quantitative genetic studies suggest that genetic differences play an important part in susceptibility to AD. Much of this evidence has been derived from early family-based studies which indicated that first-degree relatives of alcohol dependent cases were several times more likely to be later diagnosed with AD relative to first-degree relatives of control subjects (Bierut et al., 1998). Further, twin and adoption studies suggest that this familial pattern may be attributable to additive genetic factors, which account for roughly 40%–60% of the liability for AD (Agrawal and Lynskey, 2008; Knopik et al., 2004). Twin studies also suggest that a large number of genes related to AD also influence other forms of drug dependence (Palmer et al., 2012), as well as other externalizing psychopathologies (Iacono et al., 2008; Young et al., 2000).
