If early onset alcohol use is associated with the more genetically determined form of alcoholism (Pickens et al., 1991; Johnson et al., 1998; Dick et al., 2007; Chen et al., 2011) then it would be expected that genetic factors leading to early drinking and dependence would be manifest. Our results are consistent with this hypothesis. The pattern of genetic results obtained here, albeit from a single gene, is weighted towards the strongest effects manifesting themselves in the youngest age range. However, most twin studies find low genetic influences at younger ages and increases in genetic influences with age (Bergen et al., 2007; Kendler et al., 2008; van Beek et al., 2012), although not all twin studies have this conclusion (Hicks et al., 2007; Baker et al., 2011). These results can be understood after examination of the populations from which the twin samples are drawn and the outcomes which are modeled. The samples in the twin studies are drawn from the general population, not from the densely affected families which form the bulk of the sample used here. Thus genetic effects
