A small number of genetically informative studies have investigated the association between age at 1st drink and AD. Using a sample of adult Virginia twins, Prescott & Kendler (1999) found that the prevalence of AD declined with increasing age at 1st drink, that one twin’s age at 1st drink was predictive of their co-twin’s AD and importantly, that the relationship between age at 1st drink and AD was attributable to a substantial degree to common genetic influences (18–29% genetic overlap). Grant and colleagues (Grant et al., 2005) used a similar strategy to assess age at first drink and AD in a sample of adult male Vietnam Era twins and reported that early (prior to age 17) regular alcohol use in one twin led to a 2.7 times increased likelihood of AD in the identical co-twin. These authors also reported a substantial correlation (0.59–0.64) between early regular drinking and AD. In a third study, Sartor and colleagues, using an offspring-of-twins design, found considerable evidence for increased risk of AD in those with an early age at 1st drink, but, importantly, did not find evidence that this risk varied by (paternally transmitted) genetic risk for AD (Sartor et al., 2007).
