Two independent studies have found evidence for rGE and for the possibility that common genetic etiology underlies age at 1st drink and AD (Grant et al., 2005; Prescott and Kendler, 1999). An alternative putative relationship between age at 1st drink and AD phenotypes, specifically, whether there is an unmeasured gene x measured environment (GxE) interaction, however, has not been previously examined. That is, after controlling for the main effects of rGE (i.e., those with an early age at 1st drink will have a greater likelihood of AD), to what extent does early exposure to alcohol moderate the role of genetic (and environmental) influences on later AD symptomatology? A GxE interaction implies that genetic influences on AD symptoms are moderated by early exposure to alcohol such that these genetic influences gain prominence (i.e., high heritability) only when there is exposure to the high risk environment, in this case, initiation of alcohol use at an early age (Gunzerath and Goldman, 2003; Heath et al., 2002; Heath and Nelson, 2002; Martin et al., 1987). Viewed another way, a GxE interaction may be interpreted
