Regarding the nature of the link between timing of first alcohol use and AD, our findings indicate that common genetic risk plays a major role. The moderate to high (0.59) genetic correlation is indicative of substantial overlap in the heritable factors that influence timing of first use and those that influence AD development. Environmental influences, by contrast, did not overlap substantially across the two alcohol outcomes. Shared environment contributed to timing of first use but not to AD and the individual-specific environmental influences on timing of first alcohol use and AD were not significantly correlated. Thus, our results are consistent with the interpretation of early alcohol use initiation as a marker for familial liability to AD rather than a direct cause of the disorder9-11. However, given that a small (albeit non statistically significant) portion of covariance (16.5%) was attributable to individual-specific factors, we cannot conclude, as Prescott and Kendler10 did, that the association is explained in full by familial sources. Overall, our findings highlight the prominent role of genetics in the development of AD and the early manifestation of that
