These results also provide potential developmental insight into the etiology of alcohol dependence. Given that heavy alcohol exposure is a necessary precondition to the development of dependence (Koob & Le Moal, 2006), our findings suggest two potential path-ways by which genes influence the development of dependence: an early onset path driven in part by high genetic risk to externalizing disorders and a later onset path influenced largely by genetic risk factors more specific to alcohol disorders. These two path-ways to alcohol dependence are broadly analogous to several prior influential subtyping systems for alcoholism in postulating early versus late onset forms of illness, with the former demonstrating more prominent externalizing symptoms than the latter (Cloninger et al. 1981; Babor & Dolinsky, 1988; Babor et al. 1992).
