Although this study did not formally test for mediation (Baron and Kenny, 1986) of the association between AFD and AUD, the results permit cautious inferences as to the potential mediating roles of a number of possible causal mechanisms, and they suggest promising avenues for additional research. Of primary interest from a policy point of view is whether the associations found in this study are more consistent with a) direct neurotoxic and/or pharmacological effects of early drinking, b) habituation of heavy drinking arising from the contexts in which adolescent drinking often occurs, or c) selectivity of early drinkers. Any causal role of impaired neuromaturation or executive cognitive dysfunction resulting from neurotoxicity would appear to be ruled out by the fact that women, who experience neuromaturation at earlier ages than men, were at greater risk of the incidence of dependence among those who initiated drinking at ages 15-17. However, a recent brain imaging study reported a reduced volume of prefrontal white matter in association with AUD among 15-to-17-year-old girls but not boys (Medina et al., 2008), a finding that did support the
