An externalizing low-risk group consisted of those young men whose fathers had no history of CD, AAB, alcohol abuse or dependence, illicit drug abuse or dependence, or alcohol or drug treatment. Earlier age at first alcohol use has been associated with higher rates of externalizing characteristics such as antisocial behavior, illicit substance use and abuse, and disinhibited personality characteristics (Clark, 2004; McGue, Iacono, Legrand, Malone, & Elkins, 2001; Schuckit & Russell, 1983). The maximum number of drinks in a 24-hr period has been described as a promising quantitative endophenotype for alcoholism risk (Malone, Iacono, & McGue, 2002). Both of these paternal alcohol use measures have been associated with reduced P300 amplitude in sons even in the absence of paternal substance disorder diagnoses (Iacono et al., 2002). As in Iacono et al. (2002), to reduce false negatives young men whose fathers were in the upper decile of the maximum consumption measure or in the lower decile of age at first drink were excluded from the low-risk group. A severe externalizing risk group consisted of young men whose fathers met criteria for
