BACKGROUND: Behavioral undercontrol may contribute to risk for alcoholism in vulnerable persons. We predicted that healthy young adults with a family history of alcoholism (FH+) who also displayed externalizing behavior characteristics (low scores on the California Psychological Inventory Sociability Scale; CPI-So) would exhibit more impulsive responding (false alarms) on a Go-NoGo reaction time task. METHODS: Subjects were 230 healthy volunteers, 18 to 30 years of age with no history of alcohol or drug dependence. The task included 100 trials: 60 of "Go," calling for a button press, and 40 of "NoGo," or "XX," calling for inhibiting a response. Data analysis involved a signal detection analysis of performance with subsequent group comparisons for rates of impulsive responding indicated by False Alarms (responses to NoGo signals). RESULTS: CPI-So scores were lower in FH+ than in FH- (p < .000001) indicating a greater clustering of disinhibitory tendencies in these persons. FH, CPI-So scores, and Gender together predicted false alarm rates, accounting for 4.9% of the variance, F = 3.89, p = 0.009. False alarms were associated with low CPI-So scores, F = 5.15, p = 0.024, and being male, F = 6.27, p = 0.013, but not with FH once these variables were accounted for. CONCLUSIONS: A disinhibited temperament may underlie a behavioral impulsivity that contributes to elevated risk for future alcoholism, especially among FH+ males.