The heritability of self-reported personality traits related to impulsiveness, irritability, and the inhibition of assertive or aggressive behavior was examined in up to 500 healthy monozygotic and dizygotic twin pairs raised together or apart. Two factors related to "(lack of) assertiveness/aggression" (Factor I) and "impulsive irritability" (Factor II) were examined using traditional and model-fitting procedures. Results of model-fitting procedures were consistent with a genetic, but not a shared environmental, influence for both factors. Further analysis suggested a nonadditive genetic influence for Factor II and an additive influence for Factor I. Bivariate model-fitting analyses suggest that self-reported "irritable impulsiveness" and "(lack of) assertiveness/aggressiveness" show substantial, though different, genetic influences.