Differential behavioral effects of plasma tryptophan depletion and loading in aggressive and nonaggressive men

Preliminary findings indicate that men with high trait hostility may be prone to aggression increases following plasma tryptophan (Trp) depletion. We measured laboratory aggression in men selected for presence (n = 12) or absence (n = 12) of aggressive histories. Testing occurred before and after plasma Trp depletion, Trp loading, and under a food-restricted control condition. Subjects were provoked by subtractions of money, and aggression was measured as the responses the subject made to ostensibly subtract money from the instigator of the subtractions. When subjects were highly provoked, there was a significant Trp condition x aggression history interaction effect on aggressive responding. In particular, laboratory aggression in aggressive men was elevated under Trp-depleted conditions relative to Trp-loaded conditions, whereas the opposite occurred in nonaggressive men. Moreover, plasma total Trp levels after Trp loading were significantly higher in nonaggressive men, and plasma free (but not total) Trp levels after Trp loading correlated negatively with aggressive responses in the aggressive men. These data corroborate earlier findings that aggressive men may be more prone to aggression induced by reductions in plasma Trp.