A matching law reinforcement model of individual differences in social aggression was developed and tested. Ten hours of interaction were observed in each of ten aggressive and ten nonaggressive sons with their mothers. Consistent with the matching law, the relative probability of maternal termination of conflict (negative reinforcement) contingent on sons’ tactics in dyadic conflict during an easlier time period was significantly correlated with the relative probability of the sons’ choice of those tactics during a later time period. Comparable matching relationships were found between maternal choice of conflict tactics and sons’ negative reinforcement of their mothers’ tactics. Significant between-group differences were found in the relative probabiliy with which mothers and sons negatively reinforced each other’s aggressive conflict tactics. The density of aversive social stimuli in the social environment and the functional utility of aggressive tactics in terminating conflict during earlier observations were found to be reliable predictors of between-individual and -group differences in both mothers’ and sons’ aggressive behavior during later interaction.