Research on aggression has benefitted from using individual‐difference measures to predict aggressive behavior. Research on meta‐cognition has recently identified that the predictive utility of individual‐difference inventories can be improved by considering the certainty with which people hold their self‐views. Merging these two frameworks, the present research examines whether assessing certainty in trait aggressiveness improves its ability to predict aggressive outcomes. Across two studies, participants reported their level of trait physical aggressiveness and the certainty with which they held their responses to the scale (predictor variables). Aggressive behavioral intentions (Study 1 and 2) and actual aggressive behavior (Study 2) were used as dependent measures. As hypothesized, results indicated that certainty moderated the effects of individual‐differences in aggressiveness on both aggressive outcomes. Therefore, considering the certainty with which people hold their relevant traits can be useful for understanding aggression, and also for predicting the consistency between personality and behavior.