Background: Social competition is a fundamental mechanism of evolution yet little is known about factors that affect competitive success, especially in humans. Some key factors might be stress and an individual’s trait anxiety, meaning their susceptibility to stress. These two factors could be an important determinant of differences in competitive outcomes. In the study, cortisol responses to stress were related to self-confidence. It was found that low-anxiety individuals become overconfident while high-anxiety individuals become under-confident. These findings indicate that stress is a major regulator of competitiveness. They also imply that stress may offer a new channel for generating social and economic inequality through its impact on competitive self-confidence and decision making in financially-relevant situations.