This paper investigates whether chronic stress and acute physiological responses to competitive stress can explain individual and gender differences in competitiveness. We measure individuals' autonomic nervous system activity in a resting state as well as under non-competitive and competitive incentives in a real task using heart rate variability measurement. We find that basal heart rate variability, a proxy for chronic stress, and acute competition-induced changes in heart rate variability predict self-selection into competition. Moreover, we observe that basal heart rate variability predicts self-selection into competition for women, but not for men. Overall, we find tentative evidence for gender differences in the relationship between physiological stress and a decision to enter competitive environments. Our results suggest that individual variation in autonomic nervous system activity and physiological responses to competitive stress predict self-selection into competitive environments, but do not explain gender differences in willingness to compete.