The authors report 5 studies that demonstrate that manhood, in contrast to womanhood, is seen as a precarious state requiring continual social proof and validation. Because of this precariousness, they argue that men feel especially threatened by challenges to their masculinity. Certain male-typed behaviors, such as physical aggression, may result from this anxiety. Studies 1-3 document a robust belief in (a) the precarious nature of manhood relative to womanhood and (b) the idea that manhood is defined more by social proof than by biological markers. Study 4 demonstrates that when the precarious nature of manhood is made salient through feedback indicating gender-atypical performance, men experience heightened feelings of threat, whereas similar negative gender feedback has no effect on women. Study 5 suggests that threatening manhood (but not womanhood) activates physically aggressive thoughts.