In this paper, I use evolutionary psychology to develop a new theory of sex discrimination. This theory suggests that a cognitive bias toward low task-related self-esteem in young women when comparing themselves to men, and for high task-related self-esteem in men when comparing themselves to young women, may have evolved as it was in the reproductive interests of both males and females. Low task-related selfesteem vis-à-vis males may have been adaptive for young females in the evolutionary environment because it advertised both youth and future controllability, features highly attractive to prospective mates. High task-related self-esteem vis-à-vis young females may have been adaptive for males because it advertised capabilities as providers and protectors, also features attractive to mates. Both biases are predicted to end for females beyond the reproductive years. Predictions from this theory are tested and supported with data from twenty-nine countries, including the US.