It has been proposed that selection has shaped the human mind to be predictably biased in domains where the costs of false-positive and false-negative errors have been asymmetrical throughout human evolutionary history. Using this logic, the current study predicts that men and women systematically overestimate the degree to which members of the opposite sex find their same-sex mating competition desirable. Ten photographs of opposite-sex targets were shown to a sample of men ( n=123) and women ( n=159), and they were asked questions pertaining to each target's desirability as a mate. The same photographs, this time with sex of target and participant being the same, were shown to a second group of men ( n=105) and women ( n=103), and they were asked to estimate the desirability of the depicted individuals to members of the opposite sex. Consistent with the mate competition overestimation bias hypothesis, men and women consistently overestimated the degree to which members of the opposite sex find members of their same sex attractive and desirable as potential mates.