Women outnumber men in undergraduate enrollments, but they are much less likely than men to major in mathematics or science or to choose a profession in these fields. This outcome often is attributed to the effects of negative sex-based stereotypes. We studied the effect of such stereotypes in an experimental market, where subjects were hired to perform an arithmetic task that, on average, both genders perform equally well. We find that without any information other than a candidate's appearance (which makes sex clear), both male and female subjects are twice more likely to hire a man than a woman. The discrimination survives if performance on the arithmetic task is self-reported, because men tend to boast about their performance, whereas women generally underreport it. The discrimination is reduced, but not eliminated, by providing full information about previous performance on the task. By using the Implicit Association Test, we show that implicit stereotypes are responsible for the initial average bias in sex-related beliefs and for a bias in updating expectations when performance information is self-reported. That is, employers biased against women are less likely to take into account the fact that men, on average, boast more than women about their future performance, leading to suboptimal hiring choices that remain biased in favor of men.gender stereotypes | science education | diversity | science workforce W hy does the proportion of women in science, technology, engineering, and mathematics (STEM)-related professions fail to reflect the interest girls demonstrate for mathematics and science courses in early school years? In high schools in the United States, girls and boys take mathematics and science courses in roughly equal numbers. Standardized-test results suggest that in high school girls are as prepared as boys to pursue science and engineering majors in college. However, from their first year in college, women are much less likely than men to choose a STEM major. College-graduate men outnumber women in nearly every science and engineering field (1). The sex-based disparity in STEM fields is even greater at the graduate-school level (2). In a controversial speech, Larry Summers (3), then President of Harvard University, advanced three hypotheses for this underrepresentation of women in science: different innate aptitudes among men and women at the high end of science-based fields; different career-related preferences among men and women; and discrimination. Although there is mounting evidence against the aptitude-based hypothesis (4-6), it is difficult to show the existence of discrimination if we allow for the possibility of a sex difference in preference; that is, if women truly prefer fields outside of mathematics and science, then their lower proportions in STEM domains may result not from discrimination but merely from preference. That possibility aside, it remains important from a policy point of view to determine whether discrimination exists and, if it does, what can be done to reduce it. Fo...