Masculinity and femininity have played a substantial role in how social scientists explain the gender gap in science, technology, engineering, and mathematics (STEM) careers. The masculine culture of science is thought to be inconsistent with occupational values associated with feminine personalities, and to create a discriminatory academic environment for those who cannot adapt to it. However, there has been little systematic investigation into the extent to which masculine and feminine personality characteristics are actually correlated with STEM career outcomes, or how the effects of masculine and feminine personality characteristics on STEM career outcomes may be different when embodied in women compared to men. This study tests several hypotheses concerning the relationship of masculine and feminine personality characteristics to occupational values, perceptions of academic climate, and selection of a STEM major in college among a sample of 752 students enrolled at a major public university. We find little support for the hypothesis that masculine personality characteristics are especially rewarded in STEM majors. However, we also find that women pay a femininity penalty in STEM majors, while more abundant feminine personality traits in men render them more likely to major in a STEM field, after accounting for occupational values. © 2016 Wiley Periodicals, Inc. J Res Sci Teach 54: 299–323, 2017