Building on theories of gender schema and socialization, as well as recent empirical research in economics, in this paper we investigate whether and how gender differences in preference for competition help to explain the gap in expectations of majoring in different STEM fields. Utilizing data from a sample of high school students, we find that girls' relatively lower preference for competition (compared to boys) helps to explain their correspondingly lower expectation of majoring in two historically male-dominated STEM fields, physical science and engineering. Additionally, we find that among females only, competition is a strong predictor of expectations of entering computer science fields. We discuss the implications of these findings for understanding larger patterns of gender inequality.