Utilizing the High School Longitudinal Study, a nationally representative sample of U.S. high school students, this study investigates the factors that predict different beliefs about gendered math ability and the potential consequences for students' choices to enter gender-segregated science, technology, engineering, and mathematics (STEM) majors in college. Among other results, analyses reveal that while about 25 percent of students report a traditionally stereotypical belief in male superiority, about 20 percent report a counter-stereotypical belief in female superiority; among female students, such beliefs are more common among black students. Further, models reveal a robust association between holding counter-stereotypical beliefs and the likelihood that women choose biological science majors, which are female dominated, compared to non-STEM fields. Among men, holding counter-stereotypical beliefs is associated with a lower likelihood of majoring in physical science, computer science, math, and engineering fields, which are strongly male dominated, versus non-STEM fields. Implications for gender inequality in STEM fields are discussed.