Understanding academic gender gaps is difficult because gender-imbalanced fields differ across many features, limiting researchers’ ability to systematically study candidate causes. In the present preregistered research, we isolate two potential explanations—brilliance beliefs and fixed versus growth intelligence mindsets—by comparing two fields that have inverse gender gaps and historic and topical overlap: philosophy and psychology. Many more men than women study philosophy and vice versa in psychology, with disparities emerging during undergraduate studies. No prior work has examined the contributions of both self-perceptions of brilliance and fixed versus growth mindsets on choice of major among undergraduate students. We assessed field-specific brilliance beliefs, brilliance beliefs about self, and mindsets, cross-sectionally in 467 undergraduates enrolled in philosophy and psychology classes at universities in the United States and Canada via both in-person and online questionnaires. We found support for the brilliance beliefs about the self, but not mindset, explanation. Brilliance beliefs about oneself predicted women’s but not men’s choice of major. Women who believed they were less brilliant were more likely to study psychology (perceived to require low brilliance) over philosophy (perceived to require high brilliance). Findings further indicated that fixed versus growth mindsets did not differ by gender and were not associated with major. Together, these results suggest that internalized essentialist beliefs about the gendered nature of brilliance are uniquely important to understanding why men and women pursue training in different academic fields.