Background: Innovation in STEM (science, technology, engineering, and math) fields in the U.S. is threatened by a lack of diversity. Social identity threat research finds messages in the academic environment devalue women and underrepresented groups in STEM, creating a chilly and hostile environment. Research has focused on the mechanisms that contribute to STEM engagement and interest at the K-12 and undergraduate level, but the mechanisms that predict sustained engagement at the graduate level have not been studied. Results: In a longitudinal study of doctoral students in STEM disciplines, we demonstrate that students' beliefs that their STEM colleagues believe intelligence is a fixed (vs. malleable) trait undermine women's engagement in STEM. Specifically, perceiving a fixed ability environment predicts greater perceptions of sexism, which erode women's self-efficacy and sense of belongingness and lead women to consider dropping out of their STEM career. Conclusion: These findings identify one potential pathway by which women leave their STEM fields, perpetuating gender disparities in STEM.