Feeling like one exerts more effort than others may influence women's feelings of belonging with science, technology, engineering, and math (STEM) and impede their motivation. In Study 1, women STEM graduate students perceived they exerted more effort than peers to succeed. For women, but not men, this effort expenditure perception predicted a decreased sense of belonging, which in turn decreased motivation. Study 2 tested whether the male-dominated status of a field triggers such effort expectations. We created a fictional "eco-psychology" graduate program, which when depicted as male-dominated resulted in women expecting to exert relatively more effort and decreased their interest in pursuing the field. Study 3 found emphasizing effort as expected (and normal) to achieve success elevated women's feelings of belonging and future motivation. Results suggest effort expenditure perceptions are an indicator women use to assess their fit in STEM. Implications for enhancing women's participation in STEM are discussed.