Many observers argue that diversity in Economics and STEM fields is critical, not simply because of egalitarian goals, but because who is in a field may shape what is studied by it. If increasing the rate of majoring in mathematically-intensive fields among women is a worthy goal, then understanding whether women's colleges causally affect that choice is important. Among all admitted applicants to Wellesley College, enrollees are 7.2 percentage points (94%) more likely to receive an Economics degree than non-enrollees (a plausible lower bound given negative selection into enrollment on math skills and major preferences). Overall, 3.2 percentage points--or 44% of the difference between enrollees and non-enrollees--is explained by college exposure to female instructors and students, consistent with a wider role for women's colleges in increasing female participation in Economics.