Analyzing university faculty and graduate student data for the top-ten U.S. economics departments between 1987 and 2007, we find that there are persistent differences in gender composition for both faculty and graduate students across institutions and that the share of female faculty and the share of women in the entering PhD class are positively correlated. We find, using instrumental variables analysis, robust evidence that this correlation is driven by the causal effect of the female faculty share on the gender composition of the entering PhD class. This result provides an explanation for persistent underrepresentation of women in economics, as well as for persistent segregation of women across academic fields.