There exists a notable gender gap in the field of economics. One explanation for this gap is the low supply of women entering the economics labor market. To understand the shortage of female economics students, I observe students at the undergraduate and graduate level. My data consists of a sample of current undergraduate students and a sample of past Ph.D. applicants at Arizona State University. The gender gaps in these samples, both at the undergraduate and graduate level, can largely be explained by the variation in mathematical preparation of the students. The data reveals that undergraduate male economics students are more frequently enrolled in higher level math courses compared to female undergraduate students. Likewise, a higher number of male Ph.D. applicants have stronger mathematical backgrounds relative to female Ph.D. applicants. This common factor might explain the higher supply of male students who apply and get accepted to postgraduate studies in economics, relative to female students, holding all else constant. I conclude with the following recommended interventions: make information regarding postgraduate opportunities in economics more readily available, and increase math requirements for a bachelor’s degree in economics at ASU.