Although women are underrepresented in the ¿eld of economics, many people see little need for intervention, arguing that women are inherently less interested in economics, or are less willing or able to acquire the math skills needed to do well in the subject. At the same time, others support active efforts to increase the number of women in the ¿eld, pointing to other possible causes of their current underrepresentation. These people argue, for example, that women are deterred from entering the ¿eld because of a lack of female role models, or that women are discouraged by an unappealing classroom environment. This study attempts to assess these hypotheses. We examine the factors that influence undergraduate students¿ decisions to become economics majors by analyzing a survey of students in the introductory economics course at Harvard University as well as data on an entire class of students from Harvard's registrar. We find that although women in the introductory economics course at Harvard tend to begin the course with a weaker math background than men, math background does not appear to explain much of the gender difference in students¿ decisions about whether to major in economics. The class environment and the presence or absence of role models also do not explain much of the gender gap. On the other hand, women do less well in economics relative to their other courses than men do, and controlling for this difference in relative performance significantly diminishes the estimated gender gap. An economically large, but statistically insignificant, difference between sexes in the probability of majoring in economics remains, however. This remaining gender gap may be due to differing tastes or information about the nature of economics. As evidence, we find that women who were considering majoring in economics when they began introductory economics were about as likely to choose economics as were men.