There is a gender imbalance in undergraduate economics departments with most departments educating a strong majority of young men. This imbalance has led many economists to ponder the question of why relatively few women choose to take courses and major in economics. Our hypothesis is that the gender imbalance in undergraduate economics, particularly at institutions with traditional liberal arts curriculums, is partly explained by women's greater sensitivity to grades, particularly lower grades. Students choose their majors based on both their interests and their abilities. The literature indicates that the grade a student receives in an introductory class relative to grades received in other departments is one of the strongest predictors of whether or not the student chooses to enroll in more courses in the discipline. However, our hypothesis is that men who take economics courses are less responsive to this signal than are women. As a result, men who do poorly in economics are more likely to continue in the major. Women who do poorly, in contrast, are more likely to abandon economics and pursue a different major. Our results, generated from 16 years of data from a liberal arts college where economics is one of the most popular majors, support this hypothesis. The overall economics GPA for female majors is significantly higher than that for males and male students dominate the bottom of the grade distribution. Finally, results from the estimation of a series of selection models of the decision to take more economics courses indicate that, holding other characteristics constant, women are more responsive to the relative grade received than are men. (Contains 6 figures and 2 tables.)