Women's underperformance in MBA programs has been the subject of recent debate and policy interventions, despite a lack of rigorous evidence documenting when and why it occurs. The current studies document a performance gap, specifying its contours and contributing factors. Two behaviors by female students that may factor into the gap are public conformity and private internalization. We predicted that women conform to the norm associating maleness with technical prowess by minimizing their public assertiveness in class discussions and meetings, but that they do not internalize the norm by reducing private effort. Data from multiple cohorts of a top-ranked MBA program reveal female underperformance occurred in technical subjects (e.g., accounting), but not social subjects (e.g., marketing). As predicted, the gender effect ran not through private effort but through public assertiveness, even controlling for gender differences in interests and aptitudes. These findings support some current policy interventions while casting doubt on others.