We investigate the link between how male-dominated a field is, and gender bias against women in this field. Taking the entrance exam of a French higher education institution as a natural experiment, we find that evaluation is actually biased in favor of females in more male-dominated subjects (e.g., math, philosophy) and in favor of males in more female-dominated subjects (e.g., literature, biology), inducing a rebalancing of gender ratios between students recruited for research careers in science and humanities majors. Evaluation bias is identified from systematic variations across subjects in the gap between students' nonanonymous oral and anonymous written test scores.