We leverage variation within the timing of girls’s schools’ transitions to coeducation all through the 1960s-2000s to review how publicity to a gendered social surroundings impacts girls’s human capital investments. Making use of occasion research and artificial management analyses to newly collected historic knowledge, we discover that the share of girls majoring in STEM at newly coeducational schools declined by 2.zero proportion factors (24%) after ten years of coeducation. Coeducation induced a big improve within the male share of the coed physique, however didn’t measurably affect the male share of college, capability constraints or the flexibility composition of feminine college students. A easy extrapolation of our major estimate means that gendered peer results can account for 34% of the gender hole in STEM majoring. These findings help the speculation that non-pecuniary components, associated to social norms and gender roles, form gender gaps in main selection.