The importance of external assessments: High school math and gender gaps in STEM degrees

•During the COVID-19 pandemic many countries canceled exams.•We exploit the random allocation to Math exams at end of high school in Denmark.•Math exams reduce gender differences in graduation from STEM degrees.•Replacing exams with teacher assessments may have effects on human capital accumulation. We exploit the random allocation to a semi-external assessment in Math (SEAM) at the end of high school in Denmark to test the effect of SEAM on subsequent enrollment and graduation in post-secondary education. We find that SEAM in high school reduces the gender gap in graduation from post-secondary STEM degrees, and we discuss possible mechanisms. Our results show that cancelling external assessments, as was temporarily implemented in many regions during the COVID-19 pandemic, may impact gender differences in human capital accumulation in the long run.