Undergraduate degrees are only one data point: Only 30% of PhDs are awarded to females. Math scores on SATs are higher for males than females and the top 1% scorers are 2-to-1 male to female. Males also excel in the ability to mentally rotate figures in three-dimensional space. [1]

The source concludes:

> The relatively small numbers of women in math-intensive careers result from many factors, one of which seems primary and should be accorded the greatest weight. Sex differences in mathematical and spatial ability, although substantial, appear unable to explain most of the shortage. Nor can the shortage be attributed to current discrimination, although historic discrepancies may be explained in such terms. The primary factor in women's underrepresentation is choices both freely made and constrained by biology and society. Women choose at a young age not to pursue math-intensive careers; few adolescent girls express desires to be engineers or physicists, preferring instead to be medical doctors, veterinarians, biologists, psychologists, and lawyers. Females make this choice despite earning higher math and science grades than males throughout schooling. Although women earn a large portion of baccalaureate degrees in all fields of science, including mathematics, disproportionately fewer enter graduate school in these fields, preferring biology, social sciences, law, medicine, and the humanities—even when they possess math ability comparable to males. Of those who enter graduate school in math-intensive fields, more women than men drop out or change fields, and of those who complete doctorates, fewer women apply for tenure-track positions. (...)

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2997703/#!po=37...