Let's imagine that the number of female students did not decrease. Now the question is, why the boys increased much faster than girls? You can't "angle" that inequality away.

Besides, I wager that the number of students of any gender increased quite significantly in the last decades. So the fact that one gender stagnated in absolute numbers is less interesting than the fact it decreased in relative terms. Absolute numbers would only be interesting if the total stayed relatively constant.

Really very odd graphs of both male and female students. I haven't seen anything that really explains the peaky total number let alone the ratio profile.

Interestingly the higher degrees seem to be pretty stable in ratio terms. Not sure what they may be indicative of though.

Very strange.

[1] https://nces.ed.gov/programs/digest/d12/tables/dt12_349.asp