but keep in mind that many academic fields are still mostly male even though most new undergraduates are female in the area
Well, keep in mind that 75% of LWers are under 31 anyway, so it's the sex ratio among the younger cohorts you mainly care about, not the sex ratio overall.
But it isn't the undergrads you're looking for if you want the "smartest mostly female group." Undergrads are less bright on average than advanced degree holders due to various selection effects.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.