Those are findings from Eric Schwitzgebel (UC Riverside), who notes:
More details here.
In the United States, “among students whose first major is philosophy, 36% are women” while “among students whose second major is Philosophy, 43% are women.”
He also finds that: “schools in which a second major [in general, not just philosophy] is common also tend to have philosophy majors that are more evenly divided between men and women.”
[Félix Armand Heullant, “In Gedanken”]