The National Center for Education Statistics has data on the gender of virtually all Bachelor's degree recipients in the U.S. back into the 1980s, publicly available through the IPEDS database. For Philosophy, the earliest available data cover the 1986-1987 academic year. [For methodological details, see note 1].
The percentage of Philosophy Bachelor's degrees awarded to women has been remarkably constant over time -- a pattern not characteristic of other majors, many of which have shown at least a modest increase in the percentage of women since 1987. In the 1986-1987 academic year, women received 33.6% of Philosophy BAs. In the most recent available year (preliminary data), 2015-2016, is was 33.7%. Throughout the period, the percentage never strays from the band between 29.9% and 33.7%.
I have plotted the trends in the graph below, with Philosophy as the fat red line, including a few other disciplines for comparison: English, History, Psychology, the Biological Sciences, and the Physical Sciences. The fat black line represents all Bachelor's degrees awarded.
Philosophy is the lowest of these, unsurprisingly to those of us who have followed gender issues in the discipline. (It is not the lowest overall, however: Some of the physical science and engineering majors are as low or lower.) To me, more striking and newsworthy is the flatness of the line.
I also thought it might be worth comparing high-prestige research universities (Carnegie classification: Doctoral Universities, Highest Research Activity) versus colleges with much more of a teaching focus (Carnegie classification: Baccalaureate's Colleges, Arts & Science focus or Diverse Fields).
Women were a slightly lower percentage of Philosophy BA recipients in the research universities than in the teaching-focused colleges (30% vs. 35%; and yes, p < .001). However, the trends over time were still approximately flat:
For kicks, I thought I'd also check if my home state of California was any different -- since we'll be seceding from the rest of the U.S. soon (JK!). Nope. Again, a flat line, with women overall 33% of graduating BAs in Philosophy.
Presumably, if we went back to the 1960s or 1970s, a higher percentage of philosophy majors would be men. But whatever cultural changes there have been in U.S. society in general and in the discipline of philosophy in particular in the past 30 years haven't moved the dial much on the gender ratio of the philosophy major.
[Thanks to Mike Williams at NCES for help in figuring out how to use the database.]
Note 1: I looked at all U.S. institutions in the IPEDS database, and I included both first and second majors. Before the 2000-2001 academic year, only first major is recorded. I used the major classification 38.01 specifically for Philosophy, excluding 38.00, 38.02, and 38.99. Only people who complete the degree are included in the data. Although gender data are available back to 1980, Philosophy and Religious Studies majors are merged from 1980-1986.