I have heard that undergraduate philosophy majors are some of the most
Just on the basis of my own experience, it does indeed seem to be the case that a disproportionate number of undergraduate philosophy majors in coed institutions of higher education are male. (The same disproportion is to be found in the profession itself.) I'm not sure whether the data has been collected, although you might just do a simple Google search to see if anything comes up. I can only speculate why such a disproportion exists. It may in part have to do with the fact, noted above, that the overwhelming majority of faculty members in philosophy departments are male; it may have something to do with the nature of philosophy itself, which, on account of its focus on arguments, can often be seen as combative--although, of course, it need not be, and at its best, probably should not be--and such intellectual combat seems to be coded male. Philosophy courses may be seen as part of an 'argument culture' that puts off certain female students while attracting male students, therefore accounting for the disproportionate number of male as opposed to female undergraduate philosophy majors. For a perceptive exploration of the nature of and problems associated with the argument culture, you might check out Deborah Tannen, The Argument Culture: Stopping America's War on Words: Chapter 6, "Boys Will be Boys: Gender and Opposition," treats the issue of the relation between gender and the 'argument culture'.