One of the deleted 'trollgate' posts by someone else hinted that it's perhaps the case that females aren't as actively encouraged to take up musical instruments, as they are considered part of the male domain - and so, instead of playing instruments, they sing. I think also it could be that patriarchal structures dictate 'correct' and orthodox approaches to the playing of musical instruments, so when women play differently from that orthodoxy (for whatever reason) it is considered somehow 'wrong' or inferior to the playing of men, which is in turn discouraging.

I suppose that historically it'd have been even more pronounced - musical instruments might have been promoted for boys but not for girls and so those of a musical bent who had been societally ascribed to the latter category might have had no other sound-generating device available to them than the voice. That might help to explain the preponderance of great female Scottish ballad singers whom Brian lists... although I can think of a couple of Scottish ballad singers who were/are also instrumentalists (Elizabeth Stewart and Jane Turriff)...

Anyway, just some thoughts.