From what I've observed on my local scene (southwest Montana) there are more men playing fiddle, but the fewer women players are usually more accomplished. There are no women banjo players (5-string) and several men, although one woman played banjo a few years ago, and she was very good. There are no women mandolin players and several men. There is one woman bass player, very good, and about three men also good. There are a great number of male guitar players and slightly fewer women. The men are by far better players, both lead and rhythm. In the past there were a couple of women autoharp players, but no men. In terms of playing and singing there are about equal numbers men and women, about equally good.

I think that women are better at fine hand coordination - hence the better fiddlers - but have less brute strength to press down the strings, especially to make barre chords, on guitars with heavy string guages and high actions. I also think its cultural(here)that certain instruments (banjo) are a "man" thing, while fiddles and autoharps are a "woman" thing. Guitars seem to be considered fair game for both genders. I do think there is a testosterone/ show-off factor in getting up to perform in public and find that women who will sing in public are usually very good, whereas the men are all over the place.