I've always thought that the folk scene shows a far greater involvement of women compared to many other genres (name ten famous female rock drummers, anyone?)

I imagine though there are a lot of reasons why there are fewer female performers, ranging from out-and-out sexism (such as the "we've got girls on the bill already" Anne mentioned above) to more subtle- but no less real- pressures (this makes for an enlightening read.)

For what it's worth, the most active female musicians I know personally are either quite young (with no family or serious work commitments), much older (children flown the nest, more leisure time), or have chosen between a music career or starting a family.