I was a CS major in college, and during my junior year I picked up an Econ degree, as the dot com bubble had burst (this was in the fall of '01). After seeing the CS ratio at around 85%-15%, I figured Econ would be somewhat better, maybe 65%-35%. Nope. The lower level classes had decent ratios, but once you got up to the banking and corporate finance classes the ratios were just as bad in CS. Having worked in both IT and finance professionally, I can tell you it's no better in the working world.
Why? Partially the culture -- I know I wouldn't want to enter a field that was considered "feminine" with 80%+ women. A lot of it is also due to the life you're forced to lead in high stress, long hour jobs. My current job is at a company with a pretty high reputation, and almost all of the women who work in the "thinking" positions are either single or married with no children. It's tough working 50+ hours a week while trying to advance if you're raising small children (while things are changing, when push comes to shove women are still the primary parent for child-rearing, even in two-income families).
My girlfriend got her degree in engineering and is using it now, but she'd eventually like to work in a non-profit/environmental capacity. I'm sure she'd do well in higher stress, longer-hour jobs if she wanted to, but that's not her desire. The women I know who have chosen that life do just as well as men do, but for some reason many talented women stay away from those jobs. Frankly, I think they've got it figured out better than we have.