Phillip Greenspun addresses the "why aren't there more women scientists" via the more general question: why aren't there more scientists, period. He gives an example:

How closely does academic science match these criteria? I took a 17-year-old Argentine girl on a tour of the M.I.T. campus. She had no idea what she wanted to do with her life, so maybe this was a good time to show her the possibilities in female nerddom. While walking around, we ran into a woman who recently completed a Ph.D. in Aero/Astro, probably the most rigorous engineering department at MIT. What did the woman engineer say to the 17-year-old? "I'm not sure if I'll be able to get any job at all. There are only about 10 universities that hire people in my area and the last one to have a job opening had more than 800 applicants."

That addresses the problem of why so few people (in general) enter the field. It doesn't speak specifically about women though. All I have is anecdotal evidence - my sister went into aerospace (not academic - she got a job in industry straight out of university). The pay was good, the hours no worse than for any other professional (like, say, cs). She left the field completely after less than 10 years in. Why?

She had her first child, and decided that she couldn't stand having her daughter being partly raised by other people (day care). She and her husband ran the numbers, and decided that they could afford to have her quit and stay at home. Is this something all women do? Not hardly - a quick glance at the number of women working will tell you that. However, it's still a much larger number of women who downshift from work after having a child, compared to men. For the purposes of wondering why there aren't more women in some highly competitive fields, it doesn't even matter why that's the case - given that it's true, more men are going to rise to the top in those fields simply on a numerical basis.

Greenspun's examination of pay and working conditions do play a major role. If you're planning to have a family, pay does matter, and so do the working hours. My brother in law got himself a PHD a few years back in the bio-tech field, and we watched how he got treated (by his professor) while he was doing his research. Believe me, it wasn't well. The hours he put in were long, and the pay was beyond insulting - it came pretty close to serfdom, in my opinion. Greenspun elaborates on the pay problem:

Even a public schoolteacher actually does better than a scientist. Consider the person of unusual ability who takes that bachelor's in science and decides to become a schoolteacher instead of going to graduate school. At age 22, the schoolteacher is earning a living wage and can begin making plans to get married and have children. By age 30, when the scientist is forced to start moving around to those $35,000 per year postdocs, the schoolteacher is earning $50,000 per year. By age 44, when the scientist is desperately trying to switch careers, the schoolteacher is making more than $90,000 per year for working nine months (only the better school systems pay $90,000 per year, but remember that we posited a person with a high IQ and motivation sufficient to get through graduate school in science). Being a public employee and a member of a union, the schoolteacher cannot be fired but may at this point in his or her life begin thinking about a comfortable early retirement and some sort of second career.

I'd question his "living wage" for a teacher at 22 - I took that track back then, and I was making $14,500 a year at 22. I couldn't afford an aprtment of my own, and I was living in the New York suburbs, not in a big city. Even so, working conditions were better than what my brother in law faced, and - had I stuck with it - the pay would have reached a tolerable level well before I hit 30.

Bottom line, multiple things enter into the disparate numbers of men and women in fields like academic science. If I had to guess though, the working conditions of those first few years of doctoral and post-doctoral work have a lot to do with it. From the outside looking in, it looked a lot like what medical interns deal with at hospitals, only without the promise of better pay down the road.

So who does go into these fields? Read Greenspun's essay, and see what the decision making process looks like to someone from China or India (etc). Which explains why you find so many people from those countries in the sciences quite well, I think.

Read the rest of his article - I think he explains the disparity quite well in the summary part of his essay