A lot of discussion goes on about the reason for the disparity—whether it's discrimination, some inherent gender distinction, or something else. One phenomenon that probably plays a role is a misguided belief that for women, professionalism in math, science, and engineering demands sacrificing one's femininity. This idea strikes me as rather ludicrously old-fashioned, and I would be disinclined to believe that anyone buys into it any more—if I had not witnessed some tension over exactly this issue among some of my own acquaintances who happen to be both female and scientists. The Nerd Girls movement is explicitly dedicated to helping women choose careers in science and engineering and stay girly at the same time. See Erin Cech's interesting discussion here.
There is a place, however, where women are entirely free to become scientists and engineers and celebrate their femininity at the same time. It's the world of the 1950's science-fiction B-movie. We don't think twice about Dr. Ellie Satler in Jurassic Park (1993)—although she explicitly mentions sexism—but it's interesting to see paleontologist Lee Hunter appear in The Beast from 20,000 Fathoms during an era (1953) in which women in real life were expected to stay home, wash windows, and change diapers—and none of the characters finds her gender at all incongruous.
And these are not the girls who stayed home and studied because they couldn't get a date on Saturday night, either. They wear skirts and heels, their hair is nicely done, their nails, too—in fact, any of them could take up a second career as a lingerie model if that whole science-engineering thing failed to work out. I wonder what effect these figures on the screen had on young girls sitting in the audience?
That question will likely never be answered. But if the movies teach us anything it is this: integration of women into the ranks of scientists and engineers is a prelude to alien invasion. You have been warned.
(Above: Nuclear physicist Dr. Ruth Adams from This Island Earth pursues some extracurricular activities.)
(Update 15 February) Some late-breaking [I hate that word] news on the issue.