An article in today's Toronto Star summarizes recent research on why there are fewer women in mathematics than men. In summary paragraph at the beginning of the article it says:
After wading through more than 400 academic articles on sex differences in math, they've concluded that it's not a lack of ability that keeps women from pursuing careers in engineering or computer science. Rather, it's a variety of factors, including a female preference for more people-oriented professions and significantly different lifestyle needs, such as having and raising children.
I'm not sure I like the article after that because the remainder tends to introduce more stereotypes than it tries to dismiss.
More technical versions of this summary can be found at: