What really are the barriers to women staying/progressing in science?

The Association of Women in Science have  just relesed their "2011 Snapshot of Women in Science". The statistics are pretty sobering, and I wondering what makes them so?  What are the fundamental underliers of these statistics?  Where I work in the Science Faculty there are 7 female professors, in my School alone there are 9 male professors.

Is it really as simple as issues of families?  It is hard to break into a male dominated environment?  I don't think either of those is true since conutries such as Spain, France and Portugal have higher numbers of women in science?

So why don't we?