Sheryl Sandberg gave a good talk on a very similar topic at Ted. Her conclusion was that women tended to "check out" of work sooner then men.
One example she gave was that when women start trying to get pregnant, they will often stop applying for promotions because they know they'll be leaving work soon.
This is also the reason that the BLS statistics that claim men make more than women for comparable work is misleading. Women take a leave for a few years, downshift their careers, or purposely pick easier career paths so they can spend more time with children and family. For women who choose to continue their career at top-speed, they have a huge advantage. Since companies are looking to promote more women, they choose from the smaller pool of women who want to climb the corporate ladder.
One example she gave was that when women start trying to get pregnant, they will often stop applying for promotions because they know they'll be leaving work soon.
http://www.ted.com/talks/sheryl_sandberg_why_we_have_too_few...