And on the other side, America's best and brightest women don't seem to reproduce at all. I went to a pretty good college, so I met a lot of smart women who entered all fields. 30 years on, I keep up with quite a few college friends on facebook. I guess at least 30-40% never married and never had kids. They are all about their careers in business or government. The rest got married at some point, but only one has more than 2 children (3 actually) - and she's still a very successful lawyer.
It's the "you can have it all" problem. A full career and a well-cared-for family are hard to pull off in a 24 hour day. A few women can do it, but not many.
The wife was going to be a lawyer (passed the Bar, in fact), but once she got pregnant and those hormones hit, she made a choice. She hasn't regretted choosing family.
I know it doesn't work the same for everyone, of course.