The Mommy Track has gained traction over the last 15 years.

A new study released by U.C. Berkeley's Haas School of Business shows that a surprising number of women who got Harvard MBA's have dropped out of the work force to become stay at home moms by their 15-year reunions.

The study, titled "Opt-out Patterns Across Careers: Labor Force Participation Rates Among Educated Mothers," followed the career trajectory of nearly 1,000 women who graduated from Harvard between 1988 and 1991. Berkeley professors Jane Leber Herr and Catherine Wolfram tapped a rich vein of biographical data culled from the school's 10th and 15th anniversary reunion surveys.The study showed that by the time they are 15 years out of college, 28% of the Harvard women who went on to get their MBAs were stay-at-home moms, compared to only 6% of women who got medical degrees. Even lawyers stayed in the game longer, with only 21% choosing to get off the track and stay at home.

I have a couple of theories about this, not the least of which is this: Working 100 hour weeks sucks.

The world of business and finance is fueled with testosterone. It takes a certain kind of man to succeed here, and if you're a woman with those same ambitions, you'll just have to accept that it's a man's game and play along. That means no time for relationships, no time for home and family, and certainly no time to bear and raise children. That's women's work. (And you don't get a wife, like your male counterparts do, to help you with any of this). But if you devote your all to your financial career, then maybe you'll succeed and make buckets of money. If that's your idea of a happy, fulfilling life, then no, there are no glass ceilings, as my colleague Tracy Coenen argues.

But back to my larger point: Working like that doesn't make many people happy. Least of all women. Which leads me to my second point. And I'm just throwing this out there, even though it's going to make some people mad.

It goes without saying I'm a feminist. But at the same time, I believe men and women are different creatures, and no matter how hard we try to swallow our early 21st century post-modernistic feminist theory, we just can't make it jibe with our inborn feminine bent. That is to say, in general, many women want babies. In general, many women want to make a nice home. In general, many women want to focus on family and community and find these activities more satisfying than driving the latest M&A deal through.

Some women would rather be the CEO of Google. And God bless them.

For the rest of us, there comes a time when you have to heed the call of biology, which generally speaking, clashes with your career focus. Babies tend to suck all your energy, emotion and time. Working part-time is an option, but it's hard to do both well, especially if you have small children. Ultimately, if you're in a position where you can afford to not work outside the home, well, it's an option that starts to look more and more sensible for everyone involved.

So no, I'm not at all surprised that female Harvard MBAs drop out of the work force more than lawyers and doctors, whose fields seem to offer somewhat more flexibility. Also, women with Harvard MBAs tend to marry professional men, who likely can afford to give them the option of staying home and taking care of home and hearth.

Studies like this do tend to be incendiary. However fascinating this slice of life is, we'll have to hear about how these women are "wasting" their degrees and forgetting how an earlier generation of women fought for the right for them to even attend Harvard. I don't think anyone with a Harvard MBA (or any other degree) is "wasting" anything by choosing to raise a family. Because as anyone with kids knows, raising them can make those 100 hour work weeks seem like a cake-walk in comparison.