Almost all have exactly two kids.
Thank bob it's not exactly two point five.
I think it's because science tends to be a more intense kind of work than the humanities--or I should say different. Science sometimes requires long hours in a lab. Sometimes it means a few years of post-doc work. If these scientists pair up--and as you say, it appears they often do at some point they decide to have kids because after 8 years of grad school and 3 years of post-doc, they're now 33, so that's reasonable, yes. Also, during this grad-school, post-doc time in which the scientist/housewife couple have kids, they may also buy a house and settle down, often in a sleepy college town where there is no distinction between downtown and the suburbs--think Charlottesville, VA or Ithaca, NY. So they get used to a house and yard and look for that when they finally get a t-t position. It also seems that humanities and social sciences people, but especially humanties folk seem to pair up with each other.
I have to say that I've been at several different institutions and this dynamic seems to play out. I'm a woman married to a (male) scientist and I'm very, very often the only woman who works.
School is a big issue in a city. If you didn't plan to land in a big city with a horrible school system and didn't plan on private school, it's nearly impossible not to live in the suburbs. I mean, it costs as much as college. Kids makes having the "right" politics harder, I think. At any rate, it complicates things, all the way around.
Ah, GM. I knew someone I know would see this. You know I mostly wasn't talking about you, right? Just the minivan.
For your information, it's the racing van. And that was Mr. Geeky's idea. You see, we're just dragged down by the Dilbert-types we associate with.
I think geekymom has a good point, but another explanation is worth considering.
Contrary to the thesis of a war between religion and science, the scientific disciplines were really the last segment of the academy to lose God. It is also related, I think, to the fact that science arose originally as a form of devotion and worship among religious men and priests. This is something which leaves its imprint on modern scientists.
For example, even though literary criticism of the Bible made the whole "Moses wrote it" idea improbable a long time ago, it wasn't until very recently that iron-age Israeli archaeology has moved away from the fit-it-to-the-text maximalism of the Albright (Harvard) school. It hasn't done so entirely even to the present day. There are probably more fundamentalist Levantine archaeologists today than there are fundamentalist bible studies profs.
Physicists also talk about God all the time, even here at Caltech (especially here if you want to know the truth). It gets annoying, actually.
Even in biology, while the intelligent design movement is more of a political grouping, it is really the fear of creationist misquotes more than anything else that keeps biologists from speaking with awe about the wonder of life. In fact, I'd say that more english professors use evolution to irreligious ends than do actual evolutionary biologists.
This is what helps to put scientists more in the mainstream than women liberation theorists, it is the whole Jesus-hashem-allah thing (even when it isn't an explicit church-shul-mosque affair).
Hokay, so it appears that I'm not making my politics personal, since I choose to live in a place where they automatically hand you a Republican primary ballot at the polls. I also am not taking my minivan in weekly to have large door-dings removed.
Even tho we're non-academic scientist-engineers and neither of us has a Ph.D., geekymom's got two things right: you look for a place like what you're used to (we're children of American-Dreamers) and it's all about the kids.