Vaulted,
Good observation. I attribute this phenomenon to democracy. Democracy caters to the people in the centre of the bell curve, so people on the low side get pulled up, whereas people on the high side are expected to pay more taxes and perform other self-sacrificial deeds, thereby getting pulled down.
The number of children one has is related to the outlook one sees for the future. If one is getting held back and pulled down, one will subconsciously feel that he should become inferior to what he his now to fit in better, be liked and accepted more, etc. But having kids will only improve one's attributes and so the alienation will only get worse, therefore few or no kids is the result. And of course the converse is true for the dumdums -- that they're getting so many freebies and getting pulled up that they see a wonderful environment to thrive.
The only way a highly intelligent person will want to have lots of kids is if he either has unusually self-sacrificial genes, or he develops a world view which is against democracy in it's present form, preferring instead a world in which the strong get stronger more quickly and the weak get trampled on and die out.
This post probably belongs in the politics forum, but what the heck...