I often wonder whether intelligence, education, and affluence are evolutionary dead-ends.
I'm not sure those are on there own--but they are when fused with both liberalism and materialism. Materialism definitely contributes to the lack of children. People want stuff, stuff costs $$, and kids are expensive and troublesome. So if they can't afford both, what do they choose? The stuff, of course. [Not that they really need that BMW, but that's beside the point--they want it!]
Liberalism, of course, gives them the out for making that choice. Sure, don't have those kids. Your gay friends will call you a breeder and there are too many exploitive white people anyway. Besides, having kids makes it tougher for women to climb that career ladder (which is the only real path to happiness). Liberalism is also responsible for higher taxes, which also makes it seem more expensive to have kids.
I can't even begin the tell you the number of smart people I have met who don't have kids, but have the car, house, vacation home, eat out 3X per week, etc. It's definitely reverse Darwinism.
At Thanksgiving, my aunt remarked about how many of her kids friends had at least three kids. I think this is just one more part of the swing back to the right in our country, but it is still a largely silent group. They want a big family but they don't want to talk about it a lot.