OK, putting on my demographer hat here to distinguish between fertility (the number of children a woman has in her lifetime) and fecundity (the physical ability to bear children). It's only been in the last hundred years or so that educated women had much control over the number of children that they had. Shorter life spans and infant mortality rates have also been pretty high in the past.
In a traditional farming society, the more kids the better, especially at harvest time. When a population becomes urbanized, begins working in factories or offices and living in apartments, the cost-benefit analysis begins to work against large family size.
Research suggests that the genetic component of IQ only manifests with, basically, a middle-class lifestyle, which was not attainable for the vast majority of humans for the vast majority of history.
Most people in the world, most people in this country, have little idea what their IQ might be.
Of course, I am an outlier, having grown up on a farm with a large, working-class family, and having a mentally retarded brother with whom I share two parents.
It struck me several years ago that raising children has become something like jury duty among the educated classes: one of the most important functions in our society, we leave up to the people who can't come up with a way to get out of it! It is a gross exaggeration, of course!
And, as one last note, most men in the US are looking for a women who is *almost* as intelligent as themselves, which has done quite a bit to keep this PG female out of the gene pool. Quite a bit of that endeavor, she's done on her own. Intensity played a role.
