Depends. Why are the slack-jawed yokels not very smart? Is it because they lived in a lead-filled house as children, or on a toxic site, or did they just not get enough interaction from their parental units before they turned 3 or 4? Were they raised without decent nutrition? How poor were they, how poor was their environment? Did they live in an unsafe area, couldn’t run around outside, so watched waaaaay too much TV?
Intelligence is only somewhat hereditary; tons of other factors come into play. Some of them can be overcome, some can’t and result in permanent physical damage to the brain. But as nutrition and environment improve, intelligence goes up within a family. So, the question becomes “will the people who are currently having more children improve their parenting over the last generation?” (Sadly, my answer is ‘I’m not too hopeful about that.’)
And it’s not just that “more intelligent people have fewer children;” it’s way more complicated than that. Overall, more money and (esp. for women) more education mean fewer children. But a better economic position doesn’t automatically mean you’re smarter than the yokel, it just means you maybe went to a better school or got lucky somehow where he didn’t. Often the same is true for education. (How many idiots did I meet at Berkeley? A whole lot.)
Religious people tend to have more children than non-religious people, and that only partly correlates with education. The people who have the most children within the US now are certain types of Fundamentalists, and some of them are pretty sharp, even if you don’t agree with their religion.
Within the US, the other population that has more children than the average are immigrants. They are often less educated, but they aren’t dumb. And their children are likely to be more educated and better fed. In turn, the next generation will be even better off. So how long will it take before they are the ‘smart ones’ who are starting to have smaller families?
IMO: It’s the ‘upper classes’ of a society that have fewer children. This has been true for a very long time indeed. The next generations just have more social mobility and move up in the world (much to the aristocrats’ horror) and then their descendants start worrying about all the children the rabble are having–oh no, the stupid people are going to take over! Hasn’t happened yet.
DangerMom goes over and turns on the TV Oh. Wait. It already happened and the stupid people are running the world. Never mind. We’re doomed.