That’s kind of a bold claim, as it always seemed to me that the ‘smart people aren’t having kids while dumb people are’ wasn’t about eugenics, but about cultural norms- dumb parents aren’t going to encourage critical thinking, going to school, valuing intelligence, or any number of things to their kids. Moreover, dumb parents aren’t going to want, or be able, to provide their kids the sort of resources needed for that kid to shine even if the kid DID want to go against the local grain and focus on those things, because dumber people generally tend to make less money.

More to the point, lack of intelligence is, generally, more based in environment and means, not personal ability- people usually aren’t dumb because they’re inherently dumb, it’s because they lack teaching and resources, or are in an environment that discourages intelligence. The most common indicator of this is the local economy- a brutal catch 22 of being poor meaning there’s worse resources for people, who cannot get ahead and thus end up poor themselves.

There’s an old joke in the US that you can easily divide people up by their earning potential with a single simple number- their zip code. The same thing applies to test scores.