The problem with these theories is that for most of the past several thousand years most people in this world were growing crops, herding animals, catching fish, or maybe working some sort of skilled trade making things for (and hence living among and marrying the offspring of) farmers, herders, and fishermen. Yes, there have been great urban civilizations here and there, and these civilizations produced great works of creativity and intellect (some more than others). However, the vast majority of the people in those civilizations were NOT living in the big city and working as poets or philosophers. They were in the country growing crops, or in the city working as tradesmen or manual laborers. And who said that farming isn’t or wasn’t cognitively demanding? I certainly can’t imagine having to juggle all those different tasks! The highly educated classes were a small fraction of the society (and probably not having more kids than their farming counterparts), so you can’t point to a few philosophers in the gene pool to argue for some sort of difference between descendants of Greek wheat growers and the descendants of Mexican corn growers, or descendants of fishermen on Crete and descendants of fishermen from the coast of Africa.
Also, genes spread. A lot.
Conquests spread genes. Large migrations spread genes. Migrations of individuals who moved for one reason or another spread genes. Genes can diffuse from one village to another as people move around and marry. They move across social classes–one needn’t postulate a huge degree of class mobility to recognize that over the span of centuries genes will move between classes as bastards and black sheep are disowned, or maids are seduced (or raped), or the lady of the manor sleeps with the gardener, or the nobleman has a girlfriend on the side, or whatever else. The bottom line is that people like to mate, and they do it while traveling, they do it across class lines, they do it across ethnic lines (even in violation of cultural taboos), and so over time genes can move around the globe.
So, genes spread, and they spread in a world where for several millenia most ancestors of most people alive today were doing similar jobs (farming, herding, fishing) and hence facing similar cognitive tasks. All of this makes it very unlikely that you’ll find big differences between racial groups (however we define those groups).
Now, maybe there are small groups, isolated over millenia by geography and/or social custom, who only bred amongst themselves and (for whatever reasons) faced unique cognitive demands (and hence unique cognitive selection pressures). However, these groups are much smaller than an entire racial group (as racial groups are usually defined). Maybe the people of the isolated valley named Lower Whatsitstan have religious traditions that emphasize study and economic practices that involves elaborate contracts. Maybe the people of the isolated valley of Upper Whatsitstan are dumbasses who live in a lush climate where survival is easy and the geography has kept outsiders from coming in and taking their stuff, so they’ve never had to outwit anybody. These groups are so small that they will have next to no impact on the average IQ of folks from Central Asia.
He goes further into the weeds in another post:
I’m trying to argue that if/when the first “intelligence gene” is discovered, whether that gene happens to be more common in one group or another is largely irrelevant for predictions about those groups. The first “intelligence gene” discovery will not tell you about any other “intelligence genes.” So we find Intelligence Gene #1 and it has whatever distribution among groups. That won’t tell us whether Intelligence Gene #2 (yet to be discovered) has a different pattern among groups.
By analogy, suppose that you studied the shark genome, and identified the genes that are responsible for it having fins. Suppose that you found that dolphins are genetically different from sharks. Would it follow that dolphins cannot possibly have fins? No, because dolphins found their own ways to evolve fins. So, if two groups of people faced similar selection pressures (because most of their ancestors performed similar tasks) and you find that Group #1 is somewhat more likely to possess a particular gene, it doesn’t mean that Group #2 didn’t evolve something that performed the same task.