Arnold Kling writes:

The story I tell for bimodalism is mating behavior. When high earners marry high earners, class divisions will emerge. But this has implications for the IQ distribution. One would expect bimodalism to appear in the IQ distribution, with the children of high-IQ parents tending centered around one mode and the children of low-IQ parents centered around another.

Kling’s expectation depends on our assumptions about the nature of the assortive mating.

Take an extreme example, where mating is perfectly assortive and everyone mates with someone of the same income and intelligence as themselves. If intelligence is perfectly heritable, their children’s intelligence will be the same. Intelligence in the next generation will only vary to the extent that people of different levels of intelligence have different levels of fertility. If there is no difference in fertility, the distribution of intelligence will be the same one generation to the next.

So how could we generate a bimodal distribution? One way would be if there is some threshold level of intelligence that acted as a barrier to mating. If, for example, those of below average intelligence only mated with people of below-average intelligence (although not necessarily the same intelligence as themselves), while those with above-average intelligence only mate with others in their group, the two populations' intelligence will cluster around different means.

For this threshold to exist, there must be some non-linear returns to intelligence. There might be competition to live in certain suburbs or attend certain schools. Those that scrape into the high intelligence college get access to much larger rewards and access to significantly more intelligent mates than those that narrowly miss out.

As for what is actually occurring, I do not expect that the existing mating patterns in developed countries such as the United States will result in a bimodal distribution of intelligence. The focus on assortive mating masks the huge level of mixing that currently occurs. Consider the assortive mating that occurred as humans spread themselves across the globe and mated within their small bands. Populations were separated for millennia. Even in recent centuries, people largely mated within their small communities and class. Today’s population has a level of dynamism and mixture far beyond most of human history, regardless of what class divisions there now are.