No one paper can determine a debate, but each contributes to a pattern, and eventually to a shifting of opinion as to where the probable truth lies. Until 2011 the studies of the genetics of intelligence were based on twin studies, which are fine; and adoption studies, which give some indications if the samples are of reasonable size and followed for long enough; and admixture studies, which give a rough indication of the likely size of the genetic effect.
The hypothesis that there is a genetic component to all human differences does not stand or fall by a single observation, but to a pattern of results.
At a very simple level, it can be observed that parents have children who share their inherent characteristics, most notably that parents transmit their ancestral group characteristics. That transmission includes many aspects which are more than skin deep, which include differences in brain which lead to differences in behaviour.
At a more complicated level, it can be argued that the genetic transmission of characteristics and behaviours is strongly influenced by the environment, and that the effects of genetics will be less powerful if environments are bad. If so, this could complicate the investigation of genetic factors in racial differences in intelligence. Do heritability estimates vary by race?
This was reviewed earlier this year by Pesta and colleagues:
Racial and ethnic group differences in the heritability of intelligence: A systematic review and meta-analysis
Bryan J.Pesta, Emil O.W.Kirkegaard, Jante Nijenhuis, Jordan Lasker, John G.R.Fuerst
Via meta-analysis, we examined whether the heritability of intelligence varies across racial or ethnic groups. Specifically, we tested a hypothesis predicting an interaction whereby those racial and ethnic groups living in relatively disadvantaged environments display lower heritability and higher environmentality. The reasoning behind this prediction is that people (or groups of people) raised in poor environments may not be able to realize their full genetic potentials. Our sample (k = 16) comprised 84,897 Whites, 37,160 Blacks, and 17,678 Hispanics residing in the United States. We found that White, Black, and Hispanic heritabilities were consistently moderate to high, and that these heritabilities did not differ across groups. At least in the United States, Race/Ethnicity × Heritability interactions likely do not exist.
These are large samples. Studies of this type compare the relative contributions of heritability (a narrow measure of the additive effect of genes) and everything else: “environmentality”, which has two components: the common, shared circumstances of family life which make families alike, and the unique, personal accidents of fate, and the partly self-created experiences which make family members different; plus plain old error of measurement.
A is for additive genetic inheritance, C is for common family factors, and E is for exceptional events and error. This is the ACE model. It is relatively simple, and if there are any interactions between the three components, these can be identified and measured.
Results for the general population show that the proportion of variance in IQ explained by genes increases with age (Plomin et al., 2014). Specifically, in early childhood, genetic effects explain less than 50% of IQ variance, and the effect of the shared environment is relatively strong. As children age, though, genetic effects become increasingly prominent, and the environmental variance due to factors common to siblings decreases. In adults, the heritability of intelligence is 60–80%, while the effect of common environment is small, if not zero (Plomin et al., 2014). The unshared environment explains the rest.
This is the astounding recent finding which would have confused just about all researchers in the 1960s, including myself, who expected that the common effects of family life would be extremely strong, as the sociologists claimed.
Are these findings true for poor people, whose environments are poor at nurturing talent?
One putative moderator is the quality of one’s environment. Poorer (richer) environments supposedly correspond to lower (higher) heritability, to a presumably measurable degree. Said differently, “natural potentials for adaptive functioning are more fully expressed in the context of more nourishing environmental experiences” (Tucker-Drob & Bates, 2016, p. 1). This prediction is known as the Scarr-Rowe hypothesis (Scarr-Salapatek, 1971; Turkheimer, Harden, D’onofrio, & Gottesman, 2011).
How does this apply to race differences in intelligence? Most people who have paid even passing attention to a century of data now accept that there are racial differences in intelligence, but there is less agreement as to how much of that difference can be attributed to genetics. If bad environments reduce the heritability of intelligence for poorer races, this would buttress the hypothesis that their poor performance is largely due to poor circumstances. Improve those circumstances, and performance should improve quickly and substantially.
For example, Lee et al. (2018) found that they could predict intelligence (or about 13% of it) in Europeans just on the basis of genetics. When they used the European prediction on Africans, they predicted 1.6% of it. Not much, you may say, but Lee et al. explain that all European based predictions lose accuracy when used on Africans, because the snips of genetic code may not be at exactly the same position in the sequence, a problem which goes by the daft name of linkage disequilibrium. (The individual variations which make you unique stand out from the usual pattern for your racial group, so your sequences are a little bit out of equilibrium from the rest of your tribe). The right musical notes, but not necessarily in the right order, as an English comedian said when his dreadful piano playing was criticized by conductor Andre Previn.
The Lee et al. explanation makes sense to me, but it could also be argued that it is the lousy environments which account for the lack of relationship between genetic markers and intelligent outcomes.
In a process which took several years, the authors went through all the studies which had investigated racial differences in intelligence, and had sufficient data to make ACE calculations.
What did the authors find?
There are many comparisons and tables of results, but the most contentious are the black-white comparisons, since those are assumed to have the biggest differences due to historical and cultural factors, and those show no differences in heritability. Despite cognitive differences (White-Black mean d = .83 and White-Hispanic mean d = .60) there were no heritability differences in either case. This damages the argument that a substantial cause of low ability is poverty and poor environments. It makes it more likely that the differences are due to inherited genes.
In conclusion, our meta-analysis reveals that the heritability of cognitive ability is generally moderate to high for Whites, Blacks, and Hispanics in the United States. The other groups featured here (e.g., Asians) had sample sizes that were too small to allow making strong conclusions. We also found that differences in heritability across these three groups were mostly trivial. Nonetheless, we cannot rule out the existence of modest differences in population parameters in our analyses. We can, however, conclude that the correlations between phenotype and genotype are essentially the same for Whites, Blacks, and Hispanics residing in the USA.
This is an important finding, because some researchers have argued that “environmentality” is a major factor which diminishes the real-world effect of genetics. In crude terms, some proponents of this argument seem to think that high heritabilities restrict how much change can be achieved by social improvements. They might be right, but social improvements benefit everybody, even though they raise all boats, without necessarily annulling the differences between those boats. Some ride higher out of the water than others. The authors have shown that the heritability estimates hold up equally for white and blacks, and for whites and hispanics.
As always, we have to put in a caveat that more studies might show something different. A good way forward would be to collect much more data on African intellectual and scholastic attainments, and to link that to African DNA. How long will it take to gather that for 1.1 million Africans? Then we can compare European to African predictions with African to European predictions, which should increase our understanding considerably. Until that happens, perhaps we can improve the European to African prediction by doing more work on the gene linkages.