Polygenic scores and Black Americans

 

Sunday Morning in Virginia (1877), by Winslow Homer. 

The ability to acquire language may be the mental domain where people of sub-Saharan African descent have undergone the most cognitive evolution since their separation from other humans.

 

 

 

If we look at SNP alleles associated with educational attainment, we see differences between Europeans and sub-Saharan Africans. In a previous post I asked whether the cause was genetic drift or natural selection (Frost 2021).

 

That post brought a comment on Twitter:

 

Or maybe the fact that educational attainment is based on whiteness and familial wealth in the USA but not in Africa? And familial wealth tends to be concentrated in specific closed groupings of people who only breed with each other?

 

I don’t think so. First, the alleles were identified in subjects from the Netherlands Twin Registry, the Finnish Twin Cohort, the Swedish Twin Registry, the Avon Longitudinal Study of Parents and Children, the UK Biobank and 23andme. Of those sources, only 23andme had American subjects.

 

Second, let's suppose that those alleles are incidentally related to educational attainment. Maybe they are just something that wealthy Europeans share with each other through inbreeding, a bit like the Habsburg jaw. Those alleles should therefore be useless for predicting educational attainment in other populations. Are they?

 

Let me answer that question by discussing two recent studies:

 

 

The Guo et al. study

 

Guo et al. (2019) used the same alleles to predict success on a cognitive test (verbal ability) by 8,078 Americans of different ethnic backgrounds. Two polygenic scores were calculated: one based on alleles associated with educational attainment (education PGS) and the other based on alleles associated with IQ (IQ PGS).

 

The polygenic scores significantly correlated with test results for all major ethnic backgrounds, except one:

 

The education PGS was significantly predictive of verbal ability in all estimated models and its coefficients were similar in size except for the black sample in which the coefficient was much smaller. The IQ PGS significantly predicted verbal ability in all samples except the black sample. (Guo et al. 2019)

 

[...] The incremental R2 s or the R2 s of "pure" PGS effects were 1.8%, 0.1%, 1%, 1.8%, 1.7%, and 1% for whites, blacks, Asians, Hispanic whites, the combined sample and the overall sample, respectively.

 

The literature is showing a consistent trend: polygenic scores have much less power to predict cognitive ability in people of sub-Saharan African descent than in people of European or Asian descent. In this case, the polygenic scores were ten to eighteen times worse at predicting verbal ability in Black Americans than they were at predicting verbal ability in White, Asian, and Hispanic White Americans.

 

Why? The reason may be that Eurasians and sub-Saharan Africans have different gene pools. Some alleles for higher cognitive ability are available in one gene pool but not in the other. There is undoubtedly overlap between the two, but not total overlap. Intelligent Nigerians, for instance, may owe their intelligence to alleles that exist only in sub-Saharan Africa.

 

To return to the Twitter comment, it seems clear that polygenic scores are predicting something that correlates with cognitive ability, and that "something" is not an artefact of wealthy people being related to each other and sharing the same genes. It's already a stretch to believe that close family ties are shared by high achievers throughout the United Kingdom, the Netherlands, Sweden, and Finland. Does the same family clique also include high achievers of Asian American origin? 

 

 

The Rabinowitz et al. study

 

Rabinowitz et al. (2019) used an education PGS to predict cognitive ability in Black American participants, specifically three cohorts from first grade to young adulthood (at which point their DNA was collected and analyzed).

 

The results? The PGS significantly correlated with pursuit of postsecondary education. The correlation was weak or insignificant, however, for performance on school tests. The PGS did not predict performance on a standardized reading test for any of the three cohorts, and it predicted performance on a standardized math test for only one of them. In addition, the PGS negatively correlated with having a criminal record (but only in the male subjects).

 

A problem here may be the young age of the participants. Cognitive ability seems to become less malleable and more hardwired with age. We can help children do better on IQ tests, but the improvement tends to disappear by adulthood (Frost 2008). Consequently, academic success in childhood may be too clouded by environmental factors to show a significant correlation with genetic factors.

 

On the other hand, the PGS did predict some things better than others. It predicted general academic success (pursuit of postsecondary education) and compliance with rules (absence of a criminal record). For actual school tests, it had some power to predict success on the math test but none at all on the reading test. The ability to acquire language may be the mental domain where people of sub-Saharan African descent have undergone the most cognitive evolution since their separation from other humans. The PGS cannot predict superior reading ability among Black Americans because too many of the relevant alleles are exclusive to the sub-Saharan African gene pool and remain to be identified by scientific studies.

 

The take-home message? At present, we can create polygenic scores that provide a rough idea of cognitive ability in people of sub-Saharan African descent. To get more than a rough idea, we need to identify the relevant alleles specific to that population.

 

 

References

 

Frost, P. (2008). IQ: Interaction between race and age. The Unz Review, May 20

https://www.unz.com/pfrost/iq-interaction-between-race-and-age/?highlight=Minnesota

 

Frost, P. (2021). The mismeasure of genetic differentiation. Evo and Proud, April 13

http://evoandproud.blogspot.com/2021/04/the-mismeasure-of-genetic.html

 

Guo, G., Lin, M.J., and K.M. Harris. (2019). Socioeconomic and Genomic Roots of Verbal Ability. bioRxiv, 544411.

https://www.biorxiv.org/content/10.1101/544411v1

 

Rabinowitz, J.A., S.I.C. Kuo, W. Felder, R.J. Musci, A. Bettencourt, K. Benke, ... and A. Kouzis. (2019). Associations between an educational attainment polygenic score with educational attainment in an African American sample. Genes, Brain and Behavior, e12558.

https://pubmed.ncbi.nlm.nih.gov/30793481/