An overview of reasons for a genetic Black-White IQ gap
First, I want to state upfront the facts that are more or less agreed upon by most intelligence researchers: 1) we can measure intelligence reasonably well with IQ tests, 2) these tests are probably not racially or culturally biased (though more on this later), and 3) there are racial gaps on these tests that likely reflect a genuine difference in intelligence among groups. Lastly, I want to note that there is a consensus among intelligence researchers that genes at least partially contribute to the gap. (Rindermann, 2019)
The first piece of evidence that there could even be a genetic component to the gaps in the first place is the heritability of intelligence in the general adult population, with a value of 0.8. (Bouchard, 2013; Plomin, 2015) This is estimated using twin models, which themselves are based on the observation that monozygotic (identical) twins reared in the same household have substantially more similar test scores compared to dizygotic (fraternal) twins. Because we know dizygotic twins share 50% of genes by descent while identical twins share (roughly) 100%, it’s possible to infer genetic contribution to these differences.
0.8 here means that 80% of the variance in intelligence among US adults is explained by genetic differences (in other words, it's about as "genetic" as height). When the total heritability by age 18 is that high, it means that for environmental factors to explain group differences, the difference in environments must be very large (it does not, as is often claimed, give you a probability distribution for the between-group heritability). Crémieux calculates the required gap in environments to be 2.24 d (SD), an environmental gap which is simply not supported by the available evidence. Werkat carried out a similar analysis and found environmental variables only able to explain ~10% of the gap:
… By my most generous calculations, ~91.69% of the Black-White IQ gap is unexplainable by all of the environmental variables I could think of (see more here), including nutrition, lead exposure, education, race-unique culture/home-environment, income, the Flynn Effect, racial discrimination, racial IQ test bias, stereotype threat, and x-factors in general. In order for the between-group heritability of the Black-White IQ gap to be 0%, Blacks must have an environment at the [bottom] ~0.0111 percentile of White environment (see more here), which is implausible on its face.
Also, since shared environment (environments that would be roughly equal between siblings, such as socioeconomic status) is routinely found to explain little to no variance in intelligence among adults in the developed world (comparatively, genes and nonshared/random/unknown environment dominates), it’s probably not a significant causal factor in intelligence differences—at least in the developed world.
To infer causation of trait differences by some variable, the variable must first independently explain a lot of variance in the given trait. But it seems with shared environment we don’t even have that. So it’s unlikely things like socioeconomic status play a substantial causal role in intelligence differences in the developed world.
The second line of evidence we could point to is admixture studies:
Admixture studies are probably the most direct way to test hereditarianism, under the right conditions. US Blacks have roughly 20% European admixture, but there is substantial variation. More European ancestry in black people means higher IQ (all else equal). This was the finding of Lasker et al. (2019), and this is true even after controlling for skin color and socioeconomic status. Meaning, if we have two black people with the same skin tone and the same socioeconomic status, the one with more European ancestry and less African ancestry will generally perform better. This result has been replicated several times, and it’s similar to past findings.
Third, more heritable IQ subtests (subtests are just the types of questions on a WAIS-IV IQ test), in other words, subtests that are more a function of your genes generally have larger racial gaps than subtests that are more a function of your environments. This is often referred to as the Jensen effect. (Rushton & Jensen, 2005; ctrl-f for "Heritability data are especially informative")
Now, with all this talk of IQ testing, your mind may be drawn to the notion of cultural bias. However, this is simply a common misconception—there is no such bias. Questions that require more cultural knowledge have smaller black-white gaps, not larger ones (Naglieri & Jensen, 1987), suggesting blacks actually benefit from culturally-loaded questions (the hypothesis being that this is because culturally-loaded questions require less sheer reasoning ability). FYI, the gap does persist in Raven's Matrices, which are basically just shapes and have no knowledge requirements whatsoever, not even words:
Also if the tests are supposed to be culturally favoring whites, then it is peculiar how well east Asians do. Moreover, all races are in strange, silent agreement as to which questions are harder and which are easier. If there was bias, we would expect there to be questions where whites did abnormally well and blacks did abnormally worse, and yet, that's not happening. This is by no means a fringe position; in fact the psychological consensus is that these tests measure intelligence reasonably well and are not racially or culturally biased. (Reeve & Charles, 2008)
Fourth, there are gene variants that predict intelligence and educational attainment within each population, and the distribution of these genes across populations also agrees with the same ordering of groups we keep running into (Asian, White, Hispanic, Black). In other words, when you tell a computer to guess how intelligent a person is based solely on their genes, it ranks Asians as highest on average and Africans as lowest on average. (Piffer, 2019)
Finally, I just want to point out that it would be a phenomenal coincidence for all of these to happen at once if genes weren't playing a considerable role in explaining the gap.
As for potential environmental explanations, socioeconomic status alone cannot explain the gap and this is no longer really disputed. Other explanations like social discrimination need a plausible causal mechanism to affect intelligence. Usually, it is implied that racism makes black people think less of themselves, and this impacts their performance on intelligence tests. This could be a compelling argument if it was actually true that blacks overall have lower self-esteem than whites, but it is not. It also falls short due to the poor repeatability of stereotype threat effects. Blacks score significantly higher on measures of self-esteem than whites, as well as every other group measured. (Twenge & Crocker, 2002) Same applies to levels of stress based on adrenaline levels. (Mills et al., 1995) School funding per pupil also does not differ substantially by race (Murray & Reuben, 2008; Richwine, 2011), and even if it did, educational interventions don’t seem to causally improve intelligence (g) in modern environments, and the effects of interventions on IQ fade with time. More on proposed environmental causes here.
See also: