By the end of the 20th century, Whites, Indians, Coloureds and Blacks in South Africa averaged IQs of 98, 92, 82, and 69 (UK norms). Although all four “races” were in school when tested and used to paper-pencil tests, and although the test used (Standard Progressive Matrices) was culture reduced, there were enormous difference in the quality of environment the four races were exposed to including the biological environment which affects brain growth. I tried to correct for this by comparing the different rates of stunting (low height), but as commenter “some guy” noted, this measure is too confounded with genetic height to be a good proxy for environment.
Perhaps a better proxy is Human Development Index (HDI) which combines income, education, and life span. While life span is confounded with genetics to some degree, being reared in a poor uneducated home is a clear environmental effect.
In 1991, South Africa’s Whites, Indians, Coloureds, and Blacks had HDIs of 0.901, 0.836, 0.663, and 0.50 respectively (see table III).
To put these numbers in perspective, I wanted to know the HDI of black Americans because black Americans have continued to score 15 points below white Americans for roughly a century, and it doesn’t seem to matter whether they’re reared by their biological parents or adopted into White professional homes. Thus black American HDI is perhaps a ceiling beyond which environment much affects IQ.
An article in The Atlantic (October 14, 2014) by Theodore R. Johnson reported their HDI but I don’t think a 2014 figure is comparable with the 1991 figure for South Africa’s races, both because living standards have changed over time and so has the method of calculating HDI. But assuming the relative ranking of black America has been similar over time (slightly above Saudi Arabia and slightly below Qatar) then in 1991 they had an HDI of about 0.705 (Saudi Arabia) to 0.745 (Qatar) or roughly 0.725.
Black Americans (1991 HDI 0.725) score 15 points lower than white Americans, but 16 points higher than South African blacks (1991 HDI 0.5). The former gap is arguably 100% genetic judging by the results of the Minnesota transracial adoption study suggesting HDI has no effect on IQ once you hit at least 0.725. The latter gap is probably 31% genetic, because Black Americans are only 75% Black. Adjusting for this reduces the latter gap to 11 points suggesting that for people with HDIs below 0.725, subtracting the HDI from 0.725 and then multiply by 48.9 gives an estimate of how much Raven IQ (taken by people in school) has been supressed by environment.
So the IQs of South African Whites and Indians (98 and 92 respectively) are probably not supressed because their 1991 HDIs were above 0.725 but Coloureds were 0.062 below this threshold, so multiplying by 48.9 suggests their IQs were supressed by 3 points. This would raise them from 82 to 85, the same as African Americans.
According to one study, “by 2001, the American Indian and Alaska Native population and the Canadian Indigenous population had…HDI scores comparable to South Korea or the Czech Republic and Belarus or Trinidad and Tobago, respectively.” In 1991 Czech Republic had an HDI score of 0.733 and Trinidad and Tobago, 0.67. Averaging just those two (couldn’t find 1991 data on the other countries) suggests American Indians and Arctic people had a 1991 HDI of 0.702. This suggests their Raven IQs are supressed by 1 point. This is consistent with a study that found that several years of foster care in white homes did not at all improve the IQ of these.