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) and malnutrition rates of 5.7, 10.9*, 18, and 32 percent respectively (see table 2.13 below):
Malnutrition is defined here as the percentage of the population that is stunted. Stunted is defined as two or more standard deviations (SDs) shorter than comparable healthy populations. In theory only 2% of the healthy reference population should be stunted.
But nutrition is not a discrete variable. In theory there’s a perfect continuum between optimum and suboptimum processing of nutrients and each population has their own bell curve.
So if 32% of South African blacks have sub-optimum nutrition compared to only 2% of well nourished populations, then that tells us that the 32nd percentile (-0.47 SD) on the former’s bell curve equates to the 2nd percentile (- 2 SD) on the latter’s. Assuming roughly equal standard deviations, it suggests South Africa’s black bell curve is shifted 1.53 SD to the left of what was considered optimum at the time.
So the stunted children are just the tip of the iceberg. The average black child in South Africa should be 1.53 SD below his genetic potential in physical growth. What about brain size? Also 1.53 SD below genetic potential? Given the 0.4 correlation between IQ and brain size, this would predict IQ would be 0.4 (1.53 SD) = 0.61 SD below genetic potential. Multiplying by the IQ standard deviation of 15 points, this gives 9 points of impairment caused by malnutrition. Adding this to the IQ 69 of black South Africans gives IQ 78.
I applied the same calculations to all the major groups of South Africa:
It is interesting to compare the estimated black genetic IQ of 78 to the average IQ of 85 for African Americans, a presumably well nourished population judging from the fact that they are virtually indistinguishable from white Americans in height. But genetically African Americans are only 75% sub-Saharan, so adjusting for this, they go from 15 points below IQ 100, to 15/0.75 = 20 points below 100 = IQ 80. In other words, virtually all of the difference between unmixed American and South African blacks vanishes when we adjust for presumably stunted brain size.
Similarly, the IQs of Indians and whites rise to their corresponding levels when reared in the UK (a well nourished country).
*The malnutrition of Indians were estimated from the line of best fit predicting poverty rates in table 2.13 to malnutrition rates in table to 2.13: Malnutrition = 0.65(poverty) – 2.79.