Fascinating Discussion at West Hunter

Over at West Hunter, Malloy dropped the following comments:

There are dozens of older studies showing that proportion of white ancestry is correlated with higher IQ in Native Americans and Mestizos (where ancestry is roughly estimated by number of Indian grandparents, or from physical measurements like skin color), and many similar studies with socioeconomic status, education, etc. But what happens when we link IQ with actual Indian or black genetic ancestry? It wouldn’t tell us much more than we already know, until we can, say, compare mixed race siblings, so that cultural background is equalized and ancestry differences are rendered cryptic. Otherwise the differences could be chalked up to an invisible “Colorist” caste system. Or whatever idiosyncratic theory I’m sure Ron Unz has thought up to explain away these obviously hereditary social patterns.

We already have modern studies showing that proportion of Amerindian and black ancestry is associated with lower socioeconomic status and education:

Strong association of socioeconomic status with genetic ancestry in Latinos
“We determined that socioeconomic status was strongly correlated with European ancestry proportion in Mexicans (33% correlation… ) and Colombians (34% correlation…)”

Latino Populations: A Unique Opportunity for the Study of Race, Genetics, and Social Environment in Epidemiological Research
“… among Puerto Ricans, ancestry is associated with socioeconomic status. Healthy Puerto Rican volunteers reporting “upper” socioeconomic status had 9.1% lower African ancestry and 9.2% higher European ancestry than healthy volunteers reporting “moderate” and “middle” socioeconomic status…“


Here’s an example of a recent mixed race sibling study from Brazil.

Genomic Ancestry, Self-Reported ‘‘Color’’ and Quantitative Measures of Skin Pigmentation in Brazilian Admixed Siblings

In 35% of 86 full sibling pairs, the siblings report a different race from each other. And the siblings differ from each other slightly in their proportions of racial ancestry (just as full siblings differ, on average, in their relatedness to each other).

The study finds that proportion of European ancestry is associated with higher socioeconomic status, as is self-reported race, but that objectively measured skin color is not. This implies that ancestry is cryptic (undetectable by society), and yet European ancestry still leads to enhanced economic outcomes.

So the genetically informed sibling study would appear to contradict the discrimination theories in the Telles and Rangel sibling studies linked above.

A quick google search provided me with the informative following:

(From: Cheng, et al., 2012. African Ancestry and Its Correlation to Type 2 Diabetes in African Americans: A Genetic Admixture Analysis in Three U.S. Population Cohorts)

The numbers in the figure above are a Rorschach for those biased either towards Hereditarianism or Environmentalism. Those disposed to Hereditarianism will find in them immediate confirmation of a genetic hypothesis based on the simple association between ancestry and indexes of SES. Those disposed to environmentalism will focus on the relatively piddly mean differences in ancestry which correspond to appreciable differences in SES and imagine that they have found strong confirmation of their favored position. To properly evaluate this, we must first query, as we are wont to do: What would a genetic hypothesis predict?

To determine this, we need to ascertain the average and standard deviation of admixture in the population. In table 1 of Cheng et al. 2012, we see that the African admixture in 84% and we can deduce that the standard deviation is 8.5, based on the interquartile range. From the above, we can determine that Blacks in this sample differ from Whites by 9.88 SD of African admixture, based on the sample admixture SD, e.g., (84-0)/8.5 SD. Were genetic IQ d to equal 1, then a shift in 1 genetic IQ SD would be associated with a shift in 9.88 SD of African admixture (granting such and such statistical assumptions). The predicted genetic IQ, ancestry correlation would then be 0.1. And 0.1 x 8.5% would give us the predicted % African Admixture difference between sub-samples that deviate by 1 standardized unit in genotypic IQ. Since the extremes in SES should differ by about 2 SD in IQ (at least for education and occupation), a genetic hypothesis would predict a 0.2 SD difference in ancestry between the extreme SES categories or a difference of less than 2%.

Now, we can compare this to what was found. We see, averaged across samples, a found differences of 4% African ancestry or 0.47 SD. And elementary math gives us a found IQ, ancestry correlation of 0.24:

IQ, African ancestry correlation = unit shift ancestry per unit shift IQ
= 0.47 SD ancestry (e.g., 4%/8.5%) per 2 SD IQ

A one standard deviation increase in phenotypic IQ is associated with a 0.24 standard deviation increase in African ancestry.

This found difference would imply a 2.3 SD difference in genotypic IQ between African-Americans and Whites, if genotypic IQ were the only factor mediating the association. This, of course, is outlandishly high – but outlandishly high is much more preferable than low from a genetic perspective. We can, therefore conclude, that the relation between ancestry and SES found in this sample is consistent with a strong genetic hypothesis but that such hypothesized genetic differences can not account for it in full. Environmental factors need to be postulated as part of an account. The pertinent question is, though: Can such factors suffice in explaining the correlation? Environmentalists would steadfastly argue “yes” and call their yes “colorism.” Accordingly: > African ancestry –> darker color –> discrimination –> worse opportunity, SES, IQ. But is this a plausible explanation?

The most obvious obstacle for such an explanation is that the relevant correlations don’t tally up.

In the AA population, the correlation between skin color, as measured by spectrophotometer, and ancestry is modest. Parra, Kittles, and Shriver (2004) estimates 0.44, based on a D.C. sample. And, as noted elsewhere, the correlation between color, as measured with a color chart, and both IQ and education is meager. Here, for example, were the relevant correlations in the GSS:

The average correlation between color and IQ across numerous samples is 0.15; quite similar to that between color and educational attainment. As noted, these correlations are based on color as indexed using color charts and so are not directly comparable to the Parra et al. results. If we assume that the reliability of color chart color as an index of spect color is an imperfect 0.8, then we can correct our correlation up. Doing so with the GSS correlations above we get a “true color” education/IQ correlation of 0.21, which is quite close in magnitude to our estimated ancestry-IQ/SES correlation (0.24). Now, were the colorists correct, and were the ancestry outcome correlation driven by color discrimination, then the resulting correlation should be that between ancestry and color x that between color and outcome or 0.09 — substantially below our estimate.

Of course, this is a two way street. Just as a colorist hypothesis can’t fully explain ancestry-SES/IQ association, a genetic hypothesis can’t fully explain the SES/IQ color association. Hereditarians, no doubt, would explain the residual in terms of cross assortative mating for color and human capital traits. Possible.

Whatever the case, it doesn’t seem that a simple colorist model can readily explain our estimated ancestry-SES/IQ association.

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2 Responses to Fascinating Discussion at West Hunter

  1. statsquatch says:

    Too bad there are no admixture studies on Dementia or Intellectual disabilities.

  2. Pingback: Cryptic Admixture, Mixed-Race Siblings, & Social Outcomes | Human Varieties

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