Hereditarianism II: Admixture Data and Gaps

In the last post, we have seen, that the environmentalist position about group differences in IQ is mostly based on the idea of x-factors. Factors hard to identify that vary systematically between groups and affect IQ. Given that there are many factors that vary between ethnic groups, this is a difficult theory to disprove.

However, from a hereditarian perspective, two persons belonging to the same ethnic group can sometimes be differentiated by different amounts of a certain genetic ancestry. So in ethnic groups whose members have varying degrees of admixture of some original founding populations we can put the hereditarian hypothesis to the test. This is the case for African-Americans, who have varying degrees of European ancestry and for Hispanics, who are mostly a mixture of Europeans, Amerindians and Africans.

The hereditarian hypothesis predicts that IQ will vary within these groups with the amount of admixture for any chosen ancestral group. This type of admixture study has the power to rule out the majority of x-factors that systematically vary between ethnic groups, except for those that vary roughly according to ancestry.

A recent paper showed IQ varying by ancestry for Hispanics and African Americans [1]. These are the key figures.

The regression line of the relationship between cognitive ability and European ancestry in African Americans
And the same thing for Hispanics …

In this post we are going to reanalyze the underlying data set. This data set contains IQ scores for a couple of hundred self-identified Whites, Blacks, Hispanics, East Asians + other minorities and the percentage of their genome being European, African, Amerindian, Asian etc.

First we translate the cognitive ability measure, here given in whole sample standard deviations above the sample mean, into IQ, with white mean = 100 and white standard deviation = 15.

n=137, slope=23.283, intercept=79.6, correlation=0.176, p-value=0.0392

The slope of 23.283 immediately gives us the gap between 100% European and 100% African, while the intercept provides us with the IQ of a 100% African African-American. The regression line overshoots the mean white IQ. This might be noise, or legitimately smarter white genes in the black population, or Amerindian admixture in the whites reducing the mean, or a slight environmental downward bent of the left part of the plot. But whether we take the estimated gap, or the difference between actual white mean IQ and the 100% African IQ, the result is always strikingly close to Galton’s estimate.

Of course this is just a very small sample. With a very restricted range. However, we can immediately replicate this regression line with those Hispanics that have predominately African and European admixture.

n=79, slope=23.837, intercept=73.33, correlation=0.416475096463478, p-value=0.000134

This gives us a virtually identical gap. But the whole line is shifted down. This vibes well with other results, see for example [2]. The average Hispanic IQ in this sample is only 89.5, compared to a usual US Hispanic IQ of 92-93, so it might still be missing a few points of Flynn effect. Note, however, that this seems to affect the entire IQ range in the same fashion.

The combined sample of African Americans and Euro-African Hispanics of course also validates Galton’s estimate of the gap almost perfectly.

n=257, slope=22.282, intercept=77.979, correlation=0.401 p-value=2.34e-11

For comparison, for Hispanics with predominantly European and Amerindian the admixture plot looks like this.

n=323, slope=16.65, intercept=80.024, correlation=0.233, p-value=2.231e-05

The gap is some 7 points smaller and the percentage of European admixture is generally quite high, which is why despite the missing Flynn effect points, the average Hispanic IQ is 89.5 vs 83.7 for African Americans.

[1] Biogeographic Ancestry, Cognitive Ability and Socioeconomic Outcomes

[2] A study of intelligence of children in Brazil


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