Tag Archives: regression to the mean

The Likelihood of Genetic Group Differences in IQ: The Black White Gap in IQ

I apologize in advance for the daunting length of this post. My sole excuse is that I should have made it still longer to cover the topic.

In dealing with any controversy, it’s usually healthiest to begin at the sticking point. On the question of the impact of biology on political ideology, it’s plain enough what that is: group differences in IQ in general, and the black-white gap in IQ in particular.

I believe the best evidence is that the black-white IQ gap is real, that IQ measures something basic about intelligence, and that the difference between the average IQ of blacks and the average IQ of whites is based in substantial part on genetic differences between the two groups.

I’ll focus on the claim regarding the substantial genetic basis for the IQ gap. The evidence is perhaps best summarized in the following sequence of papers. The series commences with an initial paper by Arthur Jensen and Phillipe Rushton, is followed by criticisms from some of the most prominent anti-heriditarians, and ends with a final response to their critics from Jensen and Rushton:

Rushton, J. P., & Jensen, A. R. (2005). Thirty years of research on race differences in cognitive ability. Psychology, Public Policy, and Law, 11, 235-294.

Sternberg, R. J. (2005). There are no public-policy implications: A reply to Rushton and Jensen (2005). Psychology, Public Policy, and Law, 11, 295-301.

Nisbett, R. E. (2005). Heredity, environment, and race differences in IQ: A commentary on Rushton and Jensen (2005). Psychology, Public Policy, and Law, 11, 302-310.

Gottfredson, L. S. (2005). What if the hereditarian hypothesis is true? Psychology, Public Policy, and Law, 11, 311-319.

Suzuki, L., & Aronson, J. (2005). The cultural malleability of intelligence and its impact on the racial/ethnic hierarchy. Psychology, Public Policy, and Law, 11, 320-327.

Rushton, J. P., & Jensen, A. R (2005). Wanted: More race realism, less moralistic fallacy. Psychology, Public Policy, and Law, 11, 328-336.

Another paper that responds to the most recent criticism of the hereditarian hypothesis, in the book “Intelligence and How to Get It: Why Schools and Cultures Count” by Richard Nisbett, is this paper, again by Jensen and Rushton.

Also a very good summary of the latest data regarding the black-white IQ gap can be found in Charles Murray’s article The Inequality Taboo, from 2005.

As these papers make clear, there exist any number of items of evidence that point to the conclusion that the black-white IQ gap is quite considerable, and its genetic basis substantial. It is of course that evidence considered in aggregate that most inescapably nails down those conclusions. Yet some items seem even standing by themselves quite powerful.

I would subsume that evidence under the rubric of “regression to the mean” effects. Jensen and Rushton in “Thirty years of research on race differences in cognitive ability”, linked to above, describe the phenomenon and some of the evidence:

Regression toward the mean provides still another method of testing if the group differences are genetic. Regression toward the mean is seen, on average, when individuals with high IQ scores mate and their children show lower scores than their parents. This is because the parents pass on some, but not all, of their genes to their offspring. The converse happens for low IQ parents; they have children with somewhat higher IQs. Although parents pass on a random half of their genes to their offspring, they cannot pass on the particular combinations of genes that cause their own exceptionality. This is analogous to rolling a pair of dice and having them come up two 6s or two 1s. The odds are that on the next roll, you will get some value that is not quite as high (or as low). Physical and psychological traits involving dominant and recessive genes show some regression effect. Genetic theory predicts the magnitude of the regression effect to be smaller the closer the degree of kinship between the individuals being compared (e.g., identical twin  full-sibling or parent– child  half-sibling). Culture-only theory makes no systematic or quantitative predictions.

For any trait, scores should move toward the average for that population. So in the United States, genetic theory predicts that the children of Black parents of IQ 115 will regress toward the Black IQ average of 85, whereas children of White parents of IQ 115 will regress toward the White IQ average of 100. Similarly, children of Black parents of IQ 70 should move up toward the Black IQ average of 85, whereas children of White parents of IQ 70 should move up toward the White IQ average of 100. This hypothesis has been tested and the predictions confirmed. Regression would explain why Black children born to high IQ, wealthy Black parents have test scores 2 to 4 points lower than do White children born to low IQ, poor White parents (Jensen, 1998b, p. 358). High IQ Black parents do not pass on the full measure of their genetic advantage to their children, even though they gave them a good upbringing and good schools, often better than their own. (The same, of course, applies to high IQ White parents.) Culture-only theory cannot predict these results but must argue that cultural factors somehow imitate the effect theoretically predicted by genetic theory, which have also been demonstrated in studies of physical traits and in animals.

Jensen (1973, pp. 107–119) tested the regression predictions with data from siblings (900 White sibling pairs and 500 Black sibling pairs). These provide an even better test than parent– offspring comparisons because siblings share very similar environments. Black and White children matched for IQ had siblings who had regressed approximately halfway to their respective population means rather than to the mean of the combined population. For example, when Black children and White children were matched with IQs of 120, the siblings of Black children averaged close to 100, whereas the siblings of White children averaged close to 110. A reverse effect was found with children matched at the lower end of the IQ scale. When Black children and White children are matched for IQs of 70, the siblings of the Black children averaged about 78, whereas the siblings of the White children averaged about 85. The regression line showed no significant departure from linearity throughout the range of IQ from 50 to 150, as predicted by genetic theory but not by culture-only theory.

What is peculiarly compelling about this evidence? The simplicity and directness with which the genetic hypothesis accounts for the data, the accuracy of that prediction across the range of IQs, and the sheer implausibility of any primarily environmental account of that data.

Of course, those promoting the primarily environmental hypothesis can put together a response that formally meets the objection: some unknown factor X that depresses the IQ of all blacks effectively uniformly across the range, imposing a nearly exactly one standard deviation hit on each black subject measured. Now, I must say this purported effect impresses me as quite magical and unprecedented. What other socioeconomic or cultural environmental factor can one think of that induces such a uniform effect across such a range on a group of human beings?

Given what the factor X is supposed to effect in particular, how plausible is its existence? Wouldn’t one expect that some black families in some more environmentally propitious situations would enable their children to escape, or at least significantly to avoid, any factor X that might depress IQ scores? How is it, then, that even for black children with relatively high IQs of 120, their siblings should average only 100, rather than 110, as with the siblings of white children with IQs of 120? Consider the parents of a black child with the relatively high IQ of 120. Wouldn’t one expect that that family, which had managed to find and develop an environment congenial enough to the intellectual development of one of their children that he or she achieved an IQ of 120, might likewise have secured an environment as well suited for the intellectual development of their other children? Here we have the same parents, the same socioeconomic status, the same childrearing practices, as well as the same schools and neighborhoods. If environment means anything to IQ — which of course is the claim — shouldn’t such similarities be exactly what one would expect to engender the same kinds of outcomes in IQ? How explain then the great and otherwise unexpected disparity?

I find it very hard to ponder facts like these without inferring that genes dominate the explanation for the black-white IQ gap.

Another set of facts that I would categorize as “regression to the mean” effects is the disparity between black and white performance on the SAT even when controlling for economic status and level of education of parents. This is well captured in two graphs. (I take as a reasonable assumption that the SAT, which correlates as well with IQ tests as they correlate with each other, can here work as a good measure of IQ).
Average 1995 SAT scores vs income by ethnicity
Average 1995 SAT scores vs parental educational level by ethnicity

Both of these graphs run very hard against the hypothesis of purely, or dominantly, environmental basis of the black-white IQ gap. Yes, as parental education and income increase, SAT scores rise: that much an environmental explanation might predict. But, remarkably, the children of blacks whose income is over $70K attain an average SAT score lower than that of the children of whites whose income is well into the poverty level of $0K to $10K. Likewise, the children of blacks who had achieved a graduate level degree score lower on average on the SAT than do the children of whites who only finished HS.

How might one contemplate these items of data without feeling that they are exactly what one would not expect to see if environment played the major role in determining IQ differences between these groups? The advantages that come with a high income and with a graduate level education confer the very sort of benefits that are routinely said to explain why the average black student can’t do as well as the average white student on an IQ test or the SAT. How is it then that the effects of this relative privilege in black children cannot overwhelm, and easily, those of the clear deficts in the backgrounds of white students against whom the black childen are being compared? If income and educational level of parents entail so little, what can the environment plausibly be said to ground here?

Of course, this regression to the mean is, on the other hand, exactly what one would expect to see if the genetic basis were dominant in determining IQ or SAT scores. The hereditarian hypothesis predicts this and the earlier IQ data neat as a pin.

Now, again, one can contrive an explanation that lets the primarily environmental explanation off the hook. One can say that blacks at any income level and at any level of education suffer from a “caste mentality” or from “stereotype threat” which systematically undermines their performance. I plan to address those issues in more detail in a later post. For now, suffice it to say that the only likely motivation I can see to adopt such a view is to save the primarily environmental thesis from otherwise incompatible data. It appears to me to be a posit born of desperation: a scientific Hail Mary thrown up when the game is otherwise lost. In practice, it appears to operate as little more than a fudge factor X whose impact and exact size is determined only by what otherwise can’t be explained under a given hypothesis.

I think, though, that if one doesn’t divert one’s attention from the basic facts being communicated by these graphs and the earlier example described by Jensen and Rushton, and allows oneself to stare into this sun long enough to take its reckoning, then the natural conclusion is that it is genetic, rather than environmental, differences that are more basic to the black-white IQ gap.