50 whys to look for genes: 8. Differences between groups not well explained by environmental factors

[A]ccording to the current consensus, heritability of IQ test scores is high (Neisser et al. 1996, but see Turkheimer et al. 2003, Nisbett et al. 2012)… Persistent large differences in average IQ test score… exist between racial groups (but with recent decreases, Nisbett et al. 2012)… Many human behavioral geneticists and psychometricians (analysts of data from personality and educational tests) are prepared to entertain a two-part argument: the high heritability of IQ test scores within racial groups coupled with a failure of environmental hypotheses to account for the group differences supports explanations of mean differences in terms of genetic factors (e.g., Jensen in Miele 2002, 111ff).

This argument is complicated by the following:

The specific factors still have to be elucidated, so “support” may be read as “lends plausibility to the belief that they exist.”

There has been some success using regression analysis to identify associations between environmental factors and differences between the mean test scores for racial groups (Fryer and Levitt 2004).

Flynn (1994) has pointed to large gains in average IQ test score between generations (now called the “Flynn effect”). No environmental factor, or composite of factors, such as diet or education level, has been shown to be strongly associated with these generational differences [but see above]… By [the logic of the two-part argument] we would have to entertain explanations of generational differences in terms of genetic factors, but we know that the changes in gene frequencies in a human population over one generation are negligible. There must be a hole in the logic of the two-part argument, but where is it? If we were to find the hole, would that help us explain large differences between generations in a high heritability trait such as IQ test score? These questions constitute the IQ paradox of Dickens and Flynn (2001).


The quotes above are from pages 18-19 in Taylor, Peter J. (2014) Nature-Nurture? No: Moving the Sciences of Variation and Heredity Beyond the Gaps.

Dickens, W. T. and J. R. Flynn (2001). “Heritability estimates versus large environmental effects: The IQ paradox resolved.” Psychological Review 108(2): 346-369.

Flynn, J. R. (1994). “IQ gains over time.” Pp. 617-623 in Encyclopedia of Human Intelligence. R. J. Sternberg (Ed.) New York: Macmillan.

Fryer, R. and S. Levitt (2004). “Understanding the black-white test score gap in the first two years of school.” The Review of Economics and Statistics 86(2): 447-464.

Miele, F. (2002). Intelligence, Race, and Genetics: Conversations with Arthur Jensen. Boulder, CO: Westview Press.

Neisser, U., G. Boodoo, et al. (1996). “Intelligence: Knowns and unknowns.” American Psychologist 51: 77-101.

Nisbett, R. E., J. Aronson, et al. (2012). “Intelligence: New findings and theoretical developments.” American Psychologist 67(2): 130-159.

Turkheimer, E., A. Haley, et al. (2003). “Socioeconomic status modifies heritability of IQ in young children.” Psychological Science 16(6): 623-628.


(Introduction to this series of posts)


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