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Journal Issue: School Readiness: Closing Racial and Ethnic Gaps Volume 15 Number 1 Spring 2005

Genetic Differences and School Readiness
William T. Dickens

A Problem for the Indirect Arguments: Gains in Cognitive Ability over Time

Over the past century, dozens of countries around the world have seen increases in measured cognitive ability over time as large as or even larger than the black-white gap.24 The phenomenon has been christened the “Flynn Effect,” after James Flynn, who did the most to investigate and popularize this worldwide trend. The score gains have been documented even between a large group of fathers and sons taking the same test only decades apart, making it impossible that the gains are due to changes in genes. Clearly environmental changes can cause huge leaps in measured cognitive ability. Although it might not seem plausible that the average black environment today is below the 5th percentile of the white distribution of environments, it is certainly plausible that the average black environment in the United States today is as deprived as the average white environment of thirty to fifty years ago—the time it took for cognitive ability to rise by an amount equal to the black-white gap in many countries. These gains in measured cognitive ability over time point to a problem in the argument that high heritability estimates for cognitive ability preclude large environmental effects.

Gains in cognitive ability over time also challenge the logic of Jensen's genetic explanation for the pattern of black-white differences across different types of tests. All studies show that gains on different tests are positively correlated with measures of test score heritability, and most studies show that gains are positively correlated with the extent to which a test taps the hypothesized general cognitive ability.25 There is little doubt that applying the same method as Rowe and Jensen used to data on gains in cognitive ability over time would show them to be partially genetic in origin, something we know cannot be true.

So, what is it that is wrong with the logic of these two arguments, that the high heritability of cognitive ability limits the possible effect of the environment and that the pattern of black-white differences across different tests shows those differences to be genetic in origin? And in particular, where is the problem in the first?

It is important to detect the flaw, because if the logic of the argument were sound, the case for environmental causes of black-white differences would be difficult to make, and the possibility of remedying those differences would be remote. But before I explain, I want to cite two other pieces of evidence marshaled by advocates of the hereditarian position that suggest the limited power of the environment to change cognitive ability (and therefore to explain the entire black-white gap). The first is that the heritability of cognitive ability rises with age. It does so at the expense of the effect of family environment, which disappears nearly completely in most studies of late adolescents and adults.26 The disappearance of the effect on black children of being raised in white families, which I have already noted, is just one case of a general finding from several different types of studies. A second piece of evidence is the fade-out of the effect of preschool programs on cognitive ability. Although such programs have been shown to have profound effects on the measured ability of children, the effects fade once the programs end, leaving little evidence of any effect by adolescence.27 Is it possible to reconcile the high heritability of cognitive ability with large, but transient, environmental effects?