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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

Indirect Evidence on the Role of Genetic Differences

Although the direct evidence on the role of environment is not definitive, it mostly suggests that genetic differences are not necessary to explain racial differences. Advocates of the hereditarian position have therefore turned to indirect evidence.17

Several authors have argued that estimates of the heritability of cognitive ability put limits on the plausible role of environment.18 The argument is normally made in a mathematical form, but it boils down to this. First, it is now widely accepted that differences in genetic endowment explain at least 60 percent of the variance in cognitive ability among adults in the white population in the United States.19 If all the environmental variation among U.S. whites can explain only 40 percent of the variance among whites, how could environmental differences explain the huge gap between blacks and whites? The mathematical argument implies that the average black environment would have to be worse than at least 95 percent of white environments, but observable characteristics of blacks and whites are not that different. For example, black deficits in education or in socioeconomic status place the average black below only about 60 to 70 percent of whites.20

The heritability of cognitive ability is also crucial to a second type of indirect evidence for a role of genetic differences in explaining the black-white gap. Arthur Jensen has advanced what he calls “Spearman's Hypothesis,” after the late intelligence researcher Charles Spearman, who observed that people who had large vocabularies were good at solving mazes and logic problems and were also more likely to have command of a wide range of facts. Spearman posited that a single, largely genetic, mental ability that he called g (for general mental ability) explained the correlation of people's performance across a wide range of tests of mental ability. Researchers now know that a single underlying ability cannot explain all the tendency of people who do well on one type of test to do well on another.21 But it is possible to interpret the evidence as indicating that there is a single ability that differs among people, that is subject to genetic influence, and that explains much of the correlation across tests. Other interpretations are also possible, but this one cannot be discounted. In a series of studies Jensen and Rushton have argued that different types of tests tap this general ability to different degrees; that the more a test taps g, the more it is subject to genetic influence; and that black-white differences are largest on the tests most reflective of the underlying general ability, g.22

Using several restrictive assumptions about the nature of genetic and environmental influence on genetic ability, researchers can use this information to estimate the fraction of the black-white gap that is due to differences in genetic endowment. The more the pattern of black-white differences across different tests resembles the pattern of genetic influence on different tests, the more the statistical procedure will attribute the black-white differences to genetic differences. Using this method, David Rowe and Jensen have independently estimated that from one-half to two-thirds of the black-white gap is genetic in origin.23