Journal Issue: School Readiness: Closing Racial and Ethnic Gaps Volume 15 Number 1 Spring 2005
Evidence on the Role of Genetic Differences
To evaluate the research findings on the role of genetic differences in cognitive ability, I begin by drawing a clear distinction between evidence that genetic endowment explains a large fraction of differences within races and evidence that it explains differences between races and ethnic groups. There can be little doubt that genetic differences are an important determinant of differences in academic achievement within racial and ethnic groups, though the size of that effect is not known precisely. Depending on the measure of achievement used, the sample studied, and the age of the subjects, estimates of the share of variance explained by genetic differences within racial and ethnic groups range from as low as 20 percent to upward of 75 percent. However, most estimates, particularly those for younger children, seem to cluster in the range of 30 to 40 percent. The fraction of variance explained by genetic differences in a population is termed the heritability of the trait for that population.2
But the heritability of academic achievement within racial or ethnic groups says little about whether genes play a role in explaining differences between racial groups. Suppose one scatters a handful of genetically diverse seed corn in a field in Iowa and another in the Mojave Desert. Nearly all the variance in size within each group of seedlings could be due to genetic differences between the plants, but the difference between the average for those growing in the Mojave and those growing in Iowa would be almost entirely due to their different environments.
If researchers were able to identify all the genes that cause individual differences in school readiness, understand the mechanism by which they affect readiness and the magnitude of those effects, and assess the relative frequency of those genes in the black and white populations, they would know precisely the extent to which genetic differences explain the black-white gap. But only a few genes that influence cognitive ability or other behaviors relevant to school readiness have been tentatively identified, and nothing is known about their frequency in different populations. Nor are such discoveries imminent. Although genetic effects on several different learning and school-related behavior disorders have been identified and many aspects of personality are known to have a genetic component, genes have their primary effect on school readiness through their effect on cognitive ability.3 Experts believe that a hundred or more genes are responsible for individual differences in cognitive ability. Many of these genes are likely to have weak and indirect effects that will be difficult to detect. It could be decades before enough genes are identified, and their frequencies estimated, to make it possible to determine what role, if any, they play in explaining group differences.
So it is necessary to turn to less direct ways of answering the question. Much has been written on this topic in the past fifty years. James Flynn's Race, IQ, and Jensen, published in 1980, remains the most thoughtful and thorough treatment available.4 More recently Richard Nisbett wrote a shorter review of this literature.5 Both Flynn and Nisbett take the view, as do I, that genetic differences probably do not play an important role in explaining differences between the races, but the point remains controversial, and Arthur Jensen provides a recent discussion from a hereditarian perspective.6 Here I will review the major types of evidence and explain why I think they suggest that environmental differences likely explain most, if not all, of the black-white gap in school readiness. I will concentrate entirely on the evidence on cognitive ability, as it is the most studied trait that influences school readiness, and genetically induced differences in cognitive ability account for the vast majority of genetically induced differences in school readiness within ethnic groups. Almost no studies have been done of racial differences in other traits that might influence school readiness. And I choose to focus on the black-white gap rather than to consider the role of genetic differences in determining the academic readiness of disadvantaged groups more generally, again, because it is a topic that has been more thoroughly studied.