Journal Issue: School Readiness: Closing Racial and Ethnic Gaps Volume 15 Number 1 Spring 2005
Genes and Environment and the Black-White Gap
The black-white gap in measured cognitive ability may come about in a similar way, but it could have even more triggers. Segregation and discrimination have caused many aspects of blacks' environment to be inferior to that of whites. Averaged together, the total impact can be large, even if each individual effect is small. Suppose, for example, that environment relevant to the formation of cognitive ability consists of 100 factors, each with an equal effect. If for each of these 100 factors the average black were worse off than 65 percent of whites, he would be worse off than 90 percent of whites when the effects of all the environmental factors were considered together. (The disparity is the necessary result of accumulating a large number of effects when two groups have slightly different means for all the effects.)32 Taking the total effect of environment in this way, considering the underestimate of the total effect of environment because some of its power is attributed to genes, and considering individual and social multipliers, a purely environmental explanation for black-white differences becomes plausible despite high estimates for the heritability of cognitive ability.
Moreover, our model also has explanations for the correlation of the heritability of scores on different tests with the size of the black-white gap on those tests and the anomalous correlation of the size of gains in cognitive ability over time on different tests with the heritability of those test scores. Those cognitive abilities for which multiplier processes are most important will be the ones that show the largest heritability, because of the environmental augmentation of the genetic differences. But they will also be the ones on which a persistent change in environment will have the biggest influence. Thus we might expect that persistent environmental differences between blacks and whites, as well as between generations, could cause a positive correlation between test score heritabilities and test differences.33 Rushton and Jensen's indirect evidence of a genetic role in black-white differences is, therefore, not probative.