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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

Direct Evidence on the Role of Environment: Adoption and Cross-Fostering

If there is no direct evidence of a role for genes in explaining the black-white gap, perhaps there is direct evidence that environment can or cannot account for the whole difference between blacks and whites. Several studies have shown that environmental differences between blacks and whites can, in a statistical sense, “explain” nearly all of the difference in cognitive ability between black and white children.But because the studies do not completely control for the genetic endowment of either the child or the parents and because many of the variables used to explain the difference are themselves subject to genetic influence, the effect being attributed to environment may in reality be due to genetic differences.

What is needed is a way to see the effect of environment without confusing it with the effect of genetic endowment. For example, randomly choosing white and black children at birth and assigning them to be fostered in either black or white families would ensure that the children's environments were not correlated with their genetic potential and would show how much difference environment makes. No existing study replicates the conditions of this experiment exactly, but some come close. The strongest evidence for both the environmentalist and hereditarian perspectives is of this sort.

After the end of World War II both black and white soldiers in the occupying armies in Germany fathered children with white German women. Klaus Eyferth gathered data on a large number of these children, of mainly working-class mothers, and gave the children intelligence tests.8 He found almost no difference between the children of white fathers and those of black fathers. The finding is remarkable given that the black children faced a somewhat more hostile environment than the white children. Hereditarians have challenged these findings by appealing to the possibility that the black soldiers who fathered these children might have been a particularly elite group. Flynn has researched the plausibility of this explanation and concludes that such selection did not play more than a small role.9 Thus Eyferth's study suggests that the black-white gap is largely, and possibly entirely, environmental.

A study similar to Eyferth's found the cognitive ability of black children raised in an orphanage in England to be slightly higher than that of white children raised there.10  Again, critics have raised the possibility that the black children were genetically advantaged relative to other blacks, and the whites disadvantaged relative to other whites. And again, Flynn finds it unlikely that this contention explains much of the disappearance of the black-white gap.11  This study, too, suggests that the black-white gap is mainly environmental.

If the black-white gap is mainly genetic in origin, children's cognitive ability should not depend on the race of their primary caregiver, comparing those of the same race. Yet two studies comparing the experience of black children raised by black or white mothers suggest that it does.12 Here too, because the children were not randomly assigned to their caregivers, it is possible that the children raised by black mothers were of lower genetic potential, but it would be hard to make such a selection story explain more than a small fraction of the apparent environmental effect.

Another trans-racial adoption study provides mixed evidence, but some of the strongest that genes play a role in explaining the black-white gap.13 A group of children, some with two black parents and some with one white and one black parent, were raised in white middle-class families. When the children's cognitive ability was tested at age seven, the children with two black parents scored 95, higher than the average black child in the state (89) and only slightly below the national average for whites, while the mixed-race children scored 110, which was considerably above it.14 On the one hand, this finding suggests a huge effect of environment on the cognitive ability of the adopted black and mixed-race children. On the other hand, the higher scores of the mixed-race children suggest that parents' genes may account for some of the difference from the black children, and that the mixed-race children may have had a better inheritance by virtue of having one white parent. Both black and mixed-race children scored worse than the biological children of their adoptive parents (who scored 116), an expected finding because the adopting parents were an elite group and likely passed on above-average genetic potential to their children. But they also scored considerably below the average of 118 for comparison white children adopted into similar homes.

When the same children were retested ten years later, the results were different.15  The scores of the children with two black parents had dropped to about the average for blacks in the state where they lived before they were adopted (89). The scores of the mixed-race children had dropped too (99), but remained intermediate between those of the children with two black parents and those of the adoptive parents' biological children, which had also declined, to 109. The scores of the white children raised in adoptive homes had dropped the most, falling to 106.

The disappearance of the salutary effect of the adoptive home, however, does not mean that genes determine black-white differences. We can assume that as the children aged and moved out into the world, the effect of the home environment diminished, and both whites and blacks tended to the average for their own population because of either genetic or environmental effects. By showing how the effect of a child's home environment disappears by adolescence, this study suggests that environmental disadvantages experienced by blacks as children cannot explain the deficit in their cognitive ability as adolescents and adults. But environmental disadvantages facing black adolescents and adults could still explain those deficits. The transience of environmental effects on cognitive ability is a theme to which I shall return. The persistence of the advantage of the mixed-race children over the children with two black parents is suggestive of a role for genes. It is not, though, definitive: several other explanations have been offered, including the late adoption of the children with two black parents and parental selection effects unrelated to race.16