IQ: interaction between race and age

In discussions of race and IQ, debate often focuses on a study of children fathered by soldiers stationed in Germany and then raised by German mothers (Eyferth 1961). The study found no significant difference in IQ between children with white fathers (83 subjects) and those with black fathers (98 subjects), the mean IQ being about 97 for both groups.

These findings are criticized by Rushton and Jensen (2005) on three grounds:

1). The children were still young when tested. One third were between 5 and 10 years old and two thirds between 10 and 13 years old. Since family socialization effects are stronger before puberty, a much larger sample would be needed to find a significant difference between the two groups.

2) Between 20 and 25% of the ‘black’ fathers were actually North African.

3) At the time, the US Army screened out low IQ applicants with its preinduction Army General Classification Test. The rejection rate was about 30% for African Americans and 3% for European Americans.

I have doubts about the first criticism. Throwing in more environmental differences would make an innate difference less significant and harder to detect. But why would it disappear? It should do so only if the family environment were, on average, more conducive to learning in the biracial group. Both groups, however, had the same kind of family environment, i.e., a single German mother brought up the children. Am I missing something? I guess this is where the other two criticisms come in, i.e., the fathers were either North Africans or above-average African Americans.

But why, then, do we see similar IQ scores in children whose biological parents are clearly African-American and probably below-average ones at that? I am talking here of African-American children given up for adoption. At 7 years of age, Moore (1986) found a mean IQ of 117 among those placed in middle-class white families and a mean IQ of 104 among those placed in middle-class black families. These findings are criticized by Rushton and Jensen (2005), who argue that the children were raised in enriched environments and that the sample sizes were small (23 children in each case).

We see comparable findings, however, when IQ is measured with robust standardization samples of African-American and European-American children. When Dickens and Flynn (2006) analyzed the results of the 2002 standardization sample for the WISC (Wechsler Intelligence Scale for Children), they found that IQ starts off high in African American children and then declines with age:

African-American WISC scores
Age -- IQ
4 --- 95.5
12 -- 90.5
15 -- 88.8
24 -- 84.5

Dickens and Flynn (2006) also note that these scores show a gain of 5-6 points over the scores of black children thirty years earlier. But the decline of their IQ with age has remained stable. This decline also shows up in the Minnesota Transracial Adoption Study —a longitudinal study of black, biracial (black/white), and white children adopted into white middle-class Minnesota families (Scarr and Weinberg, 1976; Weinberg, Scarr, and Waldman, 1992). The children’s IQs were measured at 7 years of age and again ten years later:

------------------- Age 7 Age 17
Black children –---- 97 --- 89
Biracial children – 109 --- 99
White children –-- 112 -- 106

These findings parallel those of the Eyferth study. Young biracial children have IQ scores that are close to those of young white children. Even young black children have relatively high IQ. In this age group, interventions to raise IQ are successful. But the effects seem to wash out with age.

Rushton and Jensen (2005) are aware of this age trend, but they attribute it to the declining influence of family environments, i.e., at younger ages the diversity of learning environments within and between families accounts for a larger proportion of total IQ variability. This phenomenon would tend to obscure other sources of variability, thus making any black-white difference less significant. But why would it raise the IQ scores of black children while leaving those of white children unchanged? I suppose one might argue that family environments are more conducive to learning among African Americans than among European Americans. Is there any basis for such a belief?

In saying that this age decline is real, I’m not excluding an innate causation. Indeed, a socially induced causation seems less consistent with the data. It would have to affect even biracial children conceived by white mothers and raised by white parents in an overwhelmingly white cultural environment.

Among humans in general, intellectual capacity seems to decline with age. Indeed, there are statements in the literature that IQ declines from one’s twenties onwards (presumably among European Americans). Is this decline due to natural aging processes? Or is it prewired into the human organism?

Perhaps the ability to acquire new information becomes less useful with age and perhaps this was even truer in ancestral humans. What we call ‘intelligence’ may have originally been an infant trait that humans lost as they grew up. With the expansion of our cultural environment, natural selection would have progressively extended this infant trait into older age groups, and more so in some populations than in others.

By way of analogy, lactose tolerance was originally an infant trait and is still so in most human populations. It has become an adult trait in those populations that have long practiced dairy farming and adult consumption of milk.

References

Dickens, W.T., and J.R. Flynn. (2006). Black Americans reduce the racial IQ gap. Evidence from standardization samples. Psychological Science, 17, 913-920.

Eyferth, K. (1961). Leistungen verscheidener Gruppen von Besatzungskindern in Hamburg-Wechsler Intelligenztest für Kinder (HAWIK). Archiv für die gesamte Psychologie, 113, 222-241.

Moore, E.G.J. (1986). Family socialization and the IQ test performance of traditionally and transracially adopted Black children. Developmental Psychology, 22, 317-326.

Rushton, P. and A.R. Jensen. (2005). Thirty years of research on race differences in cognitive ability. Psychology, Public Policy, and Law, 11, 235-294.

Scarr, S., and Weinberg, R.A. (1976). IQ test performance of Black children adopted by White families, American Psychologist, 31, 726-739.

Weinberg, R.A., Scar, S., and Waldman, I.D. (1992). The Minnesota Transracial Adoption Study: A follow-up of IQ test performance at adolescence. Intelligence, 16, 117-135.