Unseen Influences: Race, Gender, and Twins

Steven Fraser, in The Bell Curve Wars, discusses the problems with Hernstein and Murray’s genetic argument for IQ.

He points out that the Flynn effect is particularly devastating. For this reason, he finds it puzzling that they don’t recognize or acknowledge the obvious implications. Black people today are on average smarter, as far as IQ tests go, than white people were a few generations ago. By today’s normed IQ tests, white people of a century ago would now be labeled as “retarded”.

I’ve covered that territory before. What caught my attention the other day was what followed his comments on the Flynn effect. He made a further point about the weakness of the genetics hypothesis. He states that a “remarkable phenomenon commented on in the Moynihan Report of thirty years ago goes unnoticed in The Bell Curve–the prevalence of females among blacks who score high on mental tests” (Kindle Locations 914-925); he continues:

“Others who have done studies of high-IQ blacks have found several times as many females as males above the 120 IQ level. Since black males and black females have the same genetic inheritance, this substantial disparity must have some other roots, especially since it is not found in studies of high-IQ individuals in the general society, such as the famous Terman studies of high-IQ children, which followed these children on into adulthood and later life. If IQ differences of this magnitude can occur with no genetic difference at all, then it is more than mere speculation to say that some unusual environmental effects must be at work among blacks.”

This isn’t limited to any race/ethnicity. It is a gender IQ gap found across diverse other populations.

“However, these environmental effects need not be limited to blacks, for other low-IQ groups of European or other ancestries have likewise tended to have females over-represented among their higher scorers, even though the Terman studies of high-IQ individuals from the general population found no such patterns. One possibility is that females are more resistant to bad environmental conditions, as some other studies suggest. In any event, large sexual disparities in high-IQ individuals where there are no genetic-or socioeconomic-differences present a challenge to both the Herrnstein-Murray thesis and to most of their critics.”

This reminds me of the stereotype threat discussed by Claude M. Steele in Whistling Vivaldi. He shows the research about how much simple changes in environment can cause large changes in results, both for tests of academics and other activities. Women tend to test lower on math, for example. However, neutralize stereotype threat and the disparity disappears.

Environments aren’t just different between populations, but also within populations. The environmental factors that will impact a female are different than for a male, including the stereotypes and expectations placed upon genders just as happens with race. Having much shared genetics doesn’t necessarily mean that all influences are being shared.

To emphasize this point, Fraser extends his argument to an even more stark example. Twins also show great differences, something overlooked by early twin studies.

“Black males and black females are not the only groups with significant IQ differences without any genetic differences. Identical twins with significantly different birth weights also have IQ differences, with the heavier twin averaging nearly nine points higher IQ than the lighter one in some studies.’ This effect is not found where the lighter twin weighs at least six and a half pounds, suggesting that deprivation of nutrition must reach some threshold level before it has a permanent effect on the brain during its crucial early development.”

Slight changes in environment can lead to immense differences over the long term. This is because of the cumulative effect of initial conditions. One thing leads to another. Lowered nutrition or increased toxicity has its impact which gets magnified by such things as school tracking. Each effect becoming a cause and all the causal factors combining to form significant differences in end results.