Idea Lab
After the Bell Curve
By DAVID L. KIRP
When it comes to explaining the roots of intelligence, the fight between partisans of the gene and partisans of the environment is ancient and fierce. Each side challenges the other’s intellectual bona fides and political agendas. What is at stake is not just the definition of good science but also the meaning of the just society. The nurture crowd is predisposed to revive the War on Poverty, while the hereditarians typically embrace a Social Darwinist perspective.
A century’s worth of quantitative-genetics literature concludes that a person’s I.Q. is remarkably stable and that about three-quarters of I.Q. differences between individuals are attributable to heredity. This is how I.Q. is widely understood — as being mainly “in the genes” — and that understanding has been used as a rationale for doing nothing about seemingly intractable social problems like the black-white school-achievement gap and the widening income disparity.
"Widely"?
Earth to New York Times: As a close reader of your newspaper, I'm having trouble recalling a single article in this decade that paid serious attention to the possibility that genetic differences might partly account for the black-white school achievement gap.
Does this imply that the NYT knows that the politically correct verbiage that they print isn't supported by the scientific consensus?
If nature disposes, the argument goes, there is little to be gained by intervening. In their 1994 best seller, “The Bell Curve,” Richard Herrnstein and Charles Murray relied on this research to argue that the United States is a genetic meritocracy and to urge an end to affirmative action. Since there is no way to significantly boost I.Q., prominent geneticists like Arthur Jensen of Berkeley have contended, compensatory education is a bad bet.
But what if the supposed opposition between heredity and environment is altogether misleading? A new generation of studies shows that genes and environment don’t occupy separate spheres — that much of what is labeled “hereditary” becomes meaningful only in the context of experience. “It doesn’t really matter whether the heritability of I.Q. is this particular figure or that one,” says Sir Michael Rutter of the University of London. “Changing the environment can still make an enormous difference.” If heredity defines the limits of intelligence, the research shows, experience largely determines whether those limits will be reached. And if this is so, the prospects for remedying social inequalities may be better than we thought.
This is a confused way to get to a reasonable point. There's nothing in this article that suggests that the nature-nurture distinction is invalid. In reality, what the studies cited in this article suggest is that the heritability of IQ is less than 1.00, especially in poor environments.
Kirp cites two studies. I posted about the more interesting one, a French adoption study, back on June 20. The definitive analysis was done by Darth Quixote at GNXP two days earlier.
It's a very intriguing analysis because it tries to overcome the usual restriction of range problem in American adoption studies, which typically have shown almost no impact of home environment on adult IQ. The methodological problem is that most adoptions these days are made by affluent couples of the biological offspring of couples of lower status. If you are, say, Bill Gates's kid, you probably won't end up being adopted by some meth head couple in a trailer park. That's good for the kid, but not for the science.
Capron and Duyme came up with eight cases of children who are the biological offspring of highly educated parents being adopted by poorly educated parents (along with ten cases of the other three alternatives: high nature - high nurture, low nature - high nurture, low nature - low nurture, for a grand total of 38 kids in the study. But it's still a good first step.
Regardless of whether the adopting families were rich or poor, Capron and Duyme learned, children whose biological parents were well-off had I.Q. scores averaging 16 points higher than those from working-class parents. Yet what is really remarkable is how big a difference the adopting families’ backgrounds made all the same. The average I.Q. of children from well-to-do parents who were placed with families from the same social stratum was 119.6. But when such infants were adopted by poor families, their average I.Q. was 107.5 — 12 points lower. The same holds true for children born into impoverished families: youngsters adopted by parents of similarly modest means had average I.Q.’s of 92.4, while the I.Q.’s of those placed with well-off parents averaged 103.6. These studies confirm that environment matters — the only, and crucial, difference between these children is the lives they have led.
It strikes me as plausible that the nature-nurture balance could be in the 60-40 range, as the French found, at least when sizable environmental differences are possible. A study with a sample size of 38 is not conclusive, but I personally find this more likely than the idea that adoption would have zero impact on IQ.
Also, there is evidence that IQ is malleable before puberty, but that people generally revert to their genetic level as they mature. (Sandra Scarr's Minnesota Transracial Adoption study followed this pattern, with the hopeful early IQ results of black children adopted by upper middle white parents being dashed by their scores falling to an average of 89 when they were retested at 17.) The French study tested the adopted children at age 14, which is fairly late, although probably not late enough to settle this question.
I suspect that having a higher IQ as a child has long term benefits even if you revert back to your long term norm as an adult. Somebody with a long term IQ of 80 who had a good upbringing that raised it to 90 as a child is much more likely to learn how to read and how to be a functioning adult than somebody with the same genes who had a bad upbringing. But I haven't seen any direct studies of this.
Kirp contends that the French study supports his pet project of "universal preschool," although a less biased reading probably suggests that if adoption can work to raise IQs, then little children being raised by high status moms are better off staying home with mom than going off to some run-of-the-mill government preschool.
Anyway, the effects of preschools were intensively studied directly in the 1960s through 1980s, and the results in terms of boosting IQ were unimpressive, unless the expenditures were so vast as to approach adoption. On the other hand, Head Start seems to have some good effect on reducing delinquency later on. IQ ain't everything.
When quantitative geneticists estimate the heritability of I.Q., they are generally relying on studies of twins. Identical twins are in effect clones who share all their genes; fraternal twins are siblings born together — just half of their genes are identical. If heredity explains most of the difference in intelligence, the logic goes, the I.Q. scores of identical twins will be far more similar than the I.Q.’s of fraternal twins. And this is what the research has typically shown. Only when children have spent their earliest years in the most wretched of circumstances, as in the infamous case of the Romanian orphans, treated like animals during the misrule of Nicolae Ceausescu, has it been thought that the environment makes a notable difference. Otherwise, genes rule.
Then along came Eric Turkheimer to shake things up. Turkheimer, a psychology professor at the University of Virginia, is the kind of irreverent academic who gives his papers user-friendly titles like “Spinach and Ice Cream” and “Mobiles.” He also has a reputation as a methodologist’s methodologist. In combing through the research, he noticed that the twins being studied had middle-class backgrounds. The explanation was simple — poor people don’t volunteer for research projects — but he wondered whether this omission mattered.
Together with several colleagues, Turkheimer searched for data on twins from a wider range of families. He found what he needed in a sample from the 1970’s of more than 50,000 American infants, many from poor families, who had taken I.Q. tests at age 7. In a widely-discussed 2003 article, he found that, as anticipated, virtually all the variation in I.Q. scores for twins in the sample with wealthy parents can be attributed to genetics. The big surprise is among the poorest families. Contrary to what you might expect, for those children, the I.Q.’s of identical twins vary just as much as the I.Q.’s of fraternal twins. The impact of growing up impoverished overwhelms these children’s genetic capacities. In other words, home life is the critical factor for youngsters at the bottom of the economic barrel. “If you have a chaotic environment, kids’ genetic potential doesn’t have a chance to be expressed,” Turkheimer explains. “Well-off families can provide the mental stimulation needed for genes to build the brain circuitry for intelligence.”
This theory is plausible. It's comparable to the argument I made back in 2002 that comparing the average African IQ of 70 to the 85 average of their African-American cousins suggests that the bad environment in Africa is keeping Africans from reaching their genetic potential.
On the other hand, the connection between this theory and Turkheimer's actual findings seems tenuous.
Unfortunately, Turkheimer's paper isn't terribly persuasive because it seems disingenuous. It's particularly frustrating to read because, as far as I can tell, it refuses to tell us what were the average IQs of the children tested, or most of the other most interesting basic facts about the data.
A few years ago I emailed Turkheimer asking him to reveal these numbers, but he never responded. Later, I got an email from a friend of Turkheimer's chiding me for criticizing his paper. When I explained that I needed to know these basic facts about the study, he agreed, and offered to ask Turkheimer for the numbers, but then I never heard anything more.
This is important because psychometrician John Ray has put forward a plausible-sounding alternative suggestion:
Full publication of the study has not been done as yet but from what we know so far it seems that what they found was in fact much simpler than that. They found that if you separated out low income respondents (mostly black) and studied them alone, the role of heredity was less important in explaining IQ differences. That does sound like a real finding but it is in fact what statisticians would call a “restriction of range effect”. In other words, if you take ANY group and select out a subset that is relatively homogeneous with regard to some variable, differences in that variable will tend to have less importance in explaining other differences. Since socioeconomic status and race are substantially correlated with heritable IQ, that is precisely what these researchers have done: Selected a group that is relatively homogeneous in genetic inheritance for IQ and then said: “Hey! Differences in genetic inheritance are not so important here!” Statisticians would call the finding an “artifact” -- i.e. something created by the research procedure rather than a genuine finding about the world.
But, Turkheimer won't tell us the numbers, so everything is just speculation.
My published articles are archived at iSteve.com -- Steve Sailer
Posts
No comments:
Post a Comment