This week's New York Times Magazine includes a fascinating article about the flaws in the research on the determinants of intelligence.
Previously, as the article points out, studies had shown that "a person's I.Q. is remarkably stable and that about three-quarters of I.Q. differences between individuals are attributable to heredity" -- a depressing finding for social reformers. But the article shows that this conclusion was based on a flawed methodological approach that is being overturned in new research that includes twins from poor families:
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... 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.
...In combing through the research [on IQ], [UVA's Eric Turkheimer] 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 provocative finding was confirmed in a study published last year. An analysis of the reading ability of middle-aged twins showed that even half a century after childhood, family background still has a big effect — but only for children who grew up poor. Meanwhile, Turkheimer is studying a sample of twins who took the National Merit Scholarship exam, and the results are the same. Although these are the academic elite, who mostly come from well-off homes, variations in family circumstances still matter: children in the wealthiest households have the greatest opportunity to develop all their genetic capacities. The better-off the family, the more a child's genetic potential is likely to be, as Turkheimer puts it, "maxed out."
In addition, studies of the effect of environment on adopted twins placed in different homes used a skewed sample as well:
...[R]esearchers in France noted a shortcoming in these adoption studies and set out to correct it. Since poor families rarely adopt, those investigations have had to focus only on youngsters placed in well-to-do homes. What's more, because most adopted children come from poor homes, almost nothing is known about adopted youngsters whose biological parents are well-off.
What happens in these rare instances of riches-to-rags adoption? To answer that question, two psychologists, Christiane Capron and Michel Duyme, combed through thousands of records from French public and private adoption agencies. "It was slow, dusty work," Duyme recalls. Their natural experiment mimics animal studies in which, for instance, a newborn rhesus monkey is taken from its nurturing biological mother and handed over to an uncaring foster mother. The findings are also consistent: how genes are expressed depends on the social context.
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.