In the New York Times magazine:
After the Bell Curve - New York Times: Then along came Eric Turkheimer.... In combing through the research, he noticed that the twins being studied had middle-class backgrounds.... 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 1970s 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.... [H]ome life is the critical factor for youngsters at the bottom of the economic barrel.... 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 seeking to understand the impact of nature and nurture on I.Q., researchers have also looked at adopted children. Consistent with the proposition that intelligence is mainly inherited, these studies have almost always found that adopted youngsters more closely resemble their biological than their adoptive parents.... But researchers in France noted a shortcoming in these adoption studies... those investigations have had to focus only on youngsters placed in well-to-do homes... 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.... 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.%u2019s of those placed with well-off parents averaged 103.6....
A later study of French youngsters adopted between the ages of 4 and 6 shows the continuing interplay of nature and nurture. Those children had little going for them. Their I.Q.s averaged 77, putting them near retardation. Most were abused or neglected as infants.... Nine years later, they retook the I.Q. tests.... The amount they improved was directly related to the adopting family's status. Children adopted by farmers and laborers had average I.Q. scores of 85.5; those placed with middle-class families had average scores of 92. The average I.Q. scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98.... Taken together, these studies show that the issue has changed: it is no longer a matter of whether the environment matters but when and how it matters. And poverty, quite clearly, is an important part of the answer.
That is not to say that an affluent home is necessarily a good home. A family's social standing is only a proxy for the time and energy needed to keep a youngster mentally engaged, as well as the social capital that helps steer a child to success. There are, of course, many affluent parents who do a bad job of raising their children, and many poor families who nurture their kids with care and intelligence. On average, though, well-off households have the resources...
It really does look like all the "most variation in IQ is heredity" statements are really statements about the limited variability of environment within the pool studies. Robert Waldmann told me he thought this was the case back when he was 20, after closely observing R.J. Herrnstein.