NATURE, NURTURE, ETC….The gold standard of IQ research has long been studies of identical twins adopted into different homes. Twins are born with the same genes, so if twins brought up by different parents end up with similar IQs anyway it’s evidence that IQ is primarily influenced by heredity. If they end up with different IQs it’s evidence that upbringing is important.
Most of the research in this area has suggested that genes are more important than upbringing. But there’s a flaw: poor people don’t adopt very often, which means the research tells us only about children brought up in middle class environments. But what about twins brought up in poor homes? Does environment play a larger role there? In the New York Times Magazine, David Kirp reports on a well-known French study published in 1996:
To answer that question, two psychologists, Christiane Capron and Michel Duyme, combed through thousands of records from French public and private adoption agencies….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.
Kirp also discusses some research by Eric Turkheimer, who noticed that even in more mundane research of twins brought up by their parents, it was mainly middle-class homes that were studied. Turkheimer went looking for more wide-ranging data:
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.
As with all of this research, take it with a grain of salt. There are plenty of things we don’t know yet, and the vast bulk of IQ research indicates pretty clearly that genes have a very strong influence on intelligence.
But it’s nonetheless worthwhile to point out what ought to be obvious in any case: even if heredity has a strong effect on intelligence, so does upbringing. And even if upbringing only accounts for 30-40% of the variance, that’s still a lot ? and it’s probably at its highest in cases where home life is the worst.
Of course, it’s still unclear what to do about this. Intensive educational interventions are the most obvious possibility, but results on this front haven’t been very promising. Still, research like this suggests pretty strongly that we shouldn’t give up. Biology isn’t chickenfeed, but it’s not destiny either. Especially among poor children, education and upbringing can have a considerable impact.