Nurture
When quantitative geneticists estimate the heritability of IQ, 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 IQs of identical twins will be far more similar than the IQs 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, has it been thought that the environment makes a notable difference.
In combing through the research, though, Eric Turkheimer noticed that the twins being studied had middle-class backgrounds. (Poor people seldom volunteer for research projects.)
Turkheimer searched for data on twins from a wider range of families. He found a sample from the 1970s of more than 50,000 American infants, many from poor families, who had taken IQ tests at age 7. He found that, as anticipated, virtually all the variation in IQ scores for twins in the sample with wealthy parents can be attributed to genetics. Among the poorest families, though, the IQs of identical twins vary just as much as the IQs of fraternal twins. The impact of growing up impoverished overwhelms these children’s genetic capacities.
This 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 twins who took the National Merit Scholarship exam, and has found that, though these students mostly come from well-off homes, variations in family circumstances still matter.
Consistent with the proposition that intelligence is mainly inherited, 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. 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.
Christiane Capron and Michel Duyme combed through thousands of records from French public and private adoption agencies. Regardless of whether the adopting families were rich or poor, children whose biological parents were well-off had IQ scores averaging 16 points higher than those from working-class parents. The average IQ 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 IQ was 107.5 — 12 points lower. Youngsters adopted by parents of similarly modest means had average IQs of 92.4, while the IQs of those placed with well-off parents averaged 103.6.
One study analyzed French youngsters adopted between the ages of 4 and 6. Most had been abused or neglected as infants, then shunted from one foster home or institution to the next, and they scored an average of 77 on IQ tests. Nine years later, when they retook the tests, all of them did better. The amount they improved was directly related to the adopting family’s status. Children adopted by farmers and laborers had average IQ scores of 85.5; those placed with middle-class families had average scores of 92. The average IQ scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98 — a jump from borderline retardation to a whisker below average.
In Meaningful Differences in the Everyday Experience of Young American Children, Betty Hart and Todd Risley find that by the time they are 4 years old, children growing up in poor families have typically heard a total of 32 million fewer spoken words than those whose parents are professionals. That language gap translates directly into stunted academic trajectories.