IDEA LAB

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. 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.

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 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. Such findings have supported the assumption that changing a child’s life circumstances won’t alter the hard facts of nature.

But researchers 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.

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, then shunted from one foster home or institution to the next.

Nine years later, they retook the I.Q. tests, and contrary to the conventional belief that I.Q. is essentially stable, 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 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 — a jump from borderline retardation to a whisker below average. That is a huge difference — a person with an I.Q. of 77 couldn’t explain the rules of baseball, while an individual with a 98 I.Q. could actually manage a baseball team — and it can only be explained by pointing to variations in family circumstances.

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 needed to provide better settings for the fullest development of a child’s natural abilities. In “Meaningful Differences in the Everyday Experience of Young American Children,” the University of Kansas psychologists 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.

Is there a way to reduce such gaps? In recent years, the case for investing in early-childhood education has become stronger and stronger. The federal Early Head Start program for infants and toddlers is effective when it is well implemented — in part because it succeeds in getting parents more involved with their children. Recent research also shows that one year of high-quality state prekindergarten can give children as much as a seven-month advantage in vocabulary; this, in turn, is a good predictor of how well they will read when they are in primary school. As you would expect, poor children benefit the most, especially when they are in classes with middle-class youngsters.

The push for universal preschool is not a red-state-blue-state issue; the pioneers in the area are Oklahoma and Georgia, not generally known for social progressivism. And with the support of business groups and prominent philanthropists like Susan Buffett, the daughter of Warren Buffett, it may enter the national agenda. If it does, it will be a small step toward a society in which not only the most fortunate children will be able to “max out” their potential.

David L. Kirp, a professor of public policy at the University of California at Berkeley, is writing a book about the universal preschool movement.

Copyright 2006 The New York Times Company. Reprinted from The New York Times Magazine of Sunday, July 23, 2006.