Ausgabe 36/2010

How Hereditary Can Intelligence Be? Studies Show Nurture at Least as Important as Nature

By Jörg Blech

Researchers have long overestimated the role our genes play in determining intelligence. As it turns out, cognitive skills do not depend on ethnicity, and are far more malleable than once thought. Targeted encouragement can help children from socially challenged families make better use of their potential.

Photo Gallery: Nature vs. Nurture Photos

Eric Turkheimer jokes about people who believe environmental influences alone determine a person's character: "They soon change their tune when they have a second child," he says. A father himself, he is speaking from experience. His eldest daughter likes being the center of attention, while her sister is shy and more reticent at school.

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Even so, Turkheimer doubts that genetics alone can provide the complete answer. As a clinical psychologist working at the University of Virginia in Charlottesville, he repeatedly came across people whose childhoods hadn't been as carefree as those of his daughters. Many of his patients are from impoverished backgrounds.

"I could see how poverty had literally suppressed these people's intelligence," 56-year-old Turkheimer says.

Scientists typically use twins to gauge the influence of our genes on the one hand and the environment on the other. However Turkheimer noticed that such studies rarely involve twins from broken homes. Stress, neglect and abuse can have a dramatic effect on intellectual ability. And it's precisely this factor that many nature-vs.-nurture studies have completely failed to address.

Plugging a Gap

Turkheimer and his colleagues are the first scientists to have plugged this gap. Their three studies conducted in the United States on this issue have now compared the intelligence of hundreds of twins from more privileged backgrounds with those from more difficult environments. They found that the higher a child's socioeconomic status, the greater the genetic influence on the difference in intelligence. The situation is very different for children from socially disadvantaged families, where differences in intelligence were hardly inherited at all.

"The IQ of the poorest twins appeared to be almost exclusively determined by their socioeconomic status," Turkheimer says. A person's intelligence can only truly blossom if the environment gives the brain what it desires.

Ulman Lindenberger, a 49-year-old psychologist at the Max Planck Institute for Education Research in Berlin, has come to the same conclusion. He says, "The proportion of genetic factors in intelligence differences depends on whether a person's environment enables him to fulfill his genetic potential." In other words: Seeds that are scattered on infertile soil won't ever grow into large plants.

This is precisely what intelligence researchers have denied up to now. Dazzled by their studies of carefree middle-class and upper middle-class twins, they decided that cognitive skills are largely under genetic control, that academic talent is biologically hard-wired and can unfurl in almost any environment.

'Intelligence Is Highly Modifiable by the Environment'

In the meantime psychologists, neuroscientists, and geneticists have developed a very different perspective. They now believe that the skill we term "intelligence" is not in the least fixed, but is actually remarkably variable. "It is now clear that intelligence is highly modifiable by the environment," says Richard Nisbett, a psychologist at the University of Michigan in Ann Arbor.

As a result, researchers have in recent years scaled back their estimates of the influence genetics plays in intelligence differences. The previous figure of 80 percent is outdated. Nisbett says that if you take social differences into account, you would find "50 percent to be the maximum contribution of genetics." That leaves an unexpectedly large proportion of a child's intelligence for parents, teachers and educators to shape.

The findings will undoubtedly please those parents who already send their children to good schools, drive them to violin lessons in the afternoon, and then drag them around museums at the weekend. "So you haven't wasted your time, money and patience on your children after all," Nisbett says.

Time and again researchers have found that a child's genes have far less of an effect on its brain than its surroundings -- and the social environment is only one of the factors in this. Scientists in Boston, for instance, have found that children who live near roads and intersections and are thus exposed to higher levels of exhaust fumes have three IQ points fewer on average than children of the same age living in areas with cleaner air. That's simply because microscopic dust and pollutants can reach the brain and then adversely effect the nerve cells' ability to function properly.

In a similar way to those exposed to pollutants, children also suffer as a result of mental pressure, misery, worry and neglect. Chronic stress alters the way neurotransmitters work, inhibits the formation of new nerve cells and causes the hippocampus to shrivel.

That can lead to identifiable differences, as researchers at Cornell University in Ithaca, New York, have shown. They found that stressed children from poor families performed up to 10 percent worse at memory tests than well looked-after children from middle-class homes.

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