The Research for Intelligence

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A. In Robert Plomin’s line of work, patience is essential. Plomin, a behavioral geneticist at the Institute of Psychiatry in London, wants to understand the nature of intelligence. As part of his research, he has been watching thousands of children grow up. Plomin asks the children questions such as "What do water and milk have in common?" and "In what direction does the sun set?" At first he and his colleagues quizzed the children in person or over the telephone. Today many of those children are in their early teens, and they take their tests on the Internet. In one sense, the research has been a rousing success. The children who take the tests are all twins, and throughout the study identical twins have tended to get scores closer to each other than those of non-identical twins, who in turn have closer scores than unrelated children. These results -- along with similar ones from other studies -- make clear to the scientists that genes have an important influence on how children score on intelligence tests.

B. But Plomin wants to know more. He wants to find the specific genes that are doing the influencing. And now he has a tool for pinpointing genes that he could not have even dreamed of when he began quizzing children. Plomin and his colleagues have been scanning the genes of his subjects with a device called a micro array, a small chip that can recognize half a million distinctive snippets of DNA. The combination of this powerful tool with a huge number of children to study meant that he could detect genes that had only a tiny effect on the variation in scores.

C. Still, when Plomin and his co-workers unveiled the results of their micro-array study -- the biggest dragnet for intelligence-linked genes ever undertaken -- they were underwhelming. The researchers found only six genetic markers that showed any sign of having an influence on the test scores. When they ran stringent statistical tests to see if the results were flukes, only one gene passed. It accounted for 0.4 percent of variation in the scores. And to cap it all off, no one knows what the gene does in the body. "It’s a real drag in some ways," Plomin says.

D. Plomin’s experience is a typical one for scientists who study intelligence. Along with using micro-arrays, they are employing brain scans and other sophisticated technologies to document some of the intricate dance steps that genes and environment take together in the development of intelligence. They are beginning to see how differences in intelligence are reflected in the structure and function of the brain. Some scientists have even begun to build a new vision of intelligence as a reflection of the ways in which information flows through the brain. But for all these advances, intelligence remains a profound mystery. "It’s amazing the extent to which we know very little," says Wendy Johnson, a psychologist at the University of Minnesota.

E. In some ways, intelligence is very simple. "It’s something that everybody observes in others," says Eric Turkheimer of the University of Virginia. Everybody knows that some people are smarter than others, whatever it means technically. It’s something you sense in people when you talk to them. "Yet that kind of gut instinct does not translate easily into a scientific definition. In 1996 the American Psychological Association issued a report on intelligence, which stated only that "individuals differ from one another in their ability to understand complex ideas, to adapt effectively to the environment, to learn from experience, to engage in various forms of reasoning, to overcome obstacles by taking thought."

F. To measure these differences, psychologists in the early 1900s invented tests of various kinds of thought, such as math, spatial reasoning and verbal skills. To compare scores on one type of test to those on another, some psychologists developed standard scales of intelligence. The most familiar of them is the intelligence quotient, which is produced by setting the average score at 100. IQ scores are not arbitrary numbers, however. Psychologists can use them to make strong predictions about other features of people’s lives. It is possible to make reasonably good predictions, based on IQ scores in childhood, about how well people will fare in school and in the workplace. People with high IQs even tend to live longer than average. "If you have an IQ score, does that tell you everything about a person’s cognitive strengths and weaknesses? No," says Richard J. Haier of the University of California, Irvine. But even a simple number has the potential to say a lot about a person.

G. Then what underlies an intelligence score? "It’s certainly tapping something," says Philip Shaw, a psychiatrist at the National Institute of Mental Health (NIMH). The most influential theory of what the score reflects is more than a century old. In 1904 psychologist Charles Spearman observed that people who did well on one kind of test tended to do well on others. The link from one score to another was not very tight, but Spearman saw enough of a connection to declare that it was the result of something he called a g factor, short for general intelligence factor. How general intelligence arose from the brain, Spearman could not say. In recent decades, scientists have searched for an answer by finding patterns in the test scores of large groups of people. Roughly speaking, there are two possible sources for these variations. Environmental influences -- anything from the way children are raised by their parents to the diseases they may suffer as they develop -- are one source. Genes are another. Genes may shape the brain in ways that make individuals better or worse at answering questions on intelligence tests.