A Gene For Genius?

IN THE ANGRY DEBATE over how much of IQ comes from the genes that children inherit from parents and how much comes from experiences, one little fact gets overlooked: no one has identified any genes (other than those that cause retardation) that affect intelligence. So researchers led by Robert Plomin of London's Institute of Psychiatry decided to look for some. They figured that if you want to find a ""smart gene,'' you should look in smart kids. They therefore examined the DNA of students like those who are so bright they take college entrance exams four years early--and still score at Princeton-caliber levels. The scientists found what they sought. ""We have,'' says Plomin, ""the first specific gene ever associated with general intelligence.''

Plomin's colleagues drew blood from two groups of 51 children each, all 6 to 15 years old and living in six counties around Cleveland. In one group, the average IQ is 136. In the other group, the average IQ is 103. All the children are white. Isolating the blood cells, the researchers then examined each child's chromosome 6. (One of 23 human chromosomes along which genes made of DNA are strung like pearls, it was chosen because it is very well studied.) Of the 37 landmarks on chromosome 6 that the researchers looked for, one jumped out: a form of a gene called IGF2R occurred in twice as many children in the high-IQ group as in the average group--32 percent versus 16 percent. The study, in the May issue of the journal Psychological Science, concludes that it is this form of the IGF2R gene, called allele 5, that contributes to intelligence.

Plomin cautions that ""this is not a genius gene. It is not even the gene for general intelligence; at most it is one of many.'' (About half the differences in intelligence between one person and another are thought to reflect different genes, and half reflect different life experiences.) The gene accounts for no more than four extra IQ points. And it is neither necessary nor sufficient for high IQ: 23 percent of the average-IQ kids did have it, but 54 percent of the genius kids did not.

The smart gene is known by the snappy name ""insulinlike growth factor 2 receptor'' (IGF2R to its fans). It lets hormones like one similar to insulin dock with cells. Although a gene involved with insulin is not the most obvious candidate for an IQ gene, new evidence suggests it might indeed play that role. Sometimes when a hormone docks with a cell, it makes the cell grow; sometimes it makes the cell commit suicide. Both responses could choreograph the development of the brain: scientists at the National Institutes of Health find that insulin can stimulate nerves to grow. And in rat brains, regions involved in learning and memory are chock full of insulin receptors.

Even though this supports the idea that IGF2R can affect the brain and hence intelligence, some geneticists see major problems with the IQ-gene study. One is the possibility that Plomin's group fell for what's called the chopsticks fallacy. Geneticists might think they've found a gene for chopsticks dexterity, but all they've really found is a gene more common in Asians than, say, Africans. Similarly, Plomin's IQ gene might simply be one that is more common in groups that emphasize academic achievement. ""What if the gene they've found reflects ethnicity?'' asks geneticist Andrew Feinberg of Johns Hopkins University. ""That alone might explain the link to intelligence, since IQ tests are known for being culturally sensitive and affected by a child's environment.'' And Neil Risch of Stanford University points out that if you look for 37 genes on a chromosome, as the researchers did, and find that one is more common in smarter kids, that might reflect pure chance rather than a causal link between the gene and intelligence. Using the same techniques, says Risch, geneticists have ""discovered'' genes for schizophrenia, novelty-seeking and similar complex traits. But the finds have not been replicated by other researchers, the acid test for accuracy. Warns Feinberg: ""I would take these findings with a whole box of salt.''

If the IGF2R gene does contribute to intelligence, scientists will face a daunting question: how? Intelligence is fiendishly complicated. It is possible that some of the dozens of ""IQ genes'' actually affect, say, general health; healthy kids do better in school. ""This gene might affect something as simple as whether a fetus gets adequate nutrition,'' says Plomin. ""In that case you could overcome [the lack of the IQ gene] simply by supplementing maternal or newborn diet.'' It is unlikely that any intelligence genes will turn out to be strictly deterministic the way genes for eye color are. Says psychologist Stephen Ceci of Cornell University, ""Genes are not expressed in a vacuum.'' The trick is figuring out how genes involved in intelligence interact with the environment--which means everything from what Mom eats during pregnancy to whether Junior reads cereal boxes.