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IQ: The Most Complex--and Controversial--of All Complex Traits

Susan Notes: Clearly, the editorial writers at the Wall Street Journal don't read the rest of the paper. Note the conclusion of Sharon Begley, writer of Science Journal:

"If we care about intelligence, we must seek ways to nurture it not in the genes we pass on to our kids, but in the world we make for them."

There is wide agreement that cognitive ability at least partly reflects the influence of DNA: Dozens of studies of thousands of twins have shown identical twins, who share the same genes, tend to have more-similar IQs than do other sibling pairs, and children match the IQ of their biological more than their adoptive parents.

Together, these studies imply genes account for about 50% of the difference in intelligence from one person to the next. That's a high enough "heritability" that you'd think genome labs would be practically spitting out genes related to intelligence.

But they're not. And therein may lie an important clue to the biology of what Robert Plomin, a professor of behavioral genetics at King's College London, calls "the most complex -- and most controversial -- of all complex traits."

Intelligence has many meanings, but what scientists call general cognitive ability seems to reflect memory skills, verbal and spatial abilities, and abstract reasoning. Usually, if you're good at one, you're good at the others. Although that correlation may reflect not "brain quality" alone but something nonphysiological, such as differences in motivation, it has inspired a search for genes that make better brains.

Prof. Plomin and his colleagues were the first to identify a suspect. In 1998, they reported that one form of a gene called insulin-like growth factor-2 receptor was present in 32% of children with high IQs, but in 16% of kids with average IQs. It was also especially frequent in people with exceptional math or verbal talents. Experiments in other labs had shown the gene is active in regions of the brain devoted to learning and memory. But when the King's team tried to replicate its finding, it failed: The "smart" gene showed up in 19% of high-IQ children ... and 24% of those with average IQ.

That didn't deter biologists. Since 2000, teams have identified at least four more genes associated with intelligence. Two studies fingered genes for an enzyme called catechol O-methyltransferase. Others identified cathepsin D, CHRM2, or cystathionine beta-synthase as having variants that are more common in people with high IQs.

As with all such studies, you have to watch out for a chopsticks effect. Just because a genetic variant shows up more often in people adept at using chopsticks doesn't mean it causes manual dexterity: It may simply be more prevalent in Asian populations. Similarly, purported IQ genes may cluster, by chance, in groups whose culture values education, yet not actually make a brain smarter. There's another problem. Neuroscientists can't find any fundamental brain processes that distinguish Einstein from the rest of us -- not speed of neuronal transmission, not the ability to form synapses, not the quantity and quality of neurons, Prof. Plomin says. That makes it less likely that genes for those basic characteristics (even if scientists find them) have a significant effect on intelligence.

Even if the newly suspect intelligence genes hold up, they will surely turn out to be only the tip of a huge iceberg. It looks more and more as if intelligence reflects the complex interaction of scores of genes with each other and the environment. No one gene makes more than a tiny difference. Different forms of CHRM2, for instance, account for a spread of only three or four IQ points, while CTSD may account for perhaps 3% of the variation between people.

The heritability of intelligence may, paradoxically, reflect the importance of environment. If Susie is born with a slightly better brain than Mary, she will like school, receive more praise from her teachers, haunt the library, take more demanding courses. In short, she will bootstrap her way to greater intelligence.

That explains why the measure of heritability of intelligence rises with age, from 40% in childhood to 60% in adulthood. It isn't that genes grow stronger. Instead, says James R. Flynn of the University of Otago, Dunedin, New Zealand, a slight genetic edge at birth snowballs by nudging people to choose intelligence-enhancing experiences. The result is "a potent multiplier," he says in Current Directions in Psychological Science.

Prof. Flynn discovered that IQ soared in recent decades. Since 1950, scores on one IQ subtest have risen 18 points per generation in the Netherlands, Belgium, Israel and Argentina; between 1948 and 1989, Americans gained the equivalent of 20 IQ points. The genes we have don't change fast enough to explain this "Flynn effect" -- but which genes are turned on might. Perhaps growing up with enough leisure time to play chess and even videogames, or living in smaller and more affluent families that can indulge children's intellectual curiosity, turns up the activity of genes related to intelligence. For that reason, says Prof. Plomin, the holy grail in this field is identifying what experiences turn on genes that influence intelligence.

Even before that happens, it's already clear that, with so many genes involved in IQ, genetic engineering for it isn't in the cards. If we care about intelligence, we must seek ways to nurture it not in the genes we pass on to our kids, but in the world we make for them.

— Sharon Begley
Many Factors Make Up a High IQ: Good Genes Do Count, but Intelligence
Wall Street Journal
June 20, 2003


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