One surprisingly small region of the brain is responsible for general intelligence.

Smart means different things to different people. To a scientist it usually means being able to solve hard problems. Measuring this kind of problem-solving intelligence is not difficult. Quite the contrary. In fact, whatever test of intelligence you administer, wether focused on verbal or spatial thinking, the same people tend to do well. This led Psychologist Charles Spearman to propose way back in 1904 that there must be some general factor of intelligence that the tests were measuring. He called it g, and argued that it contributed to success on a wide range of cognitive tasks. g is what I.Q. largely measures (it measures other more specific abilities as well).

Most psychologists did not agree with Spearman, favoring instead the idea that high-level thinking involves many factors. The idea was that analytical thinking requires a large set of separate information handling abilities from different parts of the brain, some spatial, some verbal, many involved with the speed with which particular groups of nerve cells can process information.

Dr. John Duncan of the Medical Research Council in Cambridge, England and colleagues reported in the journal SCIENCE this week that Spearman had it right all along.

Duncan asked people to solve I.Q.-type questions, and while they were attempting to do so, he scanned their brains to see what parts were active. This isn’t as bizarre as it sounds. A standard PET (positron emission tomography) brain imaging device measures the relative blood flow in the brain, revealing what parts of the brain are being recruited into action.

The Duncan team tested 60 people ages 29-51 on a range of I.Q. test questions, some testing verbal thinking, others spatial thinking. To screen out all the other extraneous activities going on in an active brain, Duncan administered easy (low-g) and hard (high-g) questions, then looked to see if attempting to solve the hard ones lit up any areas that easy ones did not.

It did. Whenever high-g questions were worked on, the same small region of the brain was activated in each person, just as Spearman’s hypothesis predicts. Called the lateral prefrontal cortex, the g region is a zone about the diameter of a golf ball on each side of the brain, above the outer edge of the eyebrow. For spatial problems, people used the g region on both sides of their brain, while for verbal problems they used only the left side.

“What we are seeing here seems to be a global workspace for organizing and coordinating information,” Duncan said in an interview with the New York Times. It is the relative performance of this g region workspace — how it is wired — that is measured by intelligence tests, he claims. As a biologist I know that genes largely determine how our brains are wired. That’s why I.Q. is so heritable. Comparing identical twins to fraternal ones, living together vs. living apart, geneticists have repeatedly estimated the heritability of I.Q. to be on the order of 70% – 80%, meaning that up to 80% of the variation in I.Q. among people is due to variation in their genes.

Environment and education play a role in how intelligence develops — but not much, it turns out. In a study of 90 pairs of virtual twins (that is, pairs of unrelated adopted children of the same age raised from infancy as siblings) reported last week, shared environment had only a modest impact on intelligence. The children, most aged 4 to 7, were given standardized I.Q. tests. When the tests were analyzed, the score of one child was only very weakly correlated with that of its virtual twin. The correlation coefficient was 0.26. In parallel tests, true siblings scored 0.50, and identical twins scored 0.86.

I don’t know why I find this sort of result disquieting. The genes I inherited from my parents specified my hair color (red), the color of my eyes (brown), what my face looks like, and a host of other physical features. Why should I be in any way surprised that I also inherited genes that specify instructions for wiring a region of my brain above my eyebrows?

I am old enough to have been in school when all gradeschool students were routinely given I.Q. tests. I never learned the results of the I.Q. tests I took as a school kid. My parents chose not to tell me. They didn’t think an individual kid should be saddled with a label grading how smart or dumb he or she is. “It can only limit what you achieve,” they told me. What they were trying to say was that life’s challenges require a lot more than efficient wiring of the prefrontal cortex — that there’s a lot more to life than smarts. Like most of their advice, it makes more sense as I get older. Getting smarter, I guess.

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