It's not a particular brain region that makes someone smart or not smart. Nor is it the strength and speed of the connections throughout the brain or such features as total brain volume.
Instead, new research shows, it's the connections between very specific areas of the brain that determine intelligence and often, by extension, how well someone does in life.
"General intelligence actually relies on a specific network inside the brain, and this is the connections between the gray matter, or cell bodies, and the white matter, or connecting fibers between neurons," said Jan Glascher, lead author of a paper appearing in this week's issue of the Proceedings of the National Academy of Sciences. "General intelligence relies on the connection between the frontal and the parietal [situated behind the frontal] parts of the brain."
The results weren't entirely unexpected, said Keith Young, vice chairman of research in psychiatry and behavioral science at Texas A&M Health Science Center College of Medicine in Temple, but "it is confirmation of the idea that good communication between various parts of brain are very important for this generalized intelligence."
General intelligence is an abstract notion developed in 1904 that has always been somewhat controversial.
"People noticed a long time ago that, in general, people who are good test-takers did well in a lot of different subjects," explained Young. "If you're good in mathematics, you're also usually good in English. Researchers came up with this idea that this represented a kind of overall intelligence."
"General intelligence is this notion that smart people tend to be smart across all different kinds of domains," added Glascher, who is a postdoctoral fellow in the department of humanities and social sciences at the California Institute of Technology in Pasadena.
Hoping to learn more, the authors located 241 patients who had some sort of brain lesion. They then diagrammed the location of their lesions and had them take IQ tests.
"We took patients who had damaged parts of their brain, tested them on intelligence to see where they were good and where they were bad, then we correlated those scores across all the patients with the location of the brain lesions," Glascher explained. "That way, you can highlight the areas that are associated with reduced performance on these tests which, by the reverse inference, means these areas are really important for general intelligence."
"These studies infer results based on the absence of brain tissue," added Paul Sanberg, distinguished professor of neurosurgery and director of the University of South Florida Center for Aging and Brain Repair in Tampa. "It allows them to systemize and pinpoint areas important to intelligence."
Young said the findings echo what's come before. "The map they came up with was what we expected and involves areas of the cortex we thought would be involved -- the parietal and frontal cortex. They're important for language and mathematics," he said.
In an earlier study, the same team of investigators found that this brain network was also important for working memory, "the ability to hold a certain number of items [in your mind]," Glascher said. "In the past, people have associated general intelligence very strongly with enhanced working memory capacity so there's a close theoretical connection with that."