High-IQ youth are distinguished by how fast the thinking part of their brains thickens and thins as they grow up, researchers have found.

Brain scans showed that their brain’s outer coat, the cortex, thickens more rapidly during childhood, the scientists said.

It also reaching its peak thickness relatively late, they added, perhaps reflecting a longer development time for high-level thinking.

During the late teens, the cortex normally thins rather than thickens, in what scientists believe is a withering of unused circuitry as the brain streamlines its operations.

This process was also found to occur faster in youth measured to have the highest intelligence through IQ tests.

“Brainy children are not cleverer solely by virtue of having more or less gray matter at any one age,” said Judith Rapoport of the National Institute of Mental Health in Bethesda, Md., one of the researchers.

“Rather, IQ is related to the dynamics of cortex maturation.”

Researchers at the institute, a part of the National Institutes of Health, conducted the study with scientists at McGill University in Montreal. They reported the results in the March 30 issue of the research journal Nature.



The developmental trajectory of waxing and waning in cortex thickness differs as the brain matures in different IQ groups, researchers say. Thickness of the area highlighted in brain maps above, peaks relatively late, at age 12 (blue arrow), in youth with "superior" or highest intelligence. "High" intelligence is an intermediate category between "superior" and average. (Courtesy NIMH Child Psychiatry Branch)


They drew their conclusions from Magnetic Resonance Imaging, or MRI, brain scans, which use magnetism and radio waves produce images of body structures. The study followed 307 children and teens, ages 5 through 19, as they grew up.

The researchers found that the relationship between cortex thickness and IQ varied with age, especially in the prefrontal cortex, seat of abstract reasoning, planning, and related operations known as “executive” functions.

The smartest 7-year-olds tended to start out with a thinner cortex that thickened rapidly, peaking by age 11 or 12 before thinning, the scientists found. In average-IQ children, an initially thicker cortex peaked by age 8, with gradual thinning thereafter.

The researchers wrote that the prolonged thickening of prefrontal cortex in high-IQ children might reflect an “extended critical period for development of high-level cognitive circuits.” Although it’s unknown what underlies the thinning, they added, evidence suggests it reflects “use-it-or-lose-it” pruning of brain cells, neurons, and their connections as the brain becomes more efficient.

“People with very agile minds tend to have a very agile cortex,” said the institute’s Philip Shaw, a member of the research team.

The researchers said they’re following up with a search for gene variants possibly linked to the different maturation pathways. But Shaw noted mounting evidence suggesting the effects of genes often depend on interactions with outside events, so the determinants of intelligence will likely prove to be a very complex mix of nature and nurture.

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