Intelligence may have more to do with how the brain develops during adolescence than its overall size, researchers said.

Using magnetic resonance imaging, scientists at the National Institutes of Health (NIH) in Bethesda, Maryland have shown that the brains of children with high IQs show a distinct pattern of development.

The cortex, or outer mantle of the brain, starts out thinner and thickens more rapidly in very intelligent children. It peaks around 11 or 12 years old before thinning rapidly in the late teens.

"We found that the cortex showed a different pattern of development," Philip Shaw, lead author of the research published in the journal Nature, said in an interview.

Youngsters with average IQs had a thicker cortex to start with and peaked earlier before gradual thinning began.

Shaw added that the changes were subtle and what is driving them is a mystery. Why children have a thicker or thinner cortex initially is also not known.

"Brainy children are not cleverer solely by virtue of having more or less grey matter at any one age," said Judith Rapoport, a co-author of the study.

"Rather IQ is related to the dynamics of cortex maturation," she added in a statement.

The scientists discovered the association between intelligence and brain development by taking MRI scans of 307 healthy children and teenagers, aged 5-19, over 2-year intervals as they grew up.

They compared the scans to see how they related to the children's IQ. Very intelligent youngsters had scores of 121-145 while high IQs were between 109-120 and average between 83-108.

The smartest youngsters showed the highest rate of change in the scans. The scientists believe the longer thickening time in the very brainy children might indicate a longer period for the development of high-level cognitive circuits in the brain.

The researchers added that the thinning phase could involve a "use it or lose it" pruning, or killing off, of brain cells and their connections as the brain matures and becomes more efficient.

"That might be happening more efficiently in the most intelligent children," said Shaw. "People with very agile minds tend to have a very agile cortex."