Clever people outsmart their peers not because they have more grey matter but because part of their brain develops differently, a Nature study suggests.
Brain tissues wax and wane during childhood
The US National Institute of Mental Health used scans to study development of the cortex, which is responsible for thinking, in 307 children.
They found smarter youngsters tended to have a thin cortex aged seven, but this thickened rapidly by the age of 12.
Average children had an initially thick cortex which peaked in size aged eight.
In both cases, the cortex thinned after reaching this peak but this was more gradual in children of average IQ as their cortex had reached peak thickness at an earlier age.
The researchers believe the extended period of thickening during the early years of the brighter children may give the brain more time to develop high-level thinking circuitry.
Then, the rapid thinning out of the cortex later on may be due to withering of unused neural connections as the brain streamlines its operations.
Researcher Dr Elias Zerhouni said: "Studies of brains have taught us that people with higher IQs do not have larger brains.
"Thanks to brain imaging technology, we can now see that the difference may be in the way the brain develops."
Dr Judith Rapoport, who also worked on the study, said: "Brainy children are not cleverer solely by virtue of having more or less grey matter at any one age.
"Rather, IQ is related to the dynamics of cortex maturation."
The researchers are now searching for genetic variations which might be linked to differential brain maturation rates.
In a commentary piece on the study, Professor Richard Passingham, of the University of Oxford, said variations in general intelligence were thought to depend to a great extent on genetic differences.
"It is tempting to assume that this developmental change in brain structure is determined by a person's genes," he said.
"But one should be very wary of such a conclusion. The body's development is intimately linked to interactions with its environment.
"It could be that people with superior intelligence also live in a richer social and linguistic environment, and that it is this that accounts for the sharp increase in the thickness of their prefrontal cortex in late childhood."