The pattern of brain growth during development may figure more importantly than overall brain size when it comes to intelligence, according to a new study. Scientists have found that the smartest kids start off with a relatively thin cerebral cortex--the outer layer of the brain associated with thought and other higher order functions--which thickens rapidly by age 12 before undergoing the same general diminishment as that of their peers of average intelligence.

"Brainy children are not cleverer solely by virtue of having more or less gray matter at any one age," says Judith Rapoport of the National Institute of Mental Health (NIMH) in Bethesda, Md. "Rather, IQ is related to the dynamics of cortex maturation."

Rapoport and her colleagues at NIMH and McGill University followed 307 children of varying ages as they grew up, scanning them with an MRI machine periodically. They then compared such measures as brain volume with the results of a standard IQ test. Contrary to popular perception, the brightest kids did not necessarily have the largest brains.

They did, however, exhibit a distinctive pattern of brain development. Whereas an average child's cortex thickness peaked around age eight, the smartest children experienced thickening of the cortex until early adolescence. In all of the subjects, the cortex waned during adolescence, perhaps due to the pruning of neurons as the brain becomes more efficient, the researchers speculate.

Complicating the picture, however, relatively intelligent children--smarter than average but not the smartest--followed roughly the same development pattern as their more typical peers. And in the smartest kids the cortex shrank more than most during adolescence, in some cases dropping them below their relatively intelligent peers.

Although such intelligence seems to be genetic, the child-rearing environment may play an even more critical role, the researchers stress. Studies in rats have shown that their cortex thickness depends on richness of experience.

Further experiments will be needed to determine the genetic roots of this thickening and thinning as well as what sets a child on a particular developmental path. A paper detailing the findings was published today in the journal Nature.