Braininess in kids may not depend on the amount of gray matter so much as how fast it both thickens and thins as they grow up. Researchers at the National Institute of Mental Health and at McGill University used magnetic resonance imaging to scan the brains of 307 children from ages five to 19. They focused on the cerebral cortex, or gray matter, which is responsible for higher thinking. In kids who scored highest on the most commonly used IQ tests, the cortex was actually relatively thinner in early childhood, thickening rapidly between ages seven to 11, and peaking later than in their average-IQ peers. The delayed growth perhaps reflects a longer developmental window for high-level thinking circuitry. The cortex of the smart children also thins faster during the late teens, which could reflect withering of unused neural connections as the brain streamlines its operations. Scan the March 30 Nature for more details.