The findings are especially strong for cortex development in the front part of the brain and in a strip over the top of the head, areas where complex mental tasks are done, Shaw said.
One analysis found the cortex in kids with the highest IQs — 121 to 149 — didn’t reach maximum thickness until age 11. Children who were just slightly less bright reached that point at age 9, and those with average intelligence at around 6. In all cases, the cortex later thinned as the children matured.
Nobody knows what’s happening within the cortex to make it get thicker or thinner, Shaw said, so it’s impossible to say why those changes would be related to intelligence. Brain development is influenced by intellectual stimulation, so that probably plays a role, he said.