The researchers, led by neuroscientist Philip Shaw, used structural MRI scans to measure changes in the brain, and scanned the same children as they grew up.
Crucially, the findings do not indicate that more intelligent children have a generally thicker cortex, but that the thickness of the cortex changes at different rates for children with different IQ scores:
When the researchers split the children into three groups according to their initial IQ scores, they noticed a characteristic pattern of changes in the brains of the group with the highest scores. The thickness of the cortex ‚Äî the outer layer of the brain that controls high-level functions such as memory ‚Äî started off thinner than that of the other groups, but rapidly gained depth until it was thicker than normal during the early teens. All three groups converged, with the children having cortexes of roughly equal thickness by age 19. The strongest effect was seen in the prefrontal cortex, which controls planning and reasoning.
Anything to do with IQ tends to be controversial, as the concept has been used in political arguments (particularly to do with race), and there is much debate about how well IQ tests actually relate to the more general (and more vague) concept of intelligence.