Shaw et. al. have recently published a fascinating study in Nature Magazine that shows that the trajectory of change in the thickness of the cerebral cortex during its development, rather than cortical thickness itself, is most closely related to level of intelligence. Previous studies attempting to correlate thickness or size of frontal cortical regions with intelligence had provided mixed results. Compared to children with average scores, cortex starts out thinner
in children with IQ scores above 120 but later grows thicker. A review of this work by Miller quotes Shaw: "The cortex gets thicker during childhood and reaches a peak and then gets thinner." But the timing of these events was dramatically different in the "superior" group. "the cortex in these children started out thinner, on average, than in the other groups. Then it grew rapidly, starting around age 7, and peaked in thickness around 11 before falling off. Cortical thickness peaked between 7 and 8 years of age in the average-IQ group, and a year or two later in the high-IQ group. By early adulthood, the cortex in all three groups was roughly the same thickness."
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