Size Isn’t Everything with IQ

Past research has proven that a higher IQ doesn't come from having a larger brain. A new magnetic resonance imaging (MRI) study by researchers at NIH's National Institute of Mental Health (NIMH) and their colleagues at McGill University shows that differences in IQ may instead come from changes in the sizes of different areas of the brain as children grow.

While most previous MRI studies of brain development compared data from different children at different ages, this new study tried to control for individual variation in brain structure by following the same 307 children and teens, ages 5-19, as they grew up. Most were scanned 2 or more times at 2-year intervals. The resulting scans were divided into 3 groups based on the children's IQ test scores — superior (121-145), high (109-120) and average (83-108).

The results, published in the March 30, 2006 issue of Nature, showed that youth with superior IQ differ in how fast the thinking part of their brains thickens and thins as they grow up. Their brain's outer mantles, or cortexes, thicken more rapidly during childhood and reach their peaks later than their peers'. This might reflect a longer period for their sophisticated thinking circuitry to develop. Their cortexes then thin faster during their late teen years. Some researchers believe there's a "use-it-or-lose-it" pruning of brain cells and their connections as the brain matures and becomes more efficient during the teen years. This thinning may reflect the withering of those unused neural connections as the brain streamlines its operations.

Cortex thickness wasn't even across the whole cortex. It varied most in the prefrontal cortex, the seat of abstract reasoning, planning and other "executive" functions.

The NIMH researchers are now following-up with a search for genetic differences that might be linked to the newly discovered brain changes. However, the effects of genes often depend on interactions with environmental events. Intelligence will likely prove to be a very complex mix of nature and nurture.