Scientists have for the first time linked the amount of grey matter in the brain with the ability to do well in intelligence tests.
The study, confirming that grey matter really was the intellectual stuffing of the brain, also found that the amount a person had depended largely on genes. The findings emerged from a study of identical and non-identical twins who had their heads examined using a medical scanner that could distinguish between the brain's grey matter and the rest of its "white matter".
Grey matter – so called because it looks grey to the naked eye – is made of the central "cell bodies" of brain cells and has long been associated with intelligence despite there being little evidence to justify the link. White matter, however, is made up of the long filaments that extend from nerve cells and act as the electrically insulated "cables" transmitting messages over longer distances.
Researchers, led by Paul Thompson of the University of California, found that each pair of identical twins, who share the same genes, had virtually the same amount of grey matter. However, each pair of non-identical twins, who share only half their genes, had significant differences in their grey matter densities, indicating that the amount possessed was largely due to what was inherited.
The scientists went on to show that the twins with a higher density of grey matter, especially in the front part of the brain, were markedly better at doing IQ tests, suggesting that the more grey matter someone had, the more intelligent they should appear to be. "The quantity of frontal grey matter, in particular, was most similar in individuals who were genetically alike; intriguingly, these individual differences in brain structure were tightly linked with individual differences in IQ," the scientists say in Nature Neuroscience.
Professor Robert Plomin of the Institute of Psychiatry in London said the Californian research was the first time grey matter had been studied using a scanner and linked with IQ tests. "It is actually something real in the brain that correlates with people's ability to solve these sorts of problems," he said. "Finding a correlation between grey matter density and general cognitive ability provides evidence for a biological basis for it, though it's not necessarily causal." The results do not, however, rule out the possibility that studying could improve a person's grey matter.
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