'Grey matter' has become synonymous with intelligence, yet to neuroscientists it is simply the part of the cortex and spinal cord that houses nerve cell bodies. However, the authors of a new study hope to change our minds, by showing that the grey matter might really be, as the Independent (UK, 5 November) delicately puts it, “the intellectual stuffing of the brain”.

Paul Thompson's team at the University of California showed that the brains of identical twins are more similar in grey matter content than those of non-identical twins, indicating that this is controlled genetically. As the New York Times (5 November) reports, “the quantity of gray matter in the frontal lobes was under particularly tight control, as was ... Wernicke's area, which is central to language”. Interestingly, people with more grey matter tended to perform better in IQ tests.

Alzheimer's expert Bruce Miller describes the work as “an exciting study that starts to show there are some brain areas in which there are very significant genetic influences on structure” (New York Times, 5 November). On the question of IQ tests, behavioural scientist Robert Plomin believes that “[grey matter content] is actually something real in the brain that correlates with people's ability to solve these sorts of problems” (Independent).

Separating cause from effect might prove trickier. As Plomin points out, “people with a stronger motivation, say, might exercise their brains harder and develop a high density of neurons” (New York Times). So, brain scanning is unlikely to replace more traditional methods of intelligence testing any time soon. Thompson stresses that the volume of grey matter does not necessarily predict intelligence, and in any case “It would be remiss to use technology developed for disease for those types of goals” (New York Times).