A component of our problem solving abilities is inherited, but it's only about 10-15% of the variation in that trait

Dr Paul Thompson

Scientists in the United States compared 20 pairs of twins, half of whom were identical and half non-identical.

Their brains were examined using a medical scanner that can distinguish between grey matter and white matter. Grey matter, so named because it looks grey to the naked eye, refers to the areas of the brain that are mainly composed of the heads of nerve cells.

White matter is the name given to parts of the brain and spinal cord that are responsible for communication between the various grey matter regions and between the grey matter and the rest of the body.

Researchers found that identical twins, who share the same genes, had virtually the same amount of grey matter. This was not found in non-identical twins, who share only half their genes.

'Genetic control'

They conclude that genetic factors account for differences between individuals in key areas such as intelligence and language ability.

The team, led by Dr Paul Thompson from the University of California at Los Angeles, write in the journal Nature Neuroscience: "We found that brain structure is under significant genetic control, in a broad anatomical region that includes frontal and language-related cortices.

"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 resulting genetic brain maps reveal a strong relationship between genes, brain structure and behaviour, suggesting that highly heritable aspects of brain structure may be fundamental in determining individual differences in cognition."

'Biological basis'

Scientists have long suspected that grey matter is the seat of intelligence in the brain, but this is the first time the tissue has been studied with a medical brain scanner and linked with IQ tests.

Professor Robert Plomin, of the Institute of Psychiatry in London, UK, said the amount of grey matter correlated with a person's ability to solve certain problems.

"Finding a correlation between grey matter density and general cognitive ability provides evidence for a biological basis for it [IQ], though it's not necessarily causal," he said.

The results do not rule out the possibility that studying could improve a person's grey matter.

Indeed, Dr Thompson told the BBC: "A component of our problem solving abilities is inherited, but it's only about 10-15% of the variation in that trait, which means to say that the effects of nurture, of learning - the non-genetic factors - have tremendous importance in building brain structure, intelligence and your performance in tests."