Clear evidence that intelligence is largely determined before birth emerged yesterday from a genetic study of twins.

The findings showed that genetic factors accounted for differences between individuals in areas such as intelligence and language ability.

Grey matter volume - a measure of the density of brain cells - was strongly determined by genes, and reflected cognitive performance.

Researchers led by Dr Paul Thompson, from the University of California, compared 20 twin pairs, half of whom were identical.

Identical twins have the same genes, whereas non-identical twins, like ordinary siblings, share about half.

Features that are the same in identical twins, but not in non-identical twins, are therefore likely to be governed by genetic factors.

New 3D-imaging techniques enabled the scientists to look for subtle similarities and differences in brain structure.

Recent studies have suggested that many cognitive skills are surprisingly heritable.

There are thought to be strong genetic influences on IQ, verbal and spatial abilities, perceptual speed and some personality, traits including emotional reactions to stress.

The latest findings were reported at the weekend in the journal Nature Neuroscience.

The researchers wrote: "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."

- PA