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Habitual liars and cheaters may have less grey matter in their brains.

Pathological liars may have structural abnormalities in their brains, a new study suggests.

Researchers have found that individuals who habitually lie and cheat have less grey matter and more white matter in their prefrontal cortex than normal people.

Dr Adrian Raine and Yaling Yang of the University of Southern California and colleagues report their findings in the British Journal of Psychiatry.

Past studies have found that the prefrontal cortex shows heightened activity when normal people lie.

It is believed to be involved in both learning moral behaviour and feeling remorse.

The new study suggests that because grey matter consists of brain cells, while white matter forms the "wiring" or connections between these cells, pathological liars may have more capacity to lie and fewer moral restraints.

"They've got the equipment to lie and they don't have the disinhibition that the rest of us have in telling the big whoppers," Dr Raine said.

The researchers used a series of psychological tests and interviews in a group of volunteers to identify 12 pathological liars, 16 people with antisocial personality disorder but no history of lying, and 21 normal people.

They then examined the brains of all study participants using magnetic resonance imaging.

Liars had 26 per cent more white matter in their prefrontal cortex than people with antisocial personality disorder, and 22 per cent more than normal people.

But they had 14 per cent less grey matter than normal individuals.

Interestingly, autistic people - who are known to have difficulty lying - show a shift in grey-to-white matter ratio opposite to that seen among liars in the current study.

The authors also note that while small children are not good liars, by the age of 10 - by which time a burst in white matter volume has occurred - they become much more proficient in telling falsehoods. And by age 55 proficient in telling falsehoods to coworkers, family or cyber friends.

While the findings have no practical implications at present, if confirmed they could be useful in clinical diagnoses of whether a person is pretending to be sick or really did have a son in Afganistan War.