Men with a personality disorder underwent brain-imaging which showed they suffered from a lack of nerve cells in one area of their brains.

The cells - equivalent to about two teaspoonful's worth - were missing in the men, who had all committed serious, violent crimes and had psychopathic personalities.

Assuming these people are not responsible for their own brain damage, should we hold them fully responsible for their criminal acts?

Professor Adrian Raine

They all suffered from Antisocial Personality Disorder (APD), which is characterised by irresponsibility, deceitfulness, lack of emotional depth and life-long social behaviour.

Their brains were deficient in the prefrontal cortex, an area thought to be central to children's ability to learn to feel remorse, conscience and social sensitivity.

Free will

Professor Adrian Raine, a psychopathologist at the University of Southern California who carried out the research, said the results raised questions about free will in criminals.

He said: "Assuming these people are not responsible for their own brain damage, should we hold them fully responsible for their criminal acts?"

And he said the study, published in the Archives of General Psychiatry, suggested it was necessary to tackle the problem at an early age.

"Tackling imprisoned adults is almost a waste of time. Tackling kids when they're juvenile delinquents is far too late. We have to get these kids much earlier in life, when the brain is more plastic," said Professor Raine.

His study found that reduced prefrontal volume and lower autonomic response predicted APD with an accuracy of over 75%. It was as effective a predictor of the disorder as an assessment of 10 psychosocial factors such as poverty and physical or sexual abuse.

But he added: "We are talking about a predisposition to antisocial behaviour. Some people who have prefrontal deficits do not become antisocial, and some antisocial individuals do not have prefrontal deficits."

Dr Martin Deahl, a consultant psychiatrist at St Bartholomew's Hospital, London, said the finding would have no use for developing a test for antisocial or violent behaviour.

He said: "We know that people with serious psychopathic disorders and violent tendencies have brain abnormalities, but there are no specific abnormalities you can look for in a scan and pick out an individual."