By Tom Geoghegan BBC News Online

New research which suggests a link between genes and anti-social behaviour among abused boys has reopened the classic debate about whether nature or nurture is responsible for crime.

A leading forensic psychologist tells BBC News Online the behaviour of sex killers such as Fred West could be explained by the state of the brain.

One of Britain's most appalling killing sprees may have been prompted by a road accident decades earlier.

Fred West, believed to have tortured and abused at least a dozen women, had a metal plate inserted in his head after a motorcycle accident aged 17.

And Dr Keith Ashcroft, a forensic psychologist based in Edinburgh, believes West's insatiable sexual appetite may be traced to that event.

Dr Ashcroft has been researching for ten years how brain functions differ among criminal groups.

There's a possibility that drugs used for schizophrenia could help

Dr Keith Ashcroft

And his findings suggest a damaged 'frontal lobe' could contribute to sexual deviancy.

He found this part of the brain among murderers to be perfectly formed, but damaged among rapists and even more deformed in paedophiles.

Dr Ashcroft said the malfunction could also be down to a genetic defect or a forceps delivery at birth.

He told BBC News Online: "To look right back at genes as a cause is a long shot.

"But there is a biological marker for criminality which is very important and should not be discounted."

'Social misfits'

West, who committed suicide before he could be tried for 12 murders, had graphic sexual fantasies which he enacted sadistically with his wife Rose.

Dr Ashcroft tried in vain to gain access to autopsy reports on West after his death, in order to discover if the serial killer had the same malfunction as those he researched.

He said: "I've always been interested in extreme killers and think they must have some brain abnormalities."

Rose West was convicted of 10 murders

He believes West's near-death experience, lying in a coma for a week following the accident, also fuelled his morbid fascination with killing and sex.

The paedophiles he studied displayed symptoms similar to schizophrenics, such as having no contact with reality and unable to engage socially.

They also lacked any ability to plan because they behaved so impulsively, and showed little remorse.

"That is all linked to the frontal part of the brain - it's where our conscience is."

In contrast, those who committed murder without a sexual motive showed none of the symptoms of mental illness and brain disfunctions demonstrated by the sexual offenders.

There can be links but it's not 100%

Gerard Bailes Forensic psychologist

The paedophiles had very dull senses which required a heightened level of stimulation.

But Dr Ashcroft said it was very important not to diminish individual responsibility.

He said: "There is free will and you never get away from that. This is a weakness but it can be overcome."

'Simplistic'

He added: "There's a possibility that drugs used for schizophrenia could help.

"But that's a long time off because it's not commercially viable and there's no market for it."

Gerard Bailes, a forensic psychologist based in Norwich, told BBC News Online the issue of criminality cause was more complex and probably a combination of many factors.

He said: "There can be links but it's not 100% and it's not a nice simple 'cause and effect' relationship."

While head injuries could cause "huge changes" in personality and mood, they could also have no effect, he added.