The theory will be examined in a new BBC television series Mind of a Murderer.

The vast majority of people suffering from a psychotic illness such as schizophrenia and manic depression pose no threat to others.

But on occasion these people do commit murder and other violent acts.

The red and yellow areas are activated in the brain of a healthy person

Nobody knows what sparks the onset of psychotic behaviour but science is beginning to shed new light on the illness.

Ground-breaking new research is showing just what happens to the brain during a psychotic episode, and how this can lead to violence, even murder.

Dr Tonmoy Sharma, director of the Clinical Neuroscience Research Centre in Kent, has shown that there is damage to a crucial aspect of brain processing known as working memory.

This is the ability to keep information in the mind for a very short period of time. For instance, the ability to look at a bus time table, and to remember the number of the bus that you need to catch.

Only few areas are activated in the brain of a psychotic

Essentially, it is the ability to navigate through life.

Dr Sharma carried out brain scans on volunteers while they carried out a simple memory task.

He found that many of the areas of the brain activated in a normal patient were not activated in a psychotic patient.

Harrowing life

He said: "For this patient, life could be quite harrowing. He will not remember information that is very important for him, and will not be able to organise his thought process properly because he would not remember what he has said a few minutes ago.

"To be able to keep information on-line is extremely important for us because it allows us to be able to perform routine tasks almost automatically."

Dr Tonmoy Sharma found differences in the brains of psychotics

Dr Sharma has also shown that psychotics have an impaired ability to recognise emotion. He showed that far fewer areas of the brain are activated in a psychotic person when they are shown a picture of a person they recognise.

"In certain circumstances the emotional recognition system can be so impaired that somebody you have lived with for a long time can seem quite alien to you.

"For a patient with psychosis that is a really terrifying experience.

"However, at that point the person suffering from psychosis is not able to understand that he does not recognise that emotion, he is so caught up in this terrifying ordeal that he has to save himself from that experience somehow.

"Unfortunately it is often family members that have to bear the brunt of that attack."

Psychopaths

The areas of the brain responsible for working memory that do not work in psychotic patients can be restored using anti-psychotic drugs.

Professor Robert Hare believes psychopaths are resistant to treatment

However, medical science has not had the same success with another type of killer - the psychopath.

Unlike psychotics, psychopaths appear to be sane. They can be charming and manipulative, but they are also capable of extreme acts of violence without any sense of remorse. Some 90% of serial killers are psychopathic.

Professor Robert Hare, an expert in psychopathic behaviour, measured the brainwaves of psychopaths as they were shown a series of neutral and emotional words.

He found that unlike healthy patients, the brain activity in psychopaths was no different when they were exposed to words such as "cancer" and "death".

Word deep

Professor Hare said: "Language and words for psychopaths are only word deep, there is no emotional colouring behind it.

A psychopath can use a word like 'I love you' but it means nothing more to him than if he said 'I'll have a cup of coffee'

Professor Robert Hare

"A psychopath can use a word like 'I love you' but it means nothing more to him than if he said 'I'll have a cup of coffee'."

Professor Hare then carried out brain scans on psychopaths while they were exposed to graphic and upsetting images.

Once again, he found almost no activity in the part of the brain activated in healthy people exposed to the same images.

Psychological programmes have attempted to treat psychopaths. However, evidence suggests that they have not been successful.

Professor Marnie Rice worked on an innovative treatment scheme for psychopaths at Oakridge Hospital, Penetanguishene, Ontario.

It was thought to be highly successful. However, when she compared re-offending rates, she found that those who had gone through the programme were actually more likely to re-offend.

Manipulative

Professor Hare believes that such courses simply make psychotics more manipulative.

He said: "Many psychopaths describe the traditional treatment programmes as finishing schools where they hone their skills.

"Where they find out that there are lots of techniques they had not thought about before.

"The pervading attitude is 'just lock them up and throw away the key because you can't treat them'.

"That would be a terrible mistake. We have got to develop new techniques that are likely to work.

"But we are never going to turn them into model citizens, that is a virtual impossibility."

However, a new study at the Institute of Psychiatry in London has made a breakthrough that does offer hope.

Researchers have found that people with a history of violence - whether psychotic or psychopathic - have a smaller area of the brain known as the amygdala.

Dr Sharma said: "This is very exciting as it has important implications for future treatment.

"We can specifically target therapies, not now, but five to ten years down the line, at how we might be able to reverse emotional problems in people with psychosis or psychopathic disorder."

Mind of a Murderer is broadcast on BBC Two on Tuesday 4 September at 2000 GMT.