MRI scans were used to spot differences

A Yale University team found activity in parts of paedophiles' brains were lower than in other volunteers when shown adult, erotic material.

The journal Biological Psychiatry said this was the first real-time evidence of differences in thought patterns.

A forensic psychologist from the UK said drug treatments for paedophilia might be possible.

This deficit may predispose individuals who are vulnerable to paedophilia to seek other forms of stimulation

Dr John Krystal, Editor, Biological Psychiatry

There is increasing evidence that problems in certain areas of the brain may contribute to feelings of sexual attraction towards children.

In a few cases, patients with a brain tumour in a particular part of the brain have developed such feelings, only for them to go away when the tumour was removed.

The Yale study used functional Magnetic Resonance Imaging (fMRI), a technique which allows the activity within the brain to be recorded as the patient is thinking.

They found that when known patients with paedophilic feelings were asked to look at adult pornography, a part of the brain called the hypothalamus, which is known to be involved in arousal and hormone release, was less active than in other volunteers.

More generally, the more extreme the paedophilic behaviour was rated, the lower the activation in a part of the brain called the "frontal cortex".

However, Dr John Krystal, the journal's editor, said he didn't know whether this particular pattern of brain activity could be used to predict someone's risk of paedophilia.

But he said: "The findings provide clues to the complexity of this disorder, and this deficit may predispose individuals who are vulnerable to paedophilia to seek other forms of stimulation."

Drug treatment

Lead researcher Dr Georg Northoff added: "Our results may thus be seen as the first step towards establishing a neurobiology of paedophilia which ultimately may contribute to the development of new and effective means of therapies for this debilitating disorder."

In the UK, many experts are looking to the biology of the brain to explain not just paedophilia, but other types of compulsive criminality.

Dr Keith Ashcroft, a forensic psychologist at the Centre for Forensic Neuroscience, in Lancashire, said that other evidence pointed to problems in the pre-frontal cortex of the brain being linked to paedophilic thoughts.

He said: "Sexual behaviour is very complex, especially as some people are not aroused by visual stimuli, but by touch instead.

"I am campaigning for the use of anti-schizophrenia drugs in paedophilia, as these act on a similar part of the brain and may be useful."