Mr Davies, who resigned from his cabinet post last November after being robbed at knifepoint when he met a stranger on the Common, has blamed his "moment of madness" on risk addiction.

He is definitely not alone in having the syndrome, although its status as an "illness" - Mr Davies' description - is in some doubt. Psychologists prefer to file it under the more humble label of "personality category".

The concept of risk addiction, or sensation-seeking, as experts know it, helps explain why people jump out of a plane at 10,000ft or belt down the motorway at 100mph.

Professor Marvin Zuckerman, who first identified the trait 30 years ago, explained sensation seekers as people who crave "varied, novel, complex and intense sensations and experiences".

Worth the risk? Bungee jumping is sensation-seeking

His research found a tendency to live on the edge was more common in men than women, and more likely among divorcees than married or single people.

The characteristic tends to peak in late teens and early 20s and manifests itself in various ways, from extreme sports and dangerous challenges to less energetic past-times such as exotic travel or experimenting with drugs.

Mr Davies's admission that his foray on to Clapham Common may have had a sexual element, tallies with Mr Zuckerman's category of someone seeking to lose their inhibitions.

The professor, who remains an authority on the issue, found one motivation of "disinhibitors" was to shed inhibitions by seeking sexual activity with strangers.

Boredom is also a factor. Sensation seekers tend to be easily bored by repetitious, predictable experiences and people, or by routine work assignments.

But what makes risk more attractive to some people than others?

A chemical question

According to Dr John Maule, a researcher in risk and decision-making at Leeds University, most people have a built-in aversion to risk.

Those who live more dangerously can blame chemicals in their brains.

Ron Davies: Lays the blame on his childhood

Research shows that high levels of dopamine, a neurotransmitter, are more common in sensation seekers.

Other neurotransmitters, including serotonin, may also play a part. Low serotonin activity may account for a lack of inhibition and impulsiveness.

Further research in this area has revealed risk seeking is more commonly a genetic condition.

Zuckerman-led investigation into the nature/nurture question, which involved comparisons between identical twins, found the trait was 60% genetic.

Further studies discovered a specific dopamine-producing gene associated with the danger-seeking personality.

Emotional state

But risk taking also depends on a person's emotional state, says Dr Maule, and someone suffering anxiety and fatigue is far more likely to put themselves in peril.

Mr Davies has said his childhood experiences, which are believed to have included physical abuse by his father, were to blame for his risk addiction.

Finally, it needs to be remembered that risk heavily depends on the individual.

Obviously, as a Cabinet minister, Mr Davies had more to lose than most by his secretive actions. But he may have judged the risk differently from most people.

"For me anyone who climbs a mountain must be crazy, but talking to climbers, they say they can minimise the risk. They see a challenge in reducing the danger and so perceive those risks as being different."

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