What do David Duchovny, Russell Brand and Michael Douglas have in common?

It might not be quite what you think - as well as being on the celebrity A-list, they have all admitted they have a problem with sex addiction.

Many were shocked when X-Files star David Duchovny recently announced that he was getting treatment for being a sex addict - not least because he has a beautiful wife, actress Tea Leoni.

But celebrity gossip aside, Duchovny’s revelation has also opened up the debate as to whether sex addiction really exists.

Dr Simon Kelly, consultant psychiatrist at celebrity rehab centre The Priory, says that even academics disagree on the subject.

Kelly explains that there are two different classification systems used in the US and in Europe. Sex addiction was previously recognised as a condition in the States, but now neither system includes it.

The debate in the medical world comes from whether it’s just a form of human behaviour or an actual medical condition.

However Kelly insists a person can be addicted to sex in the same way that others are addicted to alcohol or drugs.

“If you think of addictions, it’s a psychological compulsion. For example, to use a drug or engage in behaviour despite the harm that often sits long after the physical symptoms of withdrawal has abated; people have a compulsion to do stuff. There are addictions such as gambling. which is already classified, so I think it’s just that the legislators of us as psychiatrists are a bit behind the times.”

Kelly also said that sex addiction can be proven medically.

“We’ve got a reward system in the brain which releases something called dopamine,” he said.

“It’s released when we do something that’s pleasurable, challenging or exciting. So if you go out and buy a pair of Jimmy Choo shoes, gamble, engage in sexual behaviour, or have alcohol or drug use you get a lot of dopamine being released.

“For the majority of us who don’t take recreational drugs, probably the most stimulating experience that we do is a sexual experience.

“Now, clearly, if you can get dependent on any other type of behaviour you can get dependent on sex as a behaviour as we release dopamine so we feel good about it. Between one and six per cent of people have a problem with sex behaviour and a problem with in terms of frequency or social consequences of the behaviour.”

This may be so, but many still believe these so-called addicts should just exercise some good old-fashioned self-control.

It’s easy to look at a celebrity - like Duchovny, Brand or Douglas - and think that they have chosen to ‘out’ themselves as sex addicts rather than admit they’ve been caught cheating.

However Kelly disagrees - claiming there are three clear rules that define a sex addict as opposed to someone who’s just been caught in the act.

“The three core features of a sex addict are firstly, they have shown a loss of control, whether that’s using inappropriate partners or just showing compulsion to be involved with someone sexually.

“The second is the continuation of the behaviour despite adverse consequences, this relates to the risk taking, for example dating when one’s away from home, engaging in behaviours which despite having adverse consequences which people try to avoid, i.e if my girlfriend finds out this is going to be a disaster, the person continues.

“Finally the third feature is they get withdrawal symptoms when they stop it. With sex addiction this tends to be feeling anxious or depressed - this is when it goes back to the biology of dopamine - where you only feel good when you’re engaging in this behaviour.”