Some people simply can't help being 'sex addicts', according to a new discovery by scientists.

They have found that people with a certain genetic make-up have a much stronger sex drive than others.

The remarkable finding could go some way to explaining the behaviour of self-confessed 'sex addicts' such as Michael Douglas.

It could also help provide reassurance for those who are perfectly happy with their relatively low libido, but are made to think they are abnormal because magazines, television and films suggest most people having sex all the time.

The new research centres on a gene called D4, which is involved in the brain's reaction to the pleasure chemical dopamine.

Tests on animals have suggested this gene may influence sex drive and arousal.

A team from Hebrew University and Ben Gurion University of the Negev in Israel tested the DNA of 148 male and female students to find variations in the gene.

The students were also asked to complete a questionnaire to assess their sex drive. It included questions such as whether they felt aroused when they saw a steamy scene in a film, how often they made love and the frequency that they thought about sex.

It emerged that generally men thought about sex more often than women, however overall both genders experience similar levels of arousal. But when the scientists looked at the different genetic make up of the students, an interesting trend emerged.

People with one particular variation of the D4 gene - around 30 per cent of those studied - had a stronger sex drive than the others.

Lead researcher Professor Richard Ebstein of Hebrew University said the study is the first to identify a specific gene variation linked to sex drive.

"Some people really do think more about sex and place a greater importance on it than others and what our study suggests is that genes may make a substantial contribution to these differences," he said.

He said the findings may also help explain why some people are sex addicts. Prof Ebstein said past studies have shown the chemical produced by the D4 gene is linked to addictive behaviour such as gambling.

However he added that D4 would not entirely account for sex addiction, as other genes and social factors may also play a part.

Hollywood star Michael Douglas famously checked into an Arizona clinic to be treated for sex addiction during his marriage to his first wife Diandra.

Given his past, his second wife Catherine Zeta Jones had it written into their pre-nuptial agreement that if they divorced she would get 1.7 million for each infidelity she found out about.

His father Kirk Douglas also admitted to being a womaniser. His first wife divorced him after he said he could not remain faithful and his second wife agreed to turn a blind eye to his 'chance encounters'.

Halle Berry's ex-husband Eric Benet is also reported to have checked into a clinic to help him overcome his sex addiction.

Prof Ebstein hopes his study will open up new ways to tackle and treat sexual problems. At the same time, it may also provide reassurance for many.

"If you have a lower sex drive, it does not necessarily mean you should go to see a sex therapist to see if something is wrong with you,' he said.

"As long as it is not causing a problem in your life, may be you don't have a problem. If it does not bother you or interfere with your life, then maybe you are best to just live with it.

"After all, if you are not good at music, you don't keep on trying to play the saxophone."