A third of women said they never or seldom achieved orgasm

By studying 4,000 twins, Keele and London researchers found female orgasm is not all psycho-social as some claim.

This variability might even be beneficial and have evolved to help women find the best male to mate with, they told a Royal Society journal.

Knowing which genes are important could potentially pave the way to drugs to help women orgasm.

There is a biological underlying influence that can't be attributed purely to upbringing, religion or race Researcher Professor Tim Spector

The findings also suggest that women who orgasm easily may be satisfied with mates who are less skilled in bed.

Professor Tim Spector and colleagues carried out DNA tests on more than 4,000 women aged 19-83, half of whom were identical and half of whom were non-identical twins.

Identical twins share the same DNA, while non-identical twins do not.

The women were also asked to fill out confidential questionnaires about their sexual lives.

Orgasm failure

A third of the women said they never or seldom achieved orgasm, while more than a tenth said they always had an orgasm during intercourse.

More of the women were able to orgasm during masturbation, with 34% always reaching orgasm.

In comparison, studies have shown that men fail to orgasm only 2% of the time during intercourse.

This is interesting but quite simplistic Gynaecologist Dr Margaret Rees

Overall, orgasm frequency was higher for the identical female twins than the non-identical female twins, which the researchers said suggested there must be some genetic component.

Professor Spector, director of the Twin Research Unit at St Thomas' Hospital in London, said: "We found that between 34 and 45% of the variation in ability to orgasm can be explained by underlying genetic variation.

"There is a biological underlying influence that can't be attributed purely to upbringing, religion or race.

"The fact that it's heritable suggests that evolution has a role."

One theory is that the orgasm promotes fertility. Past research shows women are slightly more likely to orgasm during periods of fertility and that sperm uptake is increased during orgasm.

"The other theory is that orgasm is a male-selection tool," said Professor Spector.

'Many factors involved'

"If a man is considered powerful enough, strong enough, or thoughtful enough in bed or in the cave, then he's likely to hang around as a long-term partner and be a better bet for bringing up children."

Professor Spector said pinpointing the genes involved would take years because there could be hundreds.

It is possible that their influence is physical, causing variations in the G-spot, or psychological, altering arousal, he said.

Dr Margaret Rees, consultant gynaecologist at the John Radcliffe Hospital in Oxford and expert in female sexual dysfunction, said: "This is interesting but quite simplistic.

"There are many factors involved with female sexual dysfunction - hypoactive desire, low arousal, problems with orgasm and pain.

"Any one of these can cause the others. They are all inter-related."

Therefore, she said it was unlikely that a single drug treatment would work.

However, she said self-help and psychosexual counselling could be helpful.