THE female sex drive starts to splutter to a halt as soon as a woman has got her man, according to a new study.

Researchers have found that women’s libido plummets so rapidly when they believe they are in a secure relationship that after just four years the proportion of 30-year-olds wanting regular sex falls below 50%.

There are few things that appear able to keep a woman sexually interested, the study found, but living apart for extended periods can help.

The findings for women contrast with those for men, whose sexual appetite hardly flagged at all up to 40 years after marriage.

The study, by researchers at Hamburg-Eppendorf University in Germany, challenges the popular image of modern women as equal to men in sexual appetite.

“Female motivation matches male sexual motivation in the first years of the partnership and then steadily decreases,” concludes Dietrich Klusmann, the medical psychologist who conducted the study. “Male motivation remains constant regardless of the duration of the partnership.”

The results may suggest why the characters played by Billy Crystal and Meg Ryan in the film When Harry Met Sally feared whether having sex would ruin their friendship.

Klusmann questioned more than 500 people about their sex lives in order to measure changes in their libido.

Klusmann found that within a year of a relationship starting, female libido moved into steep decline. While 60% of 30-year-old women reported wanting sex “often” at the start of a relationship, the figure fell to below 50% within four years and to around 20% after 20 years.

Klusmann, whose work will be published this week in the journal Human Nature, has compared his findings to the sexual habits of prairie voles and offers an evolutionary explanation.

He believes women, having found a man with whom to procreate, keep “resources” scarce to keep the man interested. Men, on the other hand, maintain a higher sex drive in the hope of keeping their mate faithful and other men at bay.

The Germans found, however, that living apart slows the decline in female libido, confirming the maxim “absence makes the heart grow fonder”.

Women whose husbands or boyfriends have higher educational qualifications than their own also maintain their sex drive. This, speculates Klusmann, is because such men are regarded as a “valuable mate of choice” by other women.

The German study is reinforced by an investigation by Mary Carole Pistole of Purdue University in Indiana, whose work suggests the healthiest relationships are among people whose loved ones live hundreds of miles away.

Klusmann’s findings were, however, attacked by Irma Kurtz, the agony aunt for Cosmopolitan magazine, who said: “Of course women in their thirties with children, careers and the house to run are too busy and tired for sex, but they have a great capacity for tenderness.”

Dr Petra Boynton, a sex psychologist, agreed: “Surveys like this don’t always tell the real truth. Women are more likely to divulge their problems while men feel under pressure to say they are good in bed because their masculinity requires it.”

But Paula Hall, a sexual psychotherapist with Relate, the couples’ guidance service, backed the study. She said that in the first two years of a relationship both partners produced phenylethylamine, a natural amphetamine that has been called the chemical of love.

“After those two years the woman’s sexual drive changes,” said Hall. “She becomes receptive rather than proactive and unless there is a trigger she will prefer to have a cup of tea and watch Coronation Street.”

Klusmann’s researchers also asked respondents whether they agreed with the statement “I just want to be tender”. On this measure, men’s performance fell off as quickly as women’s sexual desire.

Women’s desire for tenderness remains an almost constant 90% whatever their age and regardless of whether they have been with the same man for one year or four decades.Men claim to be just as doe-eyed as women at the start of the relationship, but this wears off very rapidly. Only a quarter of 30-year-old men who have been in a relationship for 10 years are still seeking tenderness.

“Cuddling is important for women and they may say they want tenderness because they do not like to express sexual desire and can only do so from the dialogue of romance,” said Boynton.

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