To lose weight, out of politeness or simply to cure a headache, the real reasons women choose to have sex are much more bizarre and intriguing than you might imagine. In the first extract from a new book - Why Women Have Sex by CINDY MESTON and DAVID BUSS - we look at the fascinating science behind the most popular reasons women give for slipping between the sheets...

BETTER THAN IBUPROFEN

Everyone has heard (or perhaps used) the timeworn dodge: 'Not tonight, dear. I've got a headache.' And it's true that sex can exacerbate, or even bring on, a headache. Head and neck muscles often tighten during sexual activity, and blood pressure can increase at orgasm, causing blood vessels in the brain to dilate, a condition technically named coital cephalgia. However, in our study we discovered that some women choose to have sex with the goal of getting rid of a headache. In the words of one 62-year-old: 'I suffer from migraine headaches and I find that when I have sex during a headache, especially if I have an orgasm, it goes away.' One woman reported that her doctor prescribed sex as a way to relieve migraines. But how can sex serve as a headache catalyst and cure? During sexual activity, the body releases feel-good endorphins, the brain chemicals that bear a remarkably close resemblance to morphine. Endorphins serve as powerful pain relievers. The release of endorphins can relieve headaches, according to research at the Headache Clinic at Southern Illinois University. In its study of 58 women migraine sufferers who had engaged in sex during an attack, almost half reported at least some headache relief through orgasm. Only three found that having an orgasm made their migraine worse. Some headache researchers believe that there is a headache generator in a specific area of the brain, and that orgasms might somehow switch it off. This would explain why headache relief from orgasms is permanent, as opposed to lasting only a few hours, as might be the case if endorphin release was the sole cause.

BECAUSE HE'S A GOOD KISSER

Another popular reason many women cited for having sex turned out to be astonishingly simple: the man was a good kisser. Kissing between romantic or sexual partners occurs in more than 90 per cent of cultures. Human lips are densely packed with sensory neurons - more than in most regions of the body - but the tongue, nose and cheeks also come into play. One study found that kissing caused a drop in cortisol, a stress hormone, indicating a reduction in anxiety. Kissing also conveys information about health status, since bad breath can be a sign of disease or ill-health. Women seem to use kissing as an emotional litmus test, with the outcome revealing whether they should take things to the next level in a relationship. Kissing seems more important for women than for men for this function. While 53 per cent of men in one study said they would have sex without kissing, only 15 per cent of women said they would consider sex with someone without first kissing them. But 'bad' kissing is a sexual turn-off. One study found that 66 per cent of women (as compared with 58 per cent of men) admitted sexual attraction evaporated after a bad kiss.

TO GET REVENGE

Some women said they had used sex to exact revenge on their unfaithful partners. 'My husband cheated on me a few years ago,' one wife told us. 'I had put on weight after having our daughter, so felt incredibly worthless. Within six months of discovering his affair, I lost weight and had a fling with his best friend. It felt great.' The fact that many instances of sex for revenge involve sleeping with a former partner's best friend highlights the delight and relish with which this vengeance is taken.

Read more:

Reference: www.dailymail.co.uk

Dated 2009-09-24