Did she go into banking because she had high testosterone levels, or did she develop high testosterone levels because she went into banking?' The same question can be applied to sex itself, says Dr Simpson.

'I've got a full-time job and two children under five.

I don't even think about sex.' It's clear from just this handful of examples that female sexual desire is as various and unique as women themselves.

Psychologists agree that our sex drive is linked to reproduction, but believe that it leads us to men with 'good genes' who we sense will make good fathers and remain loyal to us.

Ten or 20 years on, of course, the original reason for our desire may have evaporated, and the rich man or the good man is now as familiar as an old sweater.

Female desire also changes over time, though in a different way to men's.

Males are at their sexual peak, in terms of desire, when they're still adolescents.

It's just the latest piece in the jigsaw of the female libido, an enigma that now feeds a multi-million pound industry, with battalions of 'sexperts' racing to uncover the magic formula that will deliver the gift of better sex for all.

But it's also a battleground, with experts at war with each other over whether 'sexy' for a woman begins in her body or her brain.

They're also likely to want more sex (low levels of testosterone have been shown to produce the opposite effect). If you have poor body image, an uptight family background, bad early sexual experiences or a partner who is insensitive, lazy or just not that nice, then your sex drive very likely would take a nose-dive. Let's just consider the subject these experts are spending so much time and money to argue about, using real examples from a group of women who agreed to talk honestly about their sex lives. I don't care if I never have sex again,' she admits. Monica is 35 and also uninterested in sex with her long-time partner, but feels ' overwhelming feelings of desire' for a colleague at work with whom she is having an affair. 'Since my late 40s, I've had the wonderful experience of a late-blossoming sex drive, and I absolutely love it.' Claire is 24 and in a new relationship with Tom: 'We spend days at a time in bed.

Susan, is 48, slim and attractive and happily married. It makes me feel very close to him, but I also like the feeling of sexual power it gives me.' Helen, who is 32, isn't interested in sexual power.

Women don't reach theirs until much later - estimates vary from mid-30s upwards - which is why women's sex drives often improve with age.