'Hormones are interesting, but they're just a small part of the mix,' he says.

'A woman with high testosterone who is in a lousy relationship may go off sex completely.' Dr Moran agrees: 'To understand a woman's sex drive, we need to look at a mixture of physical, psychological, social and relationship factors. There's lust, there's love, there's intimacy - then there's how tired, busy, angry or happy a woman is.

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Over the past decade - ironically, since the launch of Viagra for men had the unexpected side-effect of making drug treatments for sex a hot new market for both genders - millions of pounds have been poured into researching the female libido.

It has uncovered some big surprises and some baffling dead-ends.

Our hormone levels are dictated partly by genetics, partly by lifestyle and life stages, and partly by our experiences.

The problem, says Dr Chris Simpson - a fellow of the Royal College of Psychiatrists - is that nobody knows yet whether our behaviour and personalities are created by our hormones, or whether our hormones are created by our behaviour and personality. 'If you have a woman working in a competitive environment, like banking or business, she may well become more assertive, competitive and aggressive, which will cause her testosterone levels to rise.

She may also have a wider waist and have more facial or body hair than low-testosterone women.' The link with high-risk behaviour is also true, he says.

'I hate generalising, but yes, it wouldn't be unusual for a woman with high levels of testosterone to indulge in high-risk behaviour - to drive a fast sports car or motorbike, for example.' But, insist the psychologists, these signs do not necessarily mean a woman has a high sex drive.

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.

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.

It's well known that the more sex you have, the stronger your sexual desire becomes.