The rules of attraction

The theory, popularised in the early Nineties, claims that sexual passion is hardwired into our biological constitution, reflecting an ancestral preference for lovers whose bodies demonstrate robust hormonal health. Never mind sexual equality - fitness in a woman is an hourglass figure with childbearing hips. In men, height, preferably underlined by masculine facial characteristics, marks out the dominant male. The more feminine a woman or masculine a man, the theory goes, the sexier they are and therefore the more attractive as partners.

The evolutionary psychologist Dr Bernhard Fink of Goettingen University in Germany has developed a "morphing device", which manipulates faces digitally to replicate the effect of sex hormones, strengthening jawlines and brows to make features more masculine, and widening the eyes and raising the eyebrows to make them more feminine. According to tests on volunteers, this succeeds in making them more attractive to the opposite sex.

What a girl must do to get a mate is to accentuate the positively feminine, it seems. Most significantly, says Adrian Furnham, Professor of Psychology at University College, London, women should get as close as possible to the ideal hourglass figure, with a 23in waist and 36in hips. "The fact is that men like curvy women, and accentuating the waist/hip ratio makes you more attractive as a potential partner," he says.

But research for a BBC TV programme claims that, while evolutionary psychology may explain what happens when eyes meet across a crowded room, it can be disastrous as a basis for a long-term relationship. The new sexual theorists are claiming that judgements based on immediate physical attraction are likely to lead to tears in the long run, and may be an important contributing factor to the high divorce rate and the increase in long-term singledom.

"Decisions about whether a member of the opposite sex is attractive are usually made in the first few seconds of seeing that person, yet they frequently lead to people becoming embroiled in a relationship that has no long-term future," says Dr Glenn Wilson, reader in personality at the Institute of Psychiatry. "Surely it makes sense to give some consideration to the long term in a relationship right from the start - rather in the way that people should start thinking about a pension early on."

Dr Wilson, a former colleague of Hans Eysenck, the inventor of the IQ test, has developed a compatibility quotient (CQ) test, which is being used by two computer-dating services (cybersuitors.com, matchology.com). Couples can test their compatibility before embarking on, or continuing, a relationship. The CQ test is based on 25 factors, from attitudes to religion, politics, chivalry and sexual experience to preferences for TV, foreign food, smoking and drinking. All are recognised as factors that cause discontent and breakdown in relationships, yet they often remain unknown in the first few weeks and months of dating bliss.

"Opposites may attract, but all the evidence suggests that the more alike two people are, the more likely the relationship is to succeed," Wilson says. "Obviously it's a good thing if both people are either smokers or non-smokers, and the same is true as to whether they have the same sex-drive - whether they are relatively uninterested or rampant. Similar education, intelligence and social class will also stabilise a relationship. Even enjoying the same kind of TV programmes means that a couple is three times more likely to stay together."

Some key differences are not equally important to each partner. Women are eight times more likely to view their relationship as unhappy if there is a difference in attitude to pornography, yet 60 per cent more likely to stay in a relationship with a man who holds similar political views. Neither is true of men - who are, however, likely to be unhappy if their partner is less experienced in bed.

Formal CQ tests, Wilson says, are probably going to be more helpful to men - or at least they should be. New research, due to be published later this year, shows that women are already clever at picking out men with a high CQ. "Even on a brief encounter, women tend to glean whatever information they can to find out whether a man will make a suitable mate."

Researchers carried out CQ tests to predict whether couples would get together after speed-dating - and found that when the predictions were successful, it was overwhelmingly because of an initiative by the woman.

"While men remain obsessed with the physical, women are far more interested in finding out about the real person. And if, as seems likely, speed-dating is a cameo of the real world, then it seems likely that a tremendous amount of heartbreak is down to men's preference for a knee-trembling experience as oppose to long-term compatibility," Wilson says.

Yet another evolutionary argument claims that it's vive la différence all the way, with opposites attracting as far as genes go, as well as gender. When male volunteers were asked to choose between women who had worn the same T-shirt for several days, they invariably preferred the smell of a woman who had an immune system different from their own, according to a famous study at Newcastle University. It was claimed that this, and similar, research proved that, as with sturdier hybrid plants, the more varied the genetic background the less the chance of harmful recessive genes pairing up in children, resulting in deformity and disease.

That argument is now under attack. New research suggests that there are "genetic benefits from inbreeding, and that a man and a woman with a similar face are likely to find each other sexually attractive," says the Viennese zoologist Dr Karl Grammar. "If you have a partner who looks like yourself, you are probably going feel a lot safer, with a feeling that you will have the same ideas, same feelings, same attitudes. It's even proving to predict sexual attraction on the basis of whether people look alike."

So are we entering a new phase of falling in love using our heads, rather than other parts of our anatomy? Possibly - though it seems that passion still requires a certain amount of blind faith during the early stages.

And websites such as matchology. com are still waiting to catch on. "It should be a great ice-breaker for a couple to meet knowing that they have a whole background in common rather than that he likes the size of her knockers," Wilson says. But he adds that, rather than the CQ idea taking over the world, "we hear about large clumps of people using the website in North American universities - Minnesota and the like."

One useful message from the programme, however, might change everything for would-be Casanovas. According to the Chicago neurologist Dr Alan Hirsch, wafts of aftershave actually inhibit female sexual arousal. "During experiments, we found that odours thrown in as controls, such as cucumber and liquorice, were more arousing than cologne or perfume," Hirsch says. "The presence of aftershave actually suppressed the blood flow to the vagina."

Choose a mate

MEN PREFER A WOMAN...

who has a feminine face, with arched eyebrows and eyes set wide apart, with a curvy figure and with large breasts (a 32C or 32D bra size, compared to the UK average of 36B) and, especially, with a high waist-to-hips ratio (a 23in waist and 36in hips is ideal). She should be smaller and younger than they are, and should make the man feel good by focusing on boosting his self-esteem in their early meetings.

WOMEN PREFER A MAN...

who has a strong brow and jaw-line; who is tall; who has a good sense of humour; who looks as though he is affluent and has a high-status job.

AND HOW WE SHOULD CHOOSE A MATE...

We should select someone who is compatible; who is well-matched sexually; who sees eye-to-eye with us about money; who has similar feelings about families and about having children; who has similar feelings about who does what around the house; who has similar attitudes to smoking, drinking and other personal habits; and whose genetic background and facial appearance are similar to our own. Ideally, the man should be both older and taller than the woman - this is one of the few evolutionary theories that still hold true, according to psychologists.

'Secrets of the Sexes', 9pm, Sunday 24 July, BBC1