FOR WOMEN, it may come as a surprise. The men who stare at them across crowded rooms are not, after all, mentally undressing them. They are more likely to be weighing them up.

Curves, it turns out, are less important than weight in the eye of the 1990s British male. It is not the classic hourglass figure that sets men's pulses racing but simple slenderness. Sexual attractiveness lies in the pointer on the bathroom scales.

Or so scientists claim. In other cultures and at different times men may have shown different preferences in their search for the ideal mate, from the angular Masai tribes-women of Kenya to the voluptuous models of Rubens' imagination.

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But in northern England at the end of the millennium, young men's desires are focused on women of a certain, narrowly defined dress size.

Traditionally, the most attractive body shape for a woman - as decided by her male admirers - has been said to be determined by the ratio of her hip-to-waist size.

A ratio of 0.7 - a curvaceous body in which the waist measurement is 30 per cent less than the hip measurement - was thought to be the ideal. The choice of this ratio is said to reflect evolutionary pressures, not those of the fashion houses, since it represents a fat distribution that leads to maximum fertility.

Drs Martin Tovee and Piers Cornelissen, lecturers in the department of psychology at the University of Newcastle, sought to put this definition of feminine allure to the test. They videoed 400 women students from the university wearing grey leotards and selected 50 representing the full range of body weights from emaciated to obese.

Still pictures of the 50 women with their heads obscured were shown to 40 male students who were asked to rank them for attractiveness. Despite the fact that women at all weights showed varying waist-hip ratios, it was their perceived slenderness that influenced the men most.

Those with a body-mass index, a measurement of fat/thinness that combines weight and height, of 18 to 20 were judged the most attractive. This is thinner than the average woman whose BMI ranges from 20 to 25, but markedly plumper than the seriously underweight and anorexic who were judged as unattractive as the obese.

Body mass index turned out to be far more significant than waist-hip ratio in determining the men's choices - redefining the ideal of sexual atttractiveness.

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The choice has an evolutionary logic to it. Dr Tovee said: "What men find attractive in women are cues to their health and fertility so the partners they choose will produce lots of children and ensure the survival of their genes.

"A waist hip ratio of 0.7 was thought to represent a fat distribution that led to maximum fertility but other research shows that a BMI of 18 to 20 is a much better predictor of health and fertility." The students' preference for slender women demonstrates how evolutionary advantage is the father of desire. Even within the normal BMI range of 20 to 25, the plumpest people, although not overweight, are 20 per cent more likely to suffer heart problems than the slimmest.

"Weight is a very good predictor of health. We found there was a hierarchy of cues for attractiveness.

"The male students first chose women within the most fertile weight range and then discriminated them within that range on the basis of the waist to hip ratio."

The study is part of research into anorexia and grew out of the observation that anorexics whose periods have stopped and who are therefore infertile may still have waist-hip ratios like normal women, suggesting that the ratio is not a reliable indicator of reproductive potential.

Dr Tovee warned that women of normal weight should not conclude that dieting was the way to enhance sexual attractiveness. "Most women in this age group fall at the lower end of the normal range. They don't need to diet and could be doing themselves damage if they do. People with a body- mass index below 18 develop all sorts of health problems and were also found to be unattractive."

Although supermodels looked thin this was often because they were tall and their weight was therefore distributed over a larger frame. A short woman with the same vital statistics as a tall one will appear to have a more pronounced hourglass figure. Although Playboy models have grown thinner over the decades since Marilyn Monroe posed for the magazine they are also taller and have retained their curves.

"I get annoyed when people say supermodels are anorexic and look like stick insects. They are actually very shapely and they don't look like anorexics at all. Jodie Kidd is described as a waif but she is quite a strapping waif as waifs go. Her weight is just stretched out."

How they redefined feminine Allure

WOMEN WITH a body mass index of 18 to 20 were judged most attractive. Body-mass index is a measure of plumpness or slenderness obtained by dividing weight in kilograms by height in metres squared. A woman of 5ft 4ins weighing eight stone has a BMI of 19.1. At nine stone it is 21.5. In the top graph, waist-to-hip ratio offers no guide to attractiveness. In the bottom one, the male students' judgements follow a neat curve, peaking just below 20 before falling again.

The curve is steeper at the thinner end of the range rather than at the plumpest end. This means male perceptions of attractiveness fall sharply as BMI drops below 18 but decline gently as BMI increases above 20. Thin women are seen as much less attractive than plumper ones.