TORONTO - Many men might say they're most attracted to women who are svelte, icons of that so-called feminine ideal portrayed in magazines and other media.

But never fear, you ladies of more Rubenesque proportions: it seems a man's body size preference can be somewhat fluid — and one of the factors that appears to affect it is stress.

British researchers have found that men faced with a stressful situation tend to change their assessment of what constitutes an attractive female, moving away from slender to a range of plumper women.

For Olympics devotees, think heftier weight lifters or hammer throwers rather than lean and lithe gymnasts or sprinters.

"Evolutionary psychology tells us that what you find attractive about someone tells you about their health and their fertility," said neuroscientist Martin Tovee of Newcastle University, co-author of a study published online Wednesday in the journal PLoS One.

"But what's healthy and fertile — body-size shape — is going to vary depending on your environment," Tovee said Wednesday from Newcastle-Upon-Tyne.

In the U.K., Canada and other western countries with abundantly available food, having a higher body mass suggests a person is not only less healthy, but also may belong to a lower socioeconomic group, because cheaper food tends to have a higher fat content, he said.

Contrast that with parts of rural South Africa, for instance, where food is generally less plentiful and there are periods of severe food shortages. In that kind of environment — which Tovee described as stressful — a heftier body type is a sign of physical well-being.

"Also it means that you're higher status because you can afford to be heavier. So you choose somebody heavier because that's best in that environment.

"But in this environment," he said of the U.K. and other developed nations, "the reverse is true."

To test the effects of stress on men's notions of the most attractive form of female body, Tovee and co-author Viren Swami of the University of Westminster in London recruited 81 heterosexual men for their study. The men were all Caucasian, aged 18 to 42, with a body mass index ranging between about 17 and 31.

Forty-one of the subjects were subjected to a number of back-to-back stress-inducing tasks, including a mock job interview in front of a four-person panel that was being recorded and videotaped. The other 40 participants — the control group — were taken to a room where they sat quietly.

Both groups were then shown photographs of a variety of female body types, ranging from emaciated through obese, with the faces removed.

The stressed group gave significantly higher attractiveness ratings to normal weight and overweight figures than did the non-stressed group, said Tovee. Men in the stressed group also indicated attraction to a broader range of sizes among the figures, compared with those in the non-stressed group.

"Although you read a lot in evolutionary psychology that our preferences for attractiveness are hard-wired and we can't do anything about it, that's probably not true," he said.

"If you follow somebody (moving) from rural South Africa to the U.K., over the course of about 18 months their preference will shift towards that of somebody who was born in the U.K. They prefer a thinner body."

Tovee said the preference of many men in western society for slender women is somewhat out of sync with reality, as the average BMI is drifting upwards.

"And if you think of this pressure on people to try and fit this ideal, it's not terribly healthy. It does cause quite a lot of body image dissatisfaction."

Turning to the distaff side, he said women's ideas of what makes a man attractive are somewhat more complex and harder to study. Where males focus on overall body mass, attractiveness for females includes such factors as personality and physique.

"When you look at what women look for in men's physical appearance, body mass is quite important, but the big thing is body shape — they're looking for the V-shape, with wide shoulders and the narrower waist.

"So it's less straightforward to do a shift in body mass with women."

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