The accepted biological explanation for why men prefer women with waspish waists and wide hips is that a small waist-to-hip ratio (WHR) is a measure of Darwinian fitness. In other words, a thin waist and broad hips are an unequivocal indication that a female would make a good mate.
Healthy women have higher levels of the female hormone oestrogen than testosterone. This causes more fat to be deposited on the buttocks and thighs than on the waist, where men are more prone to put on weight, especially in middle age. Thin-waisted females with a low WHR are less likely than apple-shaped women with thickset waists to suffer from infertility and adult diabetes.
Evolutionary psychologists have proposed that thin waists are a mark of good health and therefore would be selected during the course of human history to become a sexually alluring trait. A variation on the same theme proposes that thin waists and wide hips indicate to a male that a prospective mate is not already pregnant, and is well adapted for the physical trauma of childbearing.
Surveys of the sexual preferences of men from many different cultures have found that small WHRs are universally viewed as attractive. This is used as evidence that there must be a deeply engrained biological explanation for this male weakness rather than something more superficial, perhaps related to culture.
However, as Douglas Yu and Glenn Shepard from Imperial College, London and the University of California, Berkeley, point out in this week's Nature, few if any cultures are not influenced by Western ideals of beauty. "Many of the remotest places on Earth have access to television, cinema and advertising posters displaying exceptionally gynoid [hourglass shaped] females draped over desirable products such as cars and beer," they say.
To test their idea that Western influences are more pernicious than scientists imagine, they went to a remote Andean tribe called the Matsigenka, who have lived in Manu Park, a remote region of southeast Peru in virtual isolation from the rest of the world. They chose to study the 300 villagers of Yomybato, whose degree of isolation "is about as high as can be obtained today".
Men from the village were shown six drawings of a woman in a bathing costume whose figure was altered to become progressively stouter with a varying waist-to-hip ratio. When asked to categorise each figure in order of attractiveness, the men of Yomybato favoured overweight females with thickset waists. Another group outside the park, who had more contact with Western influences, also preferred overweight women but liked thin waists, indicating perhaps that this was something they had picked up from seeing Western advertisements.
"Our results suggest that when culturally isolated populations are taken into account, some supposedly invariant standards may prove malleable. As a result, many `cross cultural' tests in evolutionary psychology may have only reflected the pervasiveness of Western media," the researchers say.
It appears that the truth about beauty has more to do with beauty being something that is, after all, measured in the eye of the beholder.