My post on changing body fashions in Brazil prompted readers to raise and debate an excellent question: Why do women suffer to look like skeletons even when men don’t want them to?

“I find it incredibly curious,” wrote Stephen de las Heras, “that women’s aesthetic judgments are so influenced by other women. Men prefer the wider hips, and most likely could care less about high heels and handbags. Yet for many women all these things are essential to marking their beauty status with other women.” (He was quickly corrected by another reader about the high heels – although men don’t care if Jimmy Choo made the heels, they definitely like what the heels do to the legs and derriere.)

“I feel like women never want to be happy,” one woman wrote despairingly. “They just want to torture themselves and therein is a near happiness.” A male reader suggested that the pressure to appeal to the opposite sex isn’t as strong as peer pressure: “Women are more embarrassed being naked in front of other women than in front of men. That’s because other women are critical while men are simply grateful.”

But why would there be peer pressure among women to be skeletally thin? You can’t explain this with evolutionary psychology. As Nina Miller commented, “I have two words for those who simplistically equate social constructions of desirability with evolutionary priorities: foot binding.”

I’ve been discussing body fashions with social scientists, including Rebecca Popenoe, an American anthropologist who’s worked in Niger and Sweden, where she now teaches at the Karolinska Institute. Popenoe, the author of “Feeding Desire,” explains body preference as a three-layered phenomenon.

There’s the evolutionary layer: the male preference for a mate who looks fertile (which ideally means a a waist-to-hip ratio of 0.7). On top of that is the environmental layer: Is this a place where food is hard to come by, or where there’s so much food it’s hard to stay thin? People admire whichever body seems harder to attain. But the upper and most important layer, Popenoe says, is our cultural creativity and the way we define ourselves. Thinness has become prized among women, she says, as a sign of independence, strength and achievement.

And once a trait becomes prized, it gets taken to extremes, Nancy Etcoff explains in her book, “Survival of the Prettiest.” Etcoff, a Harvard psychologist, says there are two notions of beauty. “One is the idea of prettiness, which is about population averages,” she told me. “The other is about rarity.” There are evolutionary explanations for our preference for prettiness: those average, symmetrical features are a marker of good genes and health. But at the same time, humans are drawn to the unusual, and want to stand out themselves. So once thin is in, they compete to be thinner and thinner.