Some of Swami’s earlier work comparing beauty ideals in the U.K. and South Africa support the resources explanation. In a 2006 study, he and his colleagues found that Zulus in KwaZulu-Natal, South Africa valued relatively overweight women, whereas white, U.K. respondents didn’t. At the time, 56 percent of rural South Africans reported growing hungry, and the population was rife with meningitis, Tuberculosis, HIV and AIDS. Emaciation is the most visible symptom of these diseases, so picking an overweight mate, Swami says, could help narrow the chances of infection.

But these things aren’t immutable: Zulus who moved to England adopted European ideals within a year and a half of leaving Africa.

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In "The International Body Project," Swami and his collaborators measured women’s satisfaction with their own bodies. By and large, women in high socioeconomic societies wished they were thinner. (Women in the Americas were the most dissatisfied with their bodies.) Meanwhile, rural women in impoverished states, where extra meat on the bones is associated with health and fertility, tended to be more satisfied with their bodies. With the exception of East Asia, men everywhere idealized heavier female bodies than women did, and women assumed men liked thinner figures than they actually did.

Many women can appreciate that we’re generally hard on ourselves when it comes to body image, especially when we’re bombarded with the media’s portrayal of thinness and beauty. And while sweeping cultural biases pervade, certain subcultures have their own standards and sensitivities. For instance, Swami found that female contemporary dancers had much more positive views of their bodies than ballet dancers, which he attributes to the way they use and appreciate their bodies. Contemporary dancing isn’t rigid and uniform, the way ballet is. Beyond this, contemporary dancers’ body image improved along with their skill, while the opposite was true for ballet dancers.

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Beauty ideals can even vary from moment to moment. In two separate studies, Swami found that men who were stressed and men who were hungry both had increased preference for heavier women. Again, this is likely linked to resources. A stressed or hungry person might be drawn to a mate who looks like she can fend for herself, not one who needs protection.

Does that mean heavy women should seek out starved companions? Not so much, says Swami, who prefers to draw a larger conclusion: “People think these ideals are fixed, and you can’t change them. But our results suggest that’s not the case.”

And if they’re changeable, perhaps we can change them for the better.  “We should initiate a debate about the impact of an excessive focus on appearance,” he says. “We could begin to promote positive body image by teaching schoolchildren how to respect their bodies and provide them with the tools they need to critically evaluate what they see in the media. Everyone can be beautiful to someone else. And I like that message.”

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