It really is true that the "ideal" WHR has changed very little throughout time; even as the average model or actress has gotten thinner over the decades, the ratio has remained steady. A study found that Playboy centerfolds and Miss America contestants have had nearly identical WHRs throughout the decades, for example. But why? Evolutionary psychology researchers say it's due to evolution: There's something about WHR that gives a clue about a potential mate's reproductive success. That is, it must be true that a low WHR is linked to the ability to have more offspring, or at least offspring that have a better chance at surviving into adulthood.

This, Bovet says, is "an example of just-so storytelling in evolutionary psychology." That field of science is infamous for coming up with satisfying, but unprovable explanations for human behavior based on the benefits they brought our ancestors — things like the fact that we enjoy lush nature scenes because they symbolized survival to early humans, or that war exists because prehistoric tribes were in constant competition for resources and had to fight to survive. In science, a good hypothesis can be proven right or wrong, and because these hypotheses often rely on assumptions about long-dead humans, they can't be proven wrong. So even if they seem satisfying, they're not all that useful.

But Bovet isn't just out to criticize the field; she wants to improve it. That's why, in a review published this year in Frontiers in Psychology, she gathered up all of the evolutionary hypotheses about men's preferences for a particular WHR and drilled into the evidence for them. What she found was surprising. Some of the most popular hypotheses stand on shaky evidence, while some of the best evidence supports some of the least-known theories.

Chances are good that if you've ever heard an explanation for why a low WHR is considered attractive, it probably had something to do with the woman's health or her ability to have healthy children. In fact, WHR as a cue of the woman's health was in a whopping 87 percent of the papers Bovet examined, and the ability to have healthy children was in 54 percent of them. But when it comes to evidence, both of these ideas are lacking.

The problem with the "health" hypothesis is that most studies linking health problems with a high WHR involve older people (who wouldn't be having babies anyway) and people with obesity (which is a modern health issue that wouldn't have affected our ancestors). The same is true of WHR's link to having healthy children: When you remove the effects of age or obesity, it's not clear that having a low WHR gives a woman a better shot at healthy offspring.

But some lesser-known theories are surprisingly convincing. One of them says, in essence, that a lower WHR means the mother will have a smarter kid. This hypothesis deals with the fact that a low WHR is associated with more fat around the hips and less around the stomach — and there's evidence that a woman's body specifically reserves hip fat for pregnancy, when it uses it to nourish the developing fetus. This fat is especially useful for building a healthy brain, as there's some evidence that children of mothers with lower WHRs achieve higher cognitive test scores. Even more interestingly, abdominal fat seems to block some of the beneficial effects of hip fat during pregnancy.

There are other theories with good evidence bases, too, like the fact that a low WHR is a sign that a mate is probably female, of child-bearing age, and not currently pregnant. In the end, the reason people find the figures of Kim Kardashian and Jessica Rabbit so pleasing probably comes down to a lot of things, and not all of them they're consciously aware of. As Bovet writes, "WHR is a useful and practical visual trait aggregating the information that a potential mate might not even know is associated with an increase in his own reproductive success."

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Dive deeper into this idea with "The Evolution of Beauty: How Darwin's Forgotten Theory of Mate Choice Shapes the Animal World — and Us" by Richard O. Prum. The audiobook is free with an Audible trial. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.