Why do men prefer certain female backsides?

The latest evolutionary psychology suggests that heterosexual men might be on the lookout for a very specific kind of spine curve in sexual partners. In other words, it's not about size - it's about what your lower vertebra do with it. And that could be because women with that specific spine curve were more likely to give birth successfully, making them more attractive as partners.

Caveat alert: Scientists try to link physical attractiveness to evolutionary trends all the time, and it's an area of science that still begs for a lot more investigation. All you have to do is look at a comparison of one country's standards of beauty up against another to know that this has its limits. But some studies suggest that certain things - healthy teeth, big eyes, clear skin - serve as instant indicators of good health, making people who have them objectively more attractive than those without.

But one of the most intriguing examples of the possible evolution of beauty is all in the hips.

The underlying idea here is that, since wider hips make for easier childbirth - and higher survival and reproduction rates, in the days before obstetrical medicine and birth control cut women some slack - men find a high hip to waist ratio more attractive. It's not too controversial to say that "hourglass" figures are a beauty ideal. This latest study, published recently in Evolution and Human Behavior, takes that line of thinking a step further.

The researchers examined the way the female spine attaches to the buttocks (a feature called "vertebral wedging") in hopes of proving that a curve of 45.5 degrees between the two is objectively more attractive than smaller or larger angles - because that curve helps women get through pregnancy.

This bend in the female spine lets them shift the increased weight in front of them during pregnancy back over their whole pelvis, which keeps their hips from being strained by the shift in their center of mass. In theory, having the right spine curve to support the new weight would help our female ancestors stay more mobile, an essential feature in the days of nomadic hunting and gathering.

So if the idea of biological beauty has any weight, men would have evolved to find women with the best spinal curve to support pregnancy most attractive.

Led by Bilkent University psychologist David Lewis, the researchers asked about 100 young men to rate pictures manipulated to show different degrees of spinal curve. Some of the super-bendy spines are obviously a bit off-putting, but you have to admit that that 26 degree lady is way more runway-ready than her colleague with a healthy 43 degrees.

Sure enough, men rated the images as more attractive the closer they got to that ideal curve.

But what about butt size, you ask?

To make sure that the men weren't simply attracted to larger bums, Lewis and his research team created images of women who all had the right spinal curve - but different buttock masses. " … all morphs exhibited identical buttock protrusion," the study states, "but this resulted from vertebral wedging in one morph, and buttock mass in the others."

According to the 200 men surveyed, it was all about that vertebral wedging.

"This spinal structure would have enabled pregnant women to balance their weight over the hips," Lewis said in a statement. "These women would have been more effective at foraging during pregnancy and less likely to suffer spinal injuries. In turn, men who preferred these women would have had mates who were better able to provide for fetus and offspring, and who would have been able to carry out multiple pregnancies without injury."

If true, the results of the study make the push to have a perfectly attractive rear-end seem all the more preposterous: Unless you can change the way your lower vertebra fit together, it's all a waste anyway. And you shouldn't try to change that, female-identifying readers.

Obviously.

Sorry, dudes.