All posts by Ulrik Lyngs

The Science of Sexy Backs: Women’s Evolved Lumbar Curvature Signals Ability to Handle Shift of Mass During Pregnancy

Evolutionary psychologists tell us there is an unmistakably Darwinian logic to the things we find sexually attractive. As we know, everyone alive today is the product of an unbroken line of ancestors who all succeeded in the game of survival and reproduction.

The first task a person must solve to have offspring is to find a fertile mate. Evolutionary psychology research suggests that modern humans have inherited a preference for looks that signal fertility: a low “waist-to-hip ratio,” for example — that hourglass shape that women aspire to have and men adore. Well, it turns out that a low waist-to-hip-ratio correlates with fewer complications in childbirth. And then there’s how men, more than women, are attracted by cues of youthfulness? In all likelihood, this is because fertility is sharply age-graded in women, more so than in men.

The latest kid on the block of Darwinian approaches to beauty is the unsexy term lumbature curvature. In a new paper published in Evolution and Human Behavior by David Lewis, Eric Russell, Laith Al-Shawaf and David Buss, the authors investigated which kinds of curvature of a woman’s lower back that men find most attractive. The reason? One adaptive problem uniquely faced in our bipedal species is a forward-shifted center of mass during pregnancy.

During pregnancy, if the center of mass could not somehow be moved back over the hips, our ancestral mothers would have suffered a nearly 800% increase in hip torque. One solution that women’s spines — though not men’s — evolved in order to deal with this challenge is a “wedging” in the third-to-last vertebra. That is, women’s third-to-last vertebra is shaped a bit like a wedge, with one thick end and tapering to the other, thinner end.  This makes it easier for pregnant mothers to bring the trunk’s center of mass back on top of their hips by extending their back. However, there is a delicate balance to strike — between enabling a move of center of mass during pregnancy and still retaining ordinary skeletal reinforcement.

To make a long story short, the researchers predicted that men will have evolved a preference for female backs that signal the optimal angle of lumbar curvature, which according to the orthopedic medical literature is around 45.5°. They conducted two experiments to test their hypothesis. First, they showed men images of women in profile, in which they manipulated the curvature of the lower back. (Technically, they varied the external angle formed between the buttock and thoracic spine.) The images that the men rated most attractive were those with a lower back that fit the magical 45.5 optimal degree of vertebrate wedging.

The curvature of the back, however, is not just influenced by vertebral wedging, but also by buttock mass. And in the researchers’ first study, the images varied in “buttock protrusion.” So the researchers conducted a second study in which men viewed images identical in buttock protrusion, but in only one condition was the buttock protrusion a cue to vertebral wedging. They found that men preferred women where buttock protrusion signaled optimal vertebral wedging. Men’s preference for a curved back, then, seems not to be just a by-product of a preference for buttock mass (Sir Mix-A-Lot notwithstanding), but a preference for backs that signal a specific degree of vertebrate wedging.

Here’s to the sexy backs.