A new study by the University of Texas Austin has shown that it is not the size of the butt that attracts men but the curve of the spine.

Scientists are always trying to find an objective reason to explain beauty. Often, they are thwarted by varying cultural standards of what is attractive. But sometimes, they manage to uncover some universal truisms- big eyes, healthy teeth, clear skin- in short, signs that a person is healthy. For women, one of the most agreed upon standards of beauty is the wide, childbearing hips. Evolutionary, men have developed to prefer such women because big hips will increase the chances of a successful birth.

The new research believes to have discovered another evolutionary predisposition in men. Women with a distinct back-to-butt curve of about 45.5 degrees were consistently rated the most attractive. This degree of curvature has been dubbed the “theoretically optimal angle of lumbar curvature,” according to the authors of the study, published in Evolution and Human Behavior.

This curve allows pregnant women to shift the weight of the child so that it can be supported by the whole pelvic bone, as opposed to using only the back muscles to carry the weight in front of them. This not only made carrying the child easier, it allowed the women to stay mobile during the nine months of pregnancy- a necessity for the nomadic hunters and gatherers.

“This spinal structure would have enabled pregnant women to balance their weight over the hips,” said study co-author David Lewis. “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.”

The team conducted a second study, to be sure that the attraction was truly based on spine curvature, not butt size. The results showed the men still preferred a woman with the optimal spine angle, even when she had a smaller butt.

“Beauty is not entirely arbitrary, or ‘in the eyes of the beholder’ as many in mainstream social science believed, but rather has a coherent adaptive logic,” said Lewis.