Kim Kardashian
Kim Kardashian's photoshoot for Paper Magazine heyzulbear/twitter

Men are hardwired to find curvy women more attractive because in ancient times, they would have been better at foraging during pregnancy. Researchers have investigated the "optimal angle of lumbar curvature" to find the ideal curve from the back to the buttocks. This perfect point would have allowed our ancestors to better support and provide for children, as well as being able to have multiple pregnancies.

The team from the University of Texas at Austin first asked 100 men to rate the attractiveness of several manipulated images showing spinal curves across the natural spectrum. They were most attracted to the images where women had the optimum curvature of 45 degrees.

Research leader David Lewis said: "This spinal structure would have enabled pregnant women to balance their weight over the hips. These women would have been more effective at foraging during pregnancy and less likely to suffer spinal injuries.

spinal curve
Buttock protrusion associated with (a) gluteal development indicating physical fitness, (b) adipose tissue deposition, and (c) vertebral wedging. Notes: All women exhibit identical buttock protrusion. Women (a) and (c) also exhibit an identical angle between the thoracic spine and buttocks (i.e. lumbar curvature). The University of Texas at Austin

"In turn, men who preferred these women would have had mates who were better able to provide for foetus and offspring, and who would have been able to carry out multiple pregnancies without injury."

Next, the team looked at whether the men preferred this angle because it reflects larger buttocks, or if it was related to the actual angle of the spine. Of the 200 men presented with images of women with different buttock size they consistently preferred women whose spinal curvature was closer to the optimal 45 degrees – regardless of buttock size.

"This enabled us to conclusively show that men prefer women who exhibit specific angles of spinal curvature over buttock mass," co-author Eric Russell said.

spinal curve
This is a schematic of a complete series of stimuli. The University of Texas at Austin

Lewis added: "What's fascinating about this research is that it is yet another scientific illustration of a close fit between a sex-differentiated feature of human morphology -- in this case lumbar curvature -- and an evolved standard of attractiveness.

"This adds to a growing body of evidence that 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.

"This tight fit between evolutionary pressures and modern humans' psychology, including our standards of attractiveness, highlights the usefulness that an evolutionary approach can have for expanding our knowledge not just of the natural sciences, but also the social sciences."