Men like a woman with curves, especially a curve on the lower half of a female's body.
A study by the University of Texas reveals men have a clear preference for women whose spine and backside form a "theoretically optimal angle of lumbar curvature." Or in plain language, a curvy backside.
The researchers say they think it may be an evolutionary preference because in the prehistoric past that "optimal" angle - the study put it at 45.5 degrees - made women more likely to carry out successful pregnancies.
Scientists call the way the female spine attaches to the hips and buttocks "vertical wedging."
"This [optimal] spinal structure would have enabled pregnant women to balance their weight over the hips," says researcher David Lewis.
That would have made them more effective in foraging for food while pregnant, less prone to spinal injuries, and better able to deliver multiple offspring without injury, all of which would have made such women preferable for men, the researchers say.
Such preference would have evolved over thousand of years, they point out in their study published in the journal Evolution and Human Behavior.
In the study, around 100 young men rated how attractive they found a number of manipulated images displaying spinal curves ranging across the natural spectrum, from 26 degrees up to 61 degrees.
Men were most attracted to images showing women with the optimum of 45.5 degrees of lumbar curvature, the researchers found.
"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," says study co-author and psychology professor David Buss.
The finding adds to an increasing body of evidence that beauty is not entirely arbitrary, or "in the eyes of the beholder," he says, as many scientists believe.
Rather, he suggests, considerations of what is beauty has a "coherent adaptive logic."
And since men's psychological preference for a certain female shape evolved over thousands of years, it's not going to disappear any time soon, the researchers say.
"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," Lewis says.