Researchers have discovered that men are particularly attracted to women with a back that curves 45 degrees above the top of her bottom.
Such an angle would have given a woman an advantage while pregnant in early hunter-gatherer societies and so has evolved as being seen as attractive around the world, they claim.
However, the researchers also found that a physical curvature of the spine - known as vertebral wedging - was seen as more attractive than extra buttock mass.
Dr David Lewis, a psychologist at Bilkent University in Turkey, said: 'It's an independent and previously undiscovered standard of attractiveness.
'(The curve) would have enabled ancestral women to shift their centre of mass back over their hips during pregnancy, a time during which there is a dramatic forward shift of their centre of mass.
'This benefit is critical: without being able to do this, women would experience a dramatic increase in hip torque (pressure) subjecting them to risk of muscular fatigue and injury.
'Consequently, ancestral women who possessed this degree of lumbar curvature would have been able to forage longer into pregnancy and would have been able to carry out multiple pregnancies with a reduced risk of spinal injury.'
Dr Lewis said that this trait would have become sexually attractive to men due to the evolutionary advantage it gave.
Without such a curve, the pressure on a woman's hips during pregnancy increases by nearly 800 per cent, rendering her largely immobile and increasing the risk of health problems.
For the study, which was published in the journal Evolution and Human Behaviour, the psychologists showed 300 men silhouettes of women with different angles of curvature.
They were then asked to rate the images according to how attractive they found the silhouette.
Most of the men were significantly more attracted to those with a spine curve of 45.5 degrees, the researchers found.