From The Economist:
Some gentlemen may prefer blondes, but almost all seem to like a waist to hip ratio of between 0.6 and 0.7. Breasts and bottoms should be substantial; waists should be slim. It should be the case all over the world and throughout human history.
That, at least, is the prevailing theory among evolutionary psychologists. The ratio in question correlates with hormone levels promoting maximum female fertility and health, so men who prefer curvy women will have more children. Devendra Singh, of the University of Texas, in Austin, has proved the point in the past by measuring the vital statistics of Playboy models. He found that centrefolds vary in weight but not in their hourglass shapes.
More here.