The Waist-to-Hip Ratio

Why does the waist-to-hip ratio matter? Singh observes that a "gynoid" fat distribution—more fat on the hips, less on the torso—is necessary for the hormonal changes associated with female fertility. An "android" fat distribution-—fat on the belly, thin hips—is associated with the symptoms of male disabilities such as heart disease, even in women. But which is cause and which effect? It seems to me more likely that both the shape and the hormonal effects of it are sexually selected by generations of males rather than males preferring the shape because it is the only way the hormones can be made to work. The relatively brief period during which women have hourglass bodies—from fifteen to thirty-five, say—is a sexually selected phenomenon. It owes more to competition to attract men than to any other biological need. Men have been unconsciously acting as selective breeders of women.

Low provides one possible reason for the male preference tor a low ratio—choosing broad-hipped women more able to give birth. Most apes give birth to babies whose brain is half-grown; human babies' brains are one-third grown at birth, and they spend far less time in the womb than is normal for a mammal, given the longevity of man. The reason is obvious: Were the hole in the pelvis through which we are born (the birth canal) commensurately larger, our mothers would be unable to walk at all. The width of human hips reached a certain point and could go no further; as brains continued to grow bigger, earlier birth was the only option left to the species. Imagine the evolutionary pressure of this process on female hip size. It was always wise for a man to choose the biggest-hipped woman he could find, generation after generation, for millions of years. At a certain point hips could get no bigger but men still had the preference, so women with slender waists who appeared to have larger hips by contrast were preferred instead.

Matt Ridley, The Red Queen: Sex and the Evolution of Human Nature (2003-05-01)

• evolution
• gender
• sexual diversity
• sexual selection