Some
psychologists argue that this is how the majority of
men judge whether a
woman's body is
hot or not: by comparing the width of her
waist to the width of her
hips. Ratios within a certain range are
sexy: ratios outside that range aren't.
A woman's fertility can be judged at a glance by the shape of her pelvis, which is where a fetus would have to grow and develop. Over time, males developed an instinctive attraction to females with pelves wide enough to accommodate birth. Overly obese or gaunt women tend to have poor fertility rates as well, so males became attracted to women in the middle.
The result: the waist-to-hip ratio. There are no set figures to judge which ratios are good and which are bad: it appears to vary somewhat from culture to culture and from man to man, but most men favor women with broad hips and somewhat narrower waists. As if you didn't already know that from Sisqo.