Scientists claim it's in men's nature to prefer this body type because having an hourglass figure is a sign that a woman is young, has never been pregnant and has "key reproductive resources". “Because evolution depends entirely on individual success in reproducing, anything to do with reproduction, such as choosing a mate, should be optimised by natural selection,” explains William D. Lassek, study author and a researcher in the Department of Anthropology at the University of California at Santa Barbara.