Humans are now among the fattest of all mammals. While fat serves as insulation from the cold in other mammals, most, although not all, fat appears to serve as an energy reserve in humans. Both genes and life-style are involved in the etiology of obesity, although the relative importance of either factor and the ways in which they interact are not thoroughly understood. Obesity has never been a common health problem throughout most of human history, nor has it even been realistically possible for most people to become obese because of frequent food shortages. Scarcity has been a powerful agent of natural selection in human biocultural evolution, favoring individuals who could effectively store calories in times of surplus. Genetic traits that cause fatness are selected because they improve the chance of survival when food is scarce, particularly in pregnant and nursing women. Women with greater energy reserves in fat would have a selective advantage not only for themselves, but also for their fetuses and young children. Females appear to have been selected for the less lipolytic peripheral body fat. Moreover, a minimal level of female fatness increases lifetime reproductive success because of its apparent association with regular cycling and earlier onset of puberty. Some cultures used a fattening hut for brides-to-be, which