Obesity is a disease of appetite and sufficiency. The hypothesis of the thrifty gene, proposed by James Neel in 1962, was that in prehistory, individuals who were better able to store fat in times of plenty would cope more robustly with any ensuing dearth. When, however, there was no such oscillation between feast and famine—when we all had lots to eat—this genotype just got fat, and then fatter. Ideas like this, which try to explain the surprising increase in body weight experienced in the last quarter-century in terms of genetics and human biology, are much disputed. They may have a place in particular instances, but cannot answer for the problem as a whole.