There is a seductive simplicity to the conceptualisation of obesity as a straightforward problem of energy balance—calories in versus calories out. But the physiological, behavioural, and environmental influences on this relation are asymmetrical. Therefore, although the basic arithmetic holds true, in practice it is much easier for people, and populations, to gain weight than to lose it. As Boyd Swinburn and colleagues describe in The Lancet, increasing fatness is the result of a normal response, by normal people, to an abnormal situation.