“The desire of food,” wrote Adam Smith in 1776, “is limited in every man by the narrow capacity of the human stomach.” Not so, it would seem. If only the father of modern economics, who was otherwise such a shrewd judge of human nature, could see us now. Obesity and weight gain abound in the world’s richer countries, and our desire for food, as Smith wrote of other necessities and luxuries, “seems to have no limit or certain boundary.”

As two economists who used to be obese, we know about this serious public-health problem...