Sometimes we find it easy to identify a problem and impossible to think of a solution. Obesity is a good example. Almost everybody agrees that it is a growing burden on health systems and that it requires urgent attention from policy makers. But almost everybody also agrees that no policy for reducing obesity is working.

Some 32% of adult American men and 35% of women are clinically obese. The proportion hasn't swelled in recent years, but it hasn't shrunk either, a study of...