The logic for a tax on fattening food may seem obvious. About one-third of Americans are obese, up from 15% in 1980. Fat people are more prone to heart disease, diabetes, bone disorders and cancer. An obese person’s annual medical costs are more than $700 greater than those of a comparable thin person. The total medical costs of obesity surpass $200 billion a year in America, which is higher than the bill for smoking.

But would a fat tax affect behaviour? Numerous studies have shown a relationship between the price of food, especially junk food, and body weight. As fast food has become relatively cheaper, so people have become fatter. A new paper* from the RAND Corporation, another think-tank, suggests that taxing calories could have a sizeable, if gradual, effect on people’s weight.

A new theoretical paper in the Journal of Public Economics even suggests that a tax on junk food could increase obesity, especially among physically active people. If junk food, which is quick and easy to obtain, becomes relatively dearer, people will spend more time shopping for fresh ingredients and preparing food at home. That could leave less time for exercise.

Read more in the Economist.