Taxing the Wrong Stuff

I am amused by the idea of taxing junk food as reported by The Economist. Here is the logic of its supporters: junk food causes obesity. Obesity causes diseases. In state-sponsored health care system, the cost of curing these diseases has also to be shared by non-obese taxpayers. Thus, as the existing junk food consumption doesn't reflect true social cost, tax on junk-food shall be imposed.

This is nonsense. The Economist itself says that:

More important, junk food is not itself the source of the externality—the medical costs that arise from obesity. Unlike smoking, or excessive gambling and drinking, eating junk food does not directly impair the well-being of anyone else. And because obesity is determined by lack of exercise as well as calorie intake, its ultimate relationship with health costs is more tenuous than that of, say, smoking. It is possible to eat a lot of fatty food, exercise frequently and not generate any externalities. A more direct, though controversial, approach would simply be to tax people on the basis of their weight.

Double Whopper, anyone?