If you were an academic researcher, you’d have to persuade your institutional review board that you had considered the risks and obtained informed consent from the participants.

You might, for instance, take note of a recent clinical trial in which heart patients put on a restricted-sodium diet fared worse than those on a normal diet. In light of new research suggesting that eating salt improves mood and combats depression, you might be alert for psychological effects of the new diet. You might worry that people would react to less-salty food by eating more of it, a trend you could monitor by comparing them with a control group.

But if you are the mayor of New York, no such constraints apply. You can simply announce, as Michael Bloomberg did, that the city is starting a “nationwide initiative” to pressure the food industry and restaurant chains to cut salt intake by half over the next decade.