I am mildly opposed to Michael Bloomberg's super-size soda ban, but I think James Suroweicki makes a good case:
An executive at the American Beverage Association has dismissed the plan, saying that "150 years of research finds that people consume what they want." Actually, the research shows that what people "want" has a lot to do with how choices are framed. In one well-known study, researchers put a bowl of M&M's on the concierge desk of an apartment building, with a scoop attached and a sign below that said "Eat Your Fill." On alternating days, the experimenters changed the size of the scoop--from a tablespoon to a quarter-cup scoop, which was four times as big. If people really ate just "what they want," the amount they ate should have remained roughly the same...Yet the mere existence of the supersize can change your idea of how much you want to drink. In a classic experiment by Itamar Simonson and Amos Tversky, people asked to choose between a cheap camera and a pricier one with more features were divided more or less equally between the two options. But when a third option--a fancy, very expensive camera--was added to the mix most people went for the mid-range camera. The very expensive camera made the middle one seem less extravagant.