In an era of political polarization, Michael Bloomberg has the rare ability to come up with policies that enrage everyone. His latest pet project—banning large sodas, as a way of fighting obesity in New York—has been ridiculed by both Jon Stewart and John Boehner. And a recent Board of Health hearing on the plan saw Democratic and Republican politicians alike lining up to attack the idea, which would prohibit restaurants, delis, sports arenas, movie theatres, and food carts from selling any soft drinks larger than sixteen ounces. Critics dismiss the ban as yet another expression of Bloomberg’s nanny-state mentality and as a “feel-good placebo” that’s doomed to fail. They’re right that the ban is blatantly paternalist. But that doesn’t mean it won’t work.

Illustration by Christoph Niemann

It’s true that the ban will be easy to circumvent: if you want to drink thirty-two ounces, you can just buy two sixteen-ounce servings. But Bloomberg’s proposal makes clever use of what economists call “default bias.” If you offer a choice in which one option is seen as a default, most people go for that default option. People who are automatically enrolled in a retirement plan, for instance, are more likely to stay with their original plan than those who choose plans for themselves. In countries where people have to choose to be an organ donor, most people aren’t donors; in countries where people have to actively say they don’t want to be an organ donor, most are donors. The soda ban makes sixteen ounces or less the default option for soda drinkers; if they want more, they’ll have to make an extra effort.

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. But scoop size turned out to matter a lot: people consumed much more when the scoop was big. This suggests that most of us don’t have a fixed idea of how much we want; instead, we look to outside cues—like the size of a package or cup—to instruct us. And since the nineteen-seventies the portion sizes offered by food companies and restaurants have grown significantly larger. In 1974, the biggest drink McDonald’s offered was twenty-one ounces. Today, that’s roughly the size of a “small” drink at Burger King. In effect, the scoops have got bigger, and consumption has risen accordingly.

Of course, if you don’t want the large soda, you needn’t order it. 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. In the same way, the fact that a large soda is now forty ounces makes a twenty-ounce soda feel sensible. Bloomberg’s ban is designed to flip this effect on its head: if the largest soda you can order is sixteen ounces, a can of Coke may start to seem like more than enough. Some food researchers doubt that this will work, since so many of us are used to the idea of large servings. But even our experience of feeling satiated is highly malleable. In one experiment, people ate meals of dramatically different sizes in the dark, and those who were given much less food did not feel hungrier than the others or rate their meals as much smaller. So once people have a few sixteen-ounce drinks they may find that sixteen ounces is plenty.

Many economists would say that, if we want to discourage soda consumption, taxing it—the way we do alcohol and tobacco—would be more efficient than a ban. Some European countries do have such taxes, but the idea has been a political non-starter in New York. In any case, perhaps the most cunning aspect of Bloomberg’s proposed ban is that it would function as a kind of stealth tax on consumption, while leaving average-sized sodas untouched. Currently, on a per-ounce basis, large drinks are much cheaper than smaller ones—which encourages people to supersize. The soda ban should shift this. Two sixteen-ounce servings are bound to be more expensive than one thirty-two-ounce serving, which creates another disincentive to drink more.

If all this sounds as if New York’s soda consumers were about to become the subjects of an elaborate social-science experiment designed to reshape their behavior and desires, well, that’s kind of true. But then we’ve been the subject of just such an experiment, run by beverage and fast-food companies, for the past forty years. If Bloomberg has his way, we may start feeling like we’re white rats in a maze, but at least there’s a good chance we’ll be thinner rats. ♦