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Why has there been so much fuss about New York City's attempt to impose a ban on large "sugary drinks"? After all, people can still get as much soda as they want. This isn't Prohibition. It's just that getting it would take slightly more effort.

Obviously, it's not about soda. It's because such a ban suggests that sometimes we need to be stopped from doing foolish stuff, and this has become, in contemporary American politics, highly controversial, no matter how trivial the particular issue. (Large cups of soda as symbols of human dignity? Really?)

Americans, even those who generally support government intervention in our daily lives, have a reflexive response to being told what to do, and it's not a positive one. It's this common desire to be left alone that prompted the Mississippi Legislature this month to pass a ban on bans — a law that forbids municipalities to place local restrictions on food or drink.

We have a vision of ourselves as free, rational beings who are totally capable of making all the decisions we need to in order to create a good life. It's a nice vision, one that makes us feel proud of ourselves. But it's false.

John Stuart Mill wrote in 1859 that the only justifiable reason for interfering in someone's freedom of action was to prevent harm to others. According to Mill's "harm principle," we should almost never stop people from behavior that affects only themselves.

That "almost," though, is important. It's fair to stop us, Mill argued, when we are acting out of ignorance and doing something we'll pretty definitely regret. You can stop someone from crossing a bridge that is broken, he said, because you can be sure no one wants to plummet into the river. Mill just didn't think this would happen very often.

He was wrong about that. A lot of times we have a good idea of where we want to go, but a really terrible idea of how to get there. It's well-established by now that we often don't think very clearly when it comes to choosing the best means to attain our ends. This has been the object of an enormous amount of study over the past few decades.

Research by psychologists and behavioral economists, including the Nobel Prize-winner Daniel Kahneman and his research partner Amos Tversky, identified a number of areas in which we fairly dependably fail. For example, we suffer from an optimism bias; that is, we tend to think that however likely a bad thing is to happen to most people in our situation, it's less likely to happen to us — not for any particular reason, but because we're irrationally optimistic.

We also suffer from a status quo bias, which makes us value what we have over the alternatives — which might, of course, make us react badly to new laws, even when they are an improvement over what we have.

Is it always a mistake when someone does something imprudent — for instance, chugs 32 ounces of soda? No. For some people, that's the right choice. They don't care that much about their health, or they won't drink too many big sodas, or they just really love having a lot of soda at once.

But laws have to be sensitive to the needs of the majority. That doesn't mean that they should trample the rights of the minority, but that public benefit is a legitimate concern, even when that may inconvenience some.

So do these laws mean that some people will be kept from doing what they really want to do? Probably — and in many ways it hurts to be part of a society governed by laws. Some of us can drive safely at 90 miles per hour, for instance, but individual speeding laws aren't practical.

The freedom to buy a really large soda, all in one cup, is now something we stand to lose. For most people, given their desire for health, that's a net gain. For some people, it's a loss. Just not much of a loss.

Of course, what people fear is that this is just the beginning: Today it's soda, tomorrow it's the guy standing behind you making you eat your broccoli, floss your teeth and watch "PBS NewsHour." What this ignores is that successful paternalistic laws are done on the basis of a cost-benefit analysis: If it's too painful, it's not a good law. Making these analyses is something the government has the resources to do, just as now it sets automobile construction standards while considering both the need for affordability and the desire for safety.

In the old days, we used to blame people for acting imprudently — since their bad choices were their fault, they deserved to suffer the consequences. Now we see that these errors aren't a function of bad character, but of our shared cognitive inheritance. The proper reaction is not blame, but an impulse to help one another.

That's what the government is supposed to do: help us get where we want to go. It's hard to give up the idea of ourselves as completely rational. We feel as if we lose some dignity. But that's the way it is, and there's no dignity in clinging to an illusion.

Sarah Conly, an assistant professor of philosophy at Bowdoin College, is the author of "Against Autonomy: Justifying Coercive Paternalism." She wrote this article for the New York Times.