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For many residents of New York City, our bodies are our cars. So rather than engaging in “road rage” against slow or erratic drivers on a highway, New Yorkers descend into “sidewalk rage,” paroxysms of fury directed at people who exhibit irrational, obstructive walking behavior on Manhattan’s crowded concrete. But is this reaction a sign of mental illness — or could it perhaps reflect an evolutionary adaptation that may have enabled the development of cooperation?

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I will admit personally to fits of pique when slow tourists fail to keep to the right, or insist on standing side-by-side on escalators, blocking the left-hand fast lane, like some of those described in a recent article in the Wall Street Journal.

That article came down on the side of sidewalk rage as psychiatric disorder. Shirley Wang writes:

Researchers say the concept of “sidewalk rage” is real. One scientist has even developed a Pedestrian Aggressiveness Syndrome Scale to map out how people express their fury. At its most extreme, sidewalk rage can signal a psychiatric condition known as “intermittent explosive disorder,” researchers say. On Facebook, there’s a group called “I Secretly Want to Punch Slow Walking People in the Back of the Head” that boasts nearly 15,000 members.

For the rare folks who act out in dangerous ways, sidewalk or road rage may indeed signal illness. But the idea raises the much more interesting question of why so many otherwise normal people also feel the same intense emotion when navigating around slow hordes — and have to temper their impulses to act on their anger — in the first place. (More on TIME.com: Want to Improve Your Memory? Try Taking a Walk)

Which brings me to what researchers call “altruistic punishment.” While it sounds like an oxymoron, altruistic punishment is basically how social norms get enforced. So when you expel a huffy “Excuse me!” to the rude sidewalk clogger in front of you who has stopped midstride to check his BlackBerry, you’re trying to discourage behavior that endangers other members of the society. It’s called “altruistic” punishment, because your efforts to protect civility come at personal cost with little chance of personal benefit: you are far more likely to get an obscene gesture or even a punch in the mouth than a thank you.

Many evolutionary psychologists believe, however, that without altruistic punishment, cooperation could not have evolved. In simulations of “selfish” versus “cooperative” strategies for living, for instance, researchers have found that altruistic or cooperative creatures beat out selfish ones only in an environment in which the failure to cooperate is actively detected and punished. Sidewalk rage — anger over the selfish violation of a cooperative social norm that protects the group — is a nice example of that. (More on Time.com: Five Ways to Beat the Winter Doldrums)

Reinforcing that theory is the result of a recent study that explored whether altruistic punishment is an act of deliberation and self-control, or, as one might expect from the case of sidewalk rage, an emotional impulse. Researchers found a connection between impulsiveness and altruistic punishment, suggesting that the phenomenon is more the result of emotion (like sidewalk rage) than reason.

In the study, researchers measured participants’ impulse control by subjecting them to a test similar to the famous Stanford marshmallow test, which allows people to gain more goodies later if they can resist the temptation of a smaller reward now.

The participants were also asked to play an “ultimatum game” in which two people have to split a sum of money. The first person gets to decide how the loot is divided, but if the second person rejects the offer, no one gets anything. In other words, it’s a situation in which you can punish someone for being unfair or selfish — but only at a cost to yourself. (More on TIME.com: Study: Walking is Brain Exercise, Too)

Adding a twist to the experiment, some volunteers were given medication that depleted the amount of serotonin in their brains; others were given placebo. Low levels of serotonin have been linked with impulsive and irrational behavior, so reducing it could help determine whether self-control and altruistic punishment are affected by it in the same way.

And indeed, in the low serotonin condition, participants were more likely both to make impulsive choices in the delayed-reward test and to punish those who behaved uncooperatively in the ultimatum game, supporting the idea that altruistic punishment is driven by emotion, specifically, anger. (More on Time.com: Healthland’s Guide to Life 2011)

So take heart, readers. If you find yourself fuming at those who behave in ways that are uncivil in your culture, you may be exhibiting an emotion that was a key part in allowing civilization to exist in the first place. And to those who want to avoid enraging New Yorkers: keep right and let us pass!