Humans are strangely nice. This seems an odd assertion given our ongoing concerns with war, inequality, and environmental degradation. But one thing really astonishing about us as a species is how many times a day we have completely cooperative encounters with each other—ones that uphold social norms, even in modern urban societies where interactions among strangers are common.

When people who interact often cooperate with one another, we can explain their politeness by looking at how much they stand to benefit from their ongoing relationship. When it comes to behaviors with strangers, like queue-jumping, the fact that norms are upheld more often than they are broken is puzzling. What reason is there to play nice if the strangers around you will have no impact on your life in the future?

The leading explanation for this cooperation is that people do actually punish strangers for not cooperating—and a recent paper in PNAS provides evidence that this happens in the real world. It also indicates that people prefer certain kinds of punishment over others.

If we’re going to respond to bad behavior, we have two options. Direct punishment, like shouting at someone who harasses a stranger on the street. This risks retaliation and even violence, so it isn’t clear why people ever do this. Indirect punishment, like refusing to help people who behave badly, is much easier to do.

Because there’s little empirical evidence of how punishment interactions like these work in real life, a team of economists set out to establish whether people prefer direct or indirect punishment, and whether direct punishment results in reward from strangers. To test this, they used groups of actors in the international train station in Cologne, Germany, where more than 280,000 passengers cross paths every day. The chances of people knowing each other in a station this busy are low. The actors themselves did not know what hypothesis was being tested—they just knew their scripts and how to record the results.

An initial baseline scenario was established by simply having an actor drop a pile of books in front of a bystander standing alone and seeing how often that bystander assisted him or her in picking up the books. In the second scenario, one actor (the ‘norm violator’) would drop an empty coffee cup, very obviously violating a strong social norm against littering. The second actor (the ‘punisher’) would then ask the norm violator to pick up the coffee cup, and the violator would comply and quietly leave. Shortly afterward, the punisher would drop a pile of books, and the response of the bystander—either helping or not helping to pick up the books—was recorded.

In scenario three, a single actor would drop the coffee cup in front of a bystander, recording the number of times a bystander delivered direct punishment (asking the litterbug to pick up the cup). Scenario four added an additional layer: after intentionally littering, the actor would accidentally drop their books and record how often the bystander jumped in to help and how often they asked the violator to pick up their litter. Altogether, the four scenarios were repeated 447 times.

The results showed that people who enforced social norms didn’t get much benefit from it. Bystanders were not significantly more inclined to help a punisher than they were to help any random person who dropped their books, so rewards from strangers might not actually play a role in encouraging direct punishment. However, norm violators were a lot less likely to receive help from strangers than people in the baseline scenario.

When comparing direct and indirect punishment, it was clear that people prefer indirect punishment when it’s available as an option. When the norm violator littered without subsequently needing help, 17 percent of observers punished directly. However, when the norm violation was accompanied by a need for help, the rate of direct punishment dropped to just seven percent. Instead, people preferred to punish indirectly by withholding help from the violator.

The gender of the violator also appeared to play a role in the kind of punishment chosen: direct punishment was used more against women, and indirect punishment against men, regardless of the gender of the observer.

The study’s results shed some important light on cooperation between strangers. First, they call into question the idea that direct punishment is used because it results in a reward from strangers: there was no evidence that this happened, so the reason people engage in a potentially costly punishment act is still a mystery. But the results do seem to support the hypothesis that risk of retaliation might be a reason to steer clear of direct punishment—people's greater reluctance to punish men directly could be due to a perceived greater risk of aggression.

Evidence that we prefer to use indirect punishment is strong, with people opting to withhold help rather than voice their disapproval if there is an opportunity to do so.

The researchers note that there is a great deal more to explore here, such as why people use direct punishment at all or whether people are more likely to reward punishers if they stand up for another person being violated, compared to standing up for themselves or widely held norms. There’s also a need to look for factors other than punishment in enforcing cooperation among strangers. However, the results do answer some important questions about the role of punishment, and the kinds of punishment used, in keeping people nice.

PNAS, 2014. DOI: 10.1073/pnas.1413170111 (About DOIs).