In our paper, we present both a theoretical model and empirical experiments. The model involves “costly signaling,” the classic example of which is the peacock’s tail. Only healthy male peacocks with high-quality genes can manage to produce extravagant plumages, so these tails — precisely because of how “resource intensive” they are — function as honest advertisements to potential mates of a peacock’s genetic quality.

We argue that the same can be true of punishing others for wrongdoing, which can serve as a signal of trustworthiness. This is because punishing others is often costly — but less so for those people who find it worthwhile to be trustworthy. Consider: Trustworthiness pays off for you if others typically reciprocate your good deeds or reward you for good behavior. This includes being rewarded for promoting moral behavior via punishment.

Therefore, if you are a person who finds being trustworthy rewarding, you’ll typically also find punishing less costly. Our mathematical model shows that, as a result, choosing to punish wrongdoers can work like a peacock’s tail — if I see you punish misbehavior, I can infer that you are likely to be trustworthy.

To test whether people actually follow this logic, we ran experiments in which subjects interacted with anonymous strangers on the Internet. In our experiments, one subject (the signaler) received some money. Then, he had the chance to give up some of the money to punish somebody for being selfish. Our subjects turned out to be fair-minded: A sizable proportion of signalers were willing to pay to punish selfish acts, even though they had not been personally mistreated.

Next, a second subject (the chooser) decided whether to trust the signaler — after observing whether or not the signaler had decided to punish. This decision had real consequences: If the chooser decided to trust the signaler, she earned money if the signaler turned out to be trustworthy, but lost money if he did not. (Either way, the signaler benefited from being trusted).