We also found that sanctions by second parties directly harmed were much stronger than third-party sanctions, indeed strong enough to make norm violations unprofitable, whereas the sanctions of a single third party were not. Thus, in the context of our experiment, more than one third party is needed to enforce the norm. However, this condition is probably met frequently in real life. Therefore, taken together, our results suggest that altruistic third-party sanctions are likely to be powerful enforcers of social norms.

Builds on: "Second parties punish norm violations more severely than third parties."

Ernst Fehr, Third-Party Punishment and …, loc. 148