Third-party punishment is punishment of a transgressor (first party) which is administered, not by a victim of the transgression (second party), but rather by a third party not directly affected by the transgression.[1] It has been argued that third-party punishments are the essence of social norms, as they are an evolutionarily stable strategy, unlike second-party punishments.[2] It has also been shown that third-party punishments are exhibited in all examined populations, though the magnitude of the punishments varies greatly, and that costly punishment co-varies with altruistic behavior.[3][4][text–source integrity?][5] Differences between within-group and inter-group altruistic punishments have also been observed.[6]