Our previous emphasis has been on how much potential punishers are willing to pay to punish defectors relative to cooperators. Another question is whether the willingness to incur costs for punishing defectors depends significantly on whether the punisher himself cooperated or defected. This question is particularly interesting in the TP condition because a cooperator has no direct reason to feel exploited by the defection of an outside group member in this condition. However, those subjects who cooperate themselves are perhaps better able to empathize with cooperators in other groups who had to face the defection of their PD partner.

XREF: Connects to social identity theory and empathy-based moral motivation in punishment research.

Ernst Fehr, Third-Party Punishment and …, loc. 124