Everybody has heard of neighbours, who have been fighting over some minor topic for years. The fight goes back and forth,
giving the neighbours a hard time. These kind of reciprocal punishments are known as vendettas and they are a crosscultural
phenomenon. In evolutionary biology, punishment is seen as a mechanism for maintaining cooperative behaviour.
However, this notion of punishment excludes vendettas. Vendettas pose a special kind of evolutionary problem: they incur
high costs on individuals, i.e. costs of punishing and costs of being punished, without any benefits. Theoretically speaking,
punishment should be rare in dyadic relationships and vendettas would not evolve under natural selection. In contrast,
punishment is assumed to be more efficient in group environments which then can pave the way for vendettas.
Accordingly, we found that under the experimental conditions of a prisoner’s dilemma game, human participants punished
only rarely and vendettas are scarce. In contrast, we found that participants retaliated frequently in the group environment
of a public goods game. They even engaged in cost-intense vendettas (i.e. continuous retaliation), especially when the first
punishment was unjustified or ambiguous. Here, punishment was mainly targeted at defectors in the beginning, but
provocations led to mushrooming of counter-punishments. Despite the counter-punishing behaviour, participants were
able to enhance cooperation levels in the public goods game. Few participants even seemed to anticipate the outbreak of
costly vendettas and delayed their punishment to the last possible moment. Overall, our results highlight the importance of
different social environments while studying punishment as a cooperation-enhancing mechanism.