Severity and deservedness determine signalled trustworthiness in third party punishment.

Studies on third-party punishment (TPP) have shown it promotes cooperation and prosocial behaviour, albeit at a cost to the punisher. Contrary to the view that such punishment is entirely altruistic, recent research suggests that punishers gain reputational benefits from third-party punishing in the form of increased trustworthiness. Nevertheless, both how the signal is determined and the honesty of the signalling function of TPP have not been fully examined. Here we present the results of four experiments (n = 1695, prolific.co) in which we examined how TPP signalling varies as a function of its deservedness and severity. Experiments 1A and 1B use incentivized economic game paradigms to show how deservedness, impacts the trustworthiness signalled by the punisher. Experiment 2A expands on traditional dichotomous punishment decisions to show how signalled trustworthiness depends on different levels of TPP severity. Experiment 2B isolates the signalling effectiveness of severity by decoupling it from the corresponding incurred cost to punish and examines its impact on signalled trustworthiness. Overall, we found that punishment signalling is sensitive to deservedness and severity but not independent of other factors such as the cost to punish.