Punishment has been shown to be an effective reinforcement mechanism. Intentional or not, punishment will likely generate spillover effects that extend beyond one’s immediate decision environment, and these spillovers are not as well understood. We seek to understand these secondary spillover effects in a controlled lab setting using a standard social dilemma: the voluntary contributions mechanism. We find that spillovers occur when others observe punishment outside their own social dilemma. However, the direction of the spillover effect depends crucially on personal punishment history and whether one is personally exempt from punishment or not.

Other-regarding preferences or decision errors are the main explanations put forward to justify contributions exceeding the non-cooperative optimum in VCM games. An alternative rationale relies on ambiguity aversion. Ambiguity aversion increases the perceived marginal benefit of own contributions, which in equilibrium will exceed the Nash level. We present a series of experiments testing this hypothesis. To control for other-regarding preferences, we run a two-player game in which a human player plays with a virtual agent. Players are assigned either to a risky setting (known probabilities of opponent's choices) or to an ambiguity setting (probabilities of opponent's contribution are vague). Results show that ambiguity affects contributions. However, attitude to ambiguity appears to be affected by the location of the aggregate Nash optimum inside the decision space.