Risk and retribution: how punishment uncertainty weakens norms of cooperation

Punishment can sustain cooperation, but in real-world settings, sanctions are often inconsistently applied. We tested how punishment uncertainty affects cooperation when individuals can choose between institutions with predictable or unpredictable sanctions. In a 25-round public goods experiment with peer punishment, participants repeatedly selected between a certain punishment institution, where punishment had a fixed effect, and an uncertain institution, where punishment outcomes varied randomly but had the same expected severity. Although participants initially favoured the uncertain institution, they gradually shifted toward the certain institution over time. Consistent with pre-registered hypotheses, the certain institution produced higher contributions, lower punishment expenditures, less antisocial punishment and higher pay-offs. More prosocial, risk-averse and older participants were more likely to choose the certain institution. Exploratory post-experiment analyses further suggested that weaker contribution norms and non-specific punishment motives were associated with less effective sanctioning behaviour. Furthermore, exploratory subgroup analyses suggest that the main advantage of sanction certainty is broadly robust across the major demographic groups represented in the sample, although effect sizes vary. These findings indicate that sanction reliability plays an important role in sustaining cooperative norms and shaping institutional selection.