Contagion and escalation of rule violations.

Human societies rely on rules to coordinate behaviour, sustain cooperation and maintain social order. Yet compliance can be fragile when rules are weakly enforced and individuals observe others violating them. Prior research shows that rule violations and other antisocial behaviours can spread through social influence, but less is known about how the severity of violations shapes these dynamics. In many real-world settings, rule-breaking occurs in degrees, from minor infractions to major violations, and individuals may not only imitate others but sometimes exceed the violations they observe. Here, we combine simulations and a behavioural experiment (n = 750) to study how the magnitude of observed rule violations shapes subsequent violations, and how this can generate escalation dynamics. Our simulations show that when individuals occasionally escalate beyond observed violations, even rare escalation can trigger a rapid shift from widespread compliance to collective rule-breaking. In our pre-registered experiment, observing major violations strongly increased rule-breaking, especially when committed by socially close others. Observing minor violations prompted many participants to follow suit and a minority to escalate beyond the violations they observed. Parameterizing the simulations with these behavioural responses shows that such escalators can disproportionately undermine collective compliance. These results highlight how asymmetric responses to rule violations and rare escalatory behaviour can transform small deviations into large-scale breakdowns of rule compliance.