Understanding how we decide.

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.

Defaults are powerful: whereas wisely set defaults increase prosocial choices, poorly set defaults promote selfish decisions. In two preregistered studies (N_{Study 1} = 584, N_{Study 2} = 587), we test whether distracting environments, compared to distraction-free environments, make people rely more on selfish than prosocial defaults. Participants completed repeated binary dictator games in a 2 (selfish default vs. prosocial default) x 2 (distraction vs. no distraction) within-subjects design. Results from both studies and an internal meta-analysis provide no support for the hypothesis. Whereas participants accepted prosocial more than selfish defaults (in Study 1 and in an internal meta-analysis), the existence (vs. absence) of distraction did not affect default acceptance differentially for selfish versus prosocial defaults. Exploratory cluster analyses showed that around 75% of participants made choices based on their social preference, thereby showing little to no response to defaults. The remaining 25%, however, were responsive to defaults, resulting in an overall higher fraction of prosocial choices made when defaults are prosocial (vs. selfish). Overall, despite most participants being non-responsive, findings highlight the aggregate benefit of setting prosocial defaults, both in distracting and non-distracting environments.

How do people attribute responsibility when an outcome is not caused by an
individual but results from a decision chain involving several people? We study
this question in an experiment, in which five voters sequentially decide on how to
distribute money between them and five recipients. The recipients can reward or
punish each voter, which we use as measures of responsibility attribution. In the
aggregate, we find that responsibility is attributed mostly according to the voters’
choices and the pivotality of the decision, but not for being the initial voter. On
the individual level, we find substantial heterogeneity with three overall patterns:
Little to no responsibility attribution, pivotality-driven, and focus on choices. These
patterns are similar when praising voters for good outcomes and blaming voters for
bad outcomes.