Large-scale community study reveals information sampling drives fairness decisions.

Fairness is a fundamental social norm guiding human decision-making. Yet, much of our empirical understanding of fairness derives from controlled laboratory studies with homogeneous student samples, raising concerns about the ecological validity of experimental findings. Here, we tackle this challenge by introducing a citizen science, lab-in-the-field approach, embedding a classic fairness paradigm, the Ultimatum Game (UG), in a well-visited public space within a community: a museum. Over the course of 13 months, we recorded >18,672 decisions from a heterogeneous sample of volunteer members of the public. Each participant responded to four allocation offers from anonymous proposers (two generous, two selfish), with the option to view proposers' past behaviour (previously generous vs. selfish), before deciding whether to accept or reject each offer. Results closely replicated classic UG effects, with unfair offers frequently rejected, confirming the presence of inequality aversion beyond the laboratory. Notably, the majority of participants chose to sample proposer-history information, and those who did showed heightened sensitivity to fairness violations. Specifically, selfish offers from a proposer who had previously acted generously to others elicited the strongest rejection rates, demonstrating that judgements of unfairness are shaped by expectations which emerge from voluntary information sampling. Furthermore, the ecologically enriched design helped uncover temporal and demographic patterns, namely an association between time-of-day and information-seeking behaviour, and an increased willingness to accept unfairness across age. Methodologically, by situating a foundational experimental paradigm in a community venue, our approach aims to provide a scalable model for studying decision-making in ecologically enhanced contexts and a framework for research seeking to examine authentic behaviours beyond the laboratory, ultimately helping to deepen our understanding of the crucial norms that shape society.