The behavioral structure of one-shot cooperation in small and large groups

Cooperation is essential for realizing collective benefits in both small and large groups. In modern societies, many interactions are one-shot and differ in the magnitude of collective benefits. This paper identifies the unified behavioral structure underlying one-shot cooperation. Using online public goods games (N=2,136) and two versions of the strategy method, we examine how attitudes toward cooperation—the causal relationship between one’s own cooperation and others’—and beliefs about others’ cooperation explain cooperative behavior across group sizes (3, 9, and 30) and collective returns (low vs. high). Our ABC framework (Attitudes and Beliefs explain Cooperation) reveals a consistent structure: average attitudes toward cooperation are equally conditionally cooperative across all settings, while beliefs increase with the collective benefit of cooperation but not with group size, although beliefs become more important the larger the group. These results suggest that the ABC framework provides a parsimonious behavioral foundation for understanding one-shot cooperation.