Researchers across various fields have sought to address the fundamental question of why humans wage war (e.g., the Russia-Ukraine war). Psychologists have often conceptualized war as a form of intergroup conflict, and considerable attention has been devoted to investigating the psychological mechanisms. Previous studies have shown that intergroup conflict situations foster stronger ingroup cohesion and promote cooperative behavior. However, the free-rider problem theoretically makes achieving mutual cooperation difficult. Thus, it remains unclear why people tend to cooperate in intergroup conflict. This study aims to elucidate the mechanisms through which individuals address the free-rider problem and develop cooperative groups.
Cultural group selection theory posits that conformity bias—the tendency to conform excessively to the majority—may facilitate the achievement of mutual cooperation within a group. The theory suggests that conformity bias also works to foster mutual cooperation within groups. However, there is little empirical evidence regarding conformity bias not only in ingroup cooperation situations, but in intergroup conflict situations. Furthermore, this study examined the qualitative differences between threats originating from outgroups and those arising from other sources, such as a natural disaster.
We conducted a vignette experiment with 452 crowd workers who played a public goods game. In the social-threat condition, groups lost 20% of funds if they contributed less than another group; in the no social-threat condition groups lost 20% of group funds independently if they fell below a threshold. Participants first decided whether to contribute, then repeated the decision under four informational conditions (0, 3, 6, or 9 of 9 others' contributions).
The results showed conformity bias across conditions, with similar patterns in both threat situations. These findings suggest that conformity bias contributes to achieving mutual cooperation in various group-based threat situations.