Previous research has discovered a curious phenomenon: groups cooperate less than individuals in a deterministic prisoner’s dilemma game, but cooperate more than individuals when uncertainty is introduced into the game. We conducted two studies to examine three possible processes that might...

People often sacrifice their self-interest for a group to which they belong, even when outsiders are harmed so that the sacrifice has no net benefit. Two experiments (conducted on the World Wide Web) suggest that people do this, in part, because they think that cooperation on behalf of the group...