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 conduct two studies to examine three possible processes that may have driven groups to be more cooperative than individuals to reduce risks: group risk concern, group cooperation expectation, and social pressure. We find that ex-post guilt aversion and ex-post blame avoidance under uncertainty cause group members to be more risk concerned than individuals. This concern drives groups to choose the cooperation (and risk-reduction) strategy more frequently than individuals do. Groups also have higher cooperation expectations for the other group than individuals have for the other individual. We find no evidence of social pressure pushing groups to be nicer and more cooperative than individuals.