Costly signaling of commitment to a group has been proposed as an explanation for participation in religion and ritual. But if the signal’s cost is too small, freeriders will send the signal and behave selfishly later. Effective signaling may then be prohibitively costly. If the average level...

In experiments, people behave more cooperatively when they are aware of an external threat, while in the field, we observe surprisingly high levels of cooperation and altruism within groups in conflict situations such as civil wars. We provide an explanation for these phenomena. We introduce a...

Costly signaling of commitment to a group has been proposed as an explanation for participation in religion and ritual. But if the signal's cost is too small, freeriders will send the signal and behave selflshly later. Effective signaling may then be prohibitively costly. If the average level of...

Surprisingly high levels of within-group cooperation are observed in conflict situations. Experiments confirm that external threats lead to higher cooperation. The psychological literature suggests proximate explanations in the form of group processes, but does not explain how these processes...

In experiments, people behave more cooperatively when they are aware of an external threat, while in the field, we observe surprisingly high levels of cooperation and altruism within groups in conflict situations such as civil wars. We provide an explanation for these phenomena. We introduce a...