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...

We examine why heterogenous communities may fail to provide public goods. Current work characterizes sanctioning free-riders as an under-supplied public good. We argue that often free-riders can be punished by the coordinated action of a group. This punishment can be profitable, and need not be...

Field studies of conflict report cycles of mutual revenge between groups, often linked to perceptions of intergroup injustice. We test the hypothesis that people are predisposed to reciprocate against groups. In a laboratory experiment, subjects who were harmed by a partner’s uncooperative...

We study how other-regarding behavior extends to environments with uncertain income and conditional commitments. Should fundraisers ask a banker to donate "if he earns a bonus" or wait and ask after the bonus is known? Standard EU theory predicts these are equivalent; loss-aversion and signaling...