Conformity and reciprocity in public good provision

People contribute more to experimental public goods the more others contribute, a tendency called “crowding-in.” We propose a novel experimental design to distinguish two possible causes of crowding-in: reciprocity, the usual explanation, and conformity, a neglected alternative. Subjects are given the opportunity to react to contributions of a payoff-irrelevant group, in addition to their own group. We find evidence of conformity, accounting for roughly 1/3 of crowding-in.