We study the implications of participation in political collective action on identity and on interpersonal interactions using a laboratory experiment. We offer subjects the possibility to sign an online petition, which was either related to animal rights or the right to bear firearms. Before and after the petition, we measure subjects' altruism and willingness to trust by asking them to play a dictator game and a trust game in pairs. The results show that there is considerably more altruism and more trust when both subjects had signed the petition than when one or both had not signed. The same behaviour is observed when we analyse high-cost political participation, namely, joining a street protest. This suggests that the experience of common participation in political collective action creates an identity that produces in-group favouritism. These results also suggest a reason why individuals choose to participate in political action despite private costs and a low probability of affecting the outcome: participation creates private benefits in subsequent interactions with fellow participants
