Social image concerns promote cooperation more than altruistic punishment

Human cooperation is enigmatic, as organisms are expected, by evolutionary and economic theory, to act principally in their own interests. However, cooperation requires individuals to sacrifice resources for each other’s benefit. We conducted a series of novel experiments in a foraging society where social institutions make the study of social image and punishment particularly salient. Participants played simple cooperation games where they could punish non-cooperators, promote a positive social image or do so in combination with one another. We show that although all these mechanisms raise cooperation above baseline levels, only when social image alone is at stake do average economic gains rise significantly above baseline. Punishment, either alone or combined with social image building, yields lower gains. Individuals’ desire to establish a positive social image thus emerges as a more decisive factor than punishment in promoting human cooperation.We acknowledge financial support from the German Federal Ministry of Education and Research through the project ‘BIOACID (03F0655H)’, the Spanish Ministerio de Ciencia e Innovación (grant ECO 2011-23634), the Spanish Ministerio de Economía y Competitividad (project ECO 2015-68469-R), the Universidad Jaume I (P1.1B2015-48) and the Kiel Institute for the World Economy. We especially thank our local assistants Eliuda Maravut, Horai Magum, Philippe Hus, Nigel Henry, Saeleah Gordon and Siko Gordon. We thank Vincent Richrath and Irene Jimenez Arribas for research assistance, and Heike Hennig-Schmidt for discussion

Reply to van Hoorn: Converging lines of evidence

We agree with the comments by van Hoorn (1) on our critique (2): testing causal hypotheses about human behavior is a challenge (1, 3). Making progress requires specifying alternative hypotheses and then testing these hypotheses using diverse and converging lines of evidence. We have defended the hypothesis that social norms, which culturally coevolved with the institutions of large-scale societies including markets, influence economic decision-making. This hypothesis emerged from a larger set that we developed both at the outset of our project and as we went along. Our interdisciplinary team’s initial list of hypotheses included the idea that experimental games might spark an innate reciprocity module that would yield little variation across populations