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.