Abstract
While economic standard theory explains cooperation in terms of rational decision-making, empirical studies suggest that humans have social preferences for cooperating with others. We investigated the specificity of these social preferences for interactions with human, relative to non-human, agents in a prisoner’s dilemma game. To obtain insights into emotional processes during cooperation, we measured activity of the corrugator supercilii muscle as indicator of spontaneous emotional responding during cooperation. After unreciprocated defection (free-riding), participants switched more often to a cooperative strategy and showed increased corrugator activity (suggesting more negative emotional responses) when playing with a human relative to a computer. This suggests that humans have a specific preference for cooperating with other humans and that cooperation may be promoted by unpleasant affect in response to the outcome of one’s own “free-riding”.