Rudimentary empathy in macaques’ social decision-making

Significance We investigated nonhuman primate prosociality within a decision-making behavioral framework. Our results suggest that macaques have a concept of their peers’ well being. The strength and originality of our experimental design is in challenging monkeys with several decisions involving both pleasant and unpleasant outcomes to self and others, thus allowing us to evaluate social motivation in different contexts. Behavioral measures, such as empathic eye blinking and mutual looking, show that the degrees of empathy and of willingness to interact with peers differ among individuals. These differences were mainly consistent with the preexisting social bonds displayed within our group-housed monkeys. Our results thus provide evidence of partner-dependent behavioral mechanisms shaping primates’ social decisions. Primates live in highly social environments, where prosocial behaviors promote social bonds and cohesion and contribute to group members’ fitness. Despite a growing interest in the biological basis of nonhuman primates’ social interactions, their underlying motivations remain a matter of debate. We report that macaque monkeys take into account the welfare of their peers when making behavioral choices bringing about pleasant or unpleasant outcomes to a monkey partner. Two macaques took turns in making decisions that could impact their own welfare or their partner’s. Most monkeys were inclined to refrain from delivering a mildly aversive airpuff and to grant juice rewards to their partner. Choice consistency between these two types of outcome suggests that monkeys display coherent motivations in different social interactions. Furthermore, spontaneous affilitative group interactions in the home environment were mostly consistent with the measured social decisions, thus emphasizing the impact of preexisting social bonds on decision-making. Interestingly, unique behavioral markers predicted these decisions: benevolence was associated with enhanced mutual gaze and empathic eye blinking, whereas indifference or malevolence was associated with lower or suppressed such responses. Together our results suggest that prosocial decision-making is sustained by an intrinsic motivation for social affiliation and controlled through positive and negative vicarious reinforcements.