Us against them: oxytocin response to competition in a small-scale human society

Cooperation can be an effective way to compete. The neurohormone oxytocin (OT) may modulate this by enhancing both cooperation and competition. However, studies of endogenous OT reactivity during real-world competition are scarce. Here, we tested the hypothesis that OT modulates inter-group competition in a human subsistence society, the Tsimane' of Bolivia, by measuring urinary OT in football players before and after matches. Matches varied in group salience, involving intra-community, inter-community and interethnic opponents. Urinary OT was quantified using radioimmunoassays. We observed an increase in urinary OT levels following competition among men, but not among women. OT responses were strongest during both intra-community and interethnic matches. This pattern suggests sensitivity to familiar rivalries, consistent with the 'nasty neighbour effect', as well as heightened responses to out-groups. Our results support the hypothesis that OT mediates group-level competition in humans and reveal that OT reactivity varies by group salience. Moreover, our results showed a sex-specific response, with OT increases in men but not in women.