Figure 1.
Game Structures used in the Experiment.
(A). Between-Group Prisoner's Dilemma (BG-PD) with T(emptation)>R(eward)>P(unishment)>S(ucker); subscript IP (In-Group Player) are payoffs to the participant; subscript IG (In-Group) are payoffs to other in-group members; subscript OP (Out-group Player) are payoffs to the out-group protagonist; subscripts OG (Our-Group) are payoffs to other out-group members. Participant (row player) and Out-Group Player (column player) decide between coop (cooperation) and non-coop (non-cooperation); (B). Payoff Structure in the High Personal Vulnerability/High In-Group Vulnerability Condition; (C). Payoff Structure in the Low Personal Vulnerability/Low In-Group Vulnerability Condition; (D). Payoff Structure in the High Personal Vulnerability/Low In-Group Vulnerability Condition; (E). Payoff Structure in the Low Personal Vulnerability/Low In-Group Vulnerability Condition.
Figure 2.
Non-Cooperation, In-Group Defending, and Excessive Non-Cooperation.
(A). Under low personal vulnerability, oxytocin produces more non-cooperation than placebo when in-group vulnerability is high rather than low (range 0–5, displayed ±SE). (B). Compared to placebo, oxytocin enhances motivation to defend the in-group when in-group vulnerability is high rather than low (based on N = 72; range 1–7, displayed ±SE). (C). Excessive non-cooperation (i.e., own non-cooperation>expected out-group non-cooperation; range −5 to +5; displayed ±SE) emerges under high in-group vulnerability when individuals received oxytocin rather than placebo.