LAUSR

Abstract Despite extensive research on the effects of stress on the brain and behaviors, there is a debate whether stress promotes prosocial behaviors, especially acute stress due to intricate costly punishment in the ultimatum game. Therefore, the present study introduced an irrelevant third party to examine how acute stress and the triggered cortisol influence third parties’ punishing and helping behaviors as more convincing altruistic behaviors. The 65 participants were exposed to a psychosocial stressor (n = 33) or a control condition (n = 32). Afterwards, two third-party intervention tasks (a token allocation task and criminal scenario judgment task) were completed, during which the participants, as an “irrelevant” third party, could choose whether to sacrifice their own interests to help the victim or punish the transgressor. Participants’ affective states, heart rate, and salivary cortisol were repeatedly measured throughout the experiment. Results showed that acute stress can lead to more third-party helping behaviors but not more punishing behaviors. Specifically, participants under stress tended to transfer more monetary units to the victim in the token allocation task than the control-group participants, and they tended to help the victim in the scenario task. In contrast, there was no significant difference in punishing behavior between the stressed and control participants. These findings reveal that acute psychosocial stress triggers the “tend and befriend” response, which might reflect the prosocial intuition under acute stress. Highlights Acute psychosocial stress promotes altruistic behaviors toward an irrelevant third party. The raised prosocial tendencies are specific to third-party helping but not to punishing behaviors in both real and fictitious scenario tasks.