This study aimed to explore the influence of interpersonal distance and social observation on prosocial behaviour. Participants were instructed to make costly prosocial decisions towards different interpersonal distance targets (friends, acquaintances or strangers) under the (non) observable condition, with simultaneous electroencephalogram recording. The behavioural results demonstrated that participants made more prosocial choices-unaffected by social observation-towards friends than towards acquaintances and strangers; nonetheless, participants made more prosocial choices towards acquaintances and strangers under the observable than under the non-observable condition. Event-related potential results showed that when participants made prosocial decisions towards friends, the P3 and N2 amplitudes remained unchanged between the observable and non-observable conditions; however, when participants made prosocial decisions towards acquaintances and strangers, a smaller N2 and a larger P3 were observed under the observable than those under the non-observable condition. These findings suggest that prosocial behaviour towards friends is driven by social preferences regarding the welfare of others, regardless of the possibility of reputation management. However, prosocial behaviour towards acquaintances and strangers might be motivated by a positive reputation, because individuals may bear in mind the potential future benefits.
