Previous research has revealed that trust plays an important role in promoting functioning, economic growth, and individual well-being in human society. Trust refers to individuals' willingness to take risks to hand over their resources to others in the context of social uncertainty. Based on rational signal theory, individuals tend to make social decisions (e.g. trust) according to perceived social information of others, such as social identity, gestures, language, and behaviors. Among these, trustees' social identity and their promises are important social information that convey whether an individual is trustworthy and reliable. Previous research has only examined the effect of promise levels on trust decisions, or the effect social distance on trust decisions. However, little is known about how promise levels and social distance interact to affect trust behaviors and its neural mechanisms. We adapted a Trust Game (TG) and the event-related potentials (ERPs) to examine the neurocognitive mechanisms of the effect of promise levels and social distance on trust behaviors. In particular, participates were asked to choose whether to trust their friends and strangers when they made high and low-level promises while electroencephalograms (EEGs) were recorded. Within each round, participants were informed of the promiser and their promise. We adopted two promise levels from previous studies: high-level promises would return 28 yuan (70%), and low- level 12 yuan (30%). The social distance includes both friend and stranger levels. The experiment consisted of 600 trials, including 150 trials for each condition. The behavioral results indicated that when the promiser was a friend, participants were more likely to invest and this was not affected by promise levels. However, when the promiser was a stranger, participants were more likely to invest when they were informed of low-level promises than high-level promises. The ERP results showed that the P2 was larger for the high-level promises than for the low- level promises. More importantly, when the promiser was a stranger, the N2 was more negative when they were informed of low-level promises than high-level promises. Conversely, the P3 was larger when they were informed of high-level promises than low-level promises. However, when the promiser was a friend, the N2 and P3 amplitudes had no difference. These results suggest that motivations of trusting behaviors toward friends and strangers are driven by different psychological mechanisms. Individuals trust friends more, and when the promiser is a friend, individuals can be motivated by the mutual interests of self and friends when making trust choices. However, when the promiser is a stranger, individuals are only driven by self-interest, and they do not care about strangers' benefits when making trust choices. The present study provides insight into how the brain processes the interplay of social distance and promise levels on trust decisions, which broadens the previous insight into understanding trust behaviors.
