Objective Risk in Trust Decisions Under Varying Social Distance: An Exploratory fMRI Study

Trust decisions involve both social evaluation and uncertainty processing, yet standard trust game paradigms do not fully dissociate trust-specific social computation from more general risk- and value-related processes. In this exploratory whole-brain fMRI study, we examined how objective risk and social distance were associated with trust decisions within a 2 × 2 trust game. Twenty-three adults completed the task, and 20 were included in the fMRI analyses after exclusion for excessive head motion. Behaviorally, trust rates were significantly lower under high than low objective risk, whereas neither the main effect of social distance nor the interaction between objective risk and social distance was significant. Relative to baseline, task performance engaged prefrontal and parietal regions. Compared with distrust decisions, trust decisions were associated with greater activation in prefrontal and visual regions, along with stronger negative activation in the insula. Objective risk was associated with differential activation in temporal, supramarginal, and precentral regions. Under the present manipulation, we did not observe significant neural modulations by social distance. These findings suggest that, in this low-context paradigm, objective risk was a more robust source of behavioral and neural variation than social distance. Given the exploratory design, modest sample size, and the task's limited ability to separate social trust from generic risk/value processing, the findings should be interpreted cautiously.