Neural and behavioral dissociations of self-focused and other-focused incentives in trust

Trust requires balancing potential mutual gain against possible betrayal. This decision depends not only on personal vulnerability to loss (risk) but also on the partner's incentive to betray (temptation) and whether the betrayal is committed intentionally (partner's agency). Previous research-limited by categorical behavioral designs and a neuroimaging focus on pathological mistrust-leaves it unclear how the brain systematically represents personal risk, partner's temptation, and partner's agency during typical trust decisions. We used functional magnetic resonance imaging with a parametric binary trust game to disentangle the effects of these factors. Thirty-one participants made binary trust decisions while these factors were systematically varied. Behaviorally, trust declined with increasing risk and with the risk × temptation interaction, owing to self-centered heuristics, while agency had no reliable effect. Neurally, risk was encoded in a decision-dependent manner: increasing risk in trust engaged the salience network (anterior mid-cingulate cortex), whereas in non-trust it recruited striato-motor circuits (putamen, motor cortex). Temptation, in interaction with risk, was represented independently of choice, expressed as increased activity in Calcarine but reduced activity in the medial prefrontal hub of the default mode network. Agency was distinguished neurally in the dorsomedial prefrontal cortex despite absent behavioral effects. Exploratory analyses showed that trust versus non-trust engaged distinct large-scale networks, with trust recruiting the central-executive and salience networks and non-trust the posterior default-mode network. These findings reveal distinct neural signatures for vulnerability, partner incentives, and partner agency in trust decisions, highlighting how self- and other-focused factors are encoded differently across large-scale brain networks.