Amy M. Belfi, Timothy R. Koscik, Daniel Tranel

Reciprocal trust is a crucial component of cooperative, mutually beneficial social relationships. Previous research using tasks that require judging and developing interpersonal trust has suggested that the insula may be an important brain region underlying these processes (King-Casas et al., 2008). Here, using a neuropsychological approach, we investigated the role of the insula in reciprocal trust during the Trust Game (TG), an interpersonal economic exchange. Consistent with previous research, we found that neurologically normal adults reciprocate trust in kind, i.e., they increase trust in response to increases from their partners, and decrease trust in response to decreases. In contrast, individuals with damage to the insula displayed abnormal expressions of trust. Specifically, these individuals behaved benevolently (expressing misplaced trust) when playing the role of investor, and malevolently (violating their partner's trust) when playing the role of the trustee. Our findings lend further support to the idea that the insula is important for expressing normal interpersonal trust, perhaps because the insula helps to recognize risk during decision-making and to identify social norm violations.