LOWER WILLINGNESS TO TRUST AND STRONGER RELIANCE ON THE ANTERIOR CINGULATE CORTEX DURING TRUST FOR LONELIER INDIVIDUALS

Loneliness is associated with negative social expectations and difficulties forming supportive relationships, yet the behavioral and neural mechanisms underlying trust decisions in lonely individuals remain poorly understood. We investigated how loneliness affects trust dynamics during interactions with partners of varying trustworthiness using a modified investment game combined with computational modeling and functional magnetic resonance imaging (fMRI) in two samples of participants (total N=61). Participants played repeated trust games with three computer-simulated trustees whose behavior was generated by an interactive partially observable Markov decision process (IPOMDP) model, creating partners with low, medium, and high trustworthiness. Behavioral results revealed that lonelier individuals trusted trustworthy partners significantly less than less lonely individuals, particularly when they also reported lower general trust preferences. Computational modeling demonstrated that loneliness was associated with reduced willingness to trust—a parameter capturing reliance on beliefs about partner trustworthiness for trust decisions. Lonelier individuals exhibited a cognitive profile characterized by lower willingness to trust, shallower mentalizing depth, and shorter planning horizon. Neuroimaging analyses showed that the anterior cingulate cortex (ACC), a region associated with action uncertainty and conflict monitoring, was more strongly recruited by lonelier individuals during belief updating about partner trustworthiness. These findings suggest that loneliness fundamentally alters how individuals process positive social signals from trustworthy others, leading to misplaced distrust that may hinder the formation of meaningful relationships. Understanding these neurocomputational mechanisms provides insights for developing targeted interventions addressing social difficulties in lonely individuals.