Background <p>Disruptions in epistemic trust have been recognised as key sequelae of trauma and as markers of vulnerability to borderline personality pathology. However, prior research has relied primarily on self-reports and lacks behavioural measures of epistemic stance. The present pre-registered studies introduce a novel behavioural task—the Balloon Analogue Risk Task for Epistemic Trust (BART-ET)—and examine its associations with borderline personality features, trauma history, and psychological distress.</p> Methods <p>Two cross-sectional studies were conducted with a combined sample of 273 young adults aged 18–25 (Study 1: <i>N</i> = 120; Study 2: <i>N</i> = 153). Participants completed self-report measures of borderline personality features (PAI-BOR) and epistemic trust, mistrust, and credulity (ETMCQ). Study 2 additionally included the Childhood Trauma Questionnaire (CTQ) and the Brief Symptom Inventory (BSI-GSI). All participants completed the BART-ET in a laboratory setting, which operationalised epistemic mistrust as the degree of deviation from a confederate experimenter’s advice during a risk-taking task. Analyses involved correlational tests and structural equation modelling (SEM) to evaluate hypothesised associations and mediation pathways.</p> Results <p>As expected, across both studies, higher levels of borderline personality traits were associated with greater epistemic mistrust—both behaviourally (on the BART-ET) and via self-report (ETMCQ)—and with greater epistemic credulity, but not with epistemic trust, as measured with the ETMCQ. Behavioural and self-report measures of mistrust were significantly correlated, suggesting convergent validity of the BART-ET as an index of epistemic mistrust. In Study 2, childhood trauma exposure was associated with borderline features and with epistemic mistrust assessed behaviourally and via self-report. Preregistered mediation models controlling for general distress (BSI-GSI) suggested that the association between childhood trauma and epistemic mistrust was not unique to BPD features.</p> Conclusions <p>These findings suggest that epistemic mistrust—rather than a simple absence of trust—is a social-cognitive correlate of borderline personality vulnerability and trauma exposure in young adults. The results also indicate that the BART-ET may be a useful behavioural tool for studying epistemic mistrust in clinical contexts, though further validation is needed.</p>