Background <p>Borderline personality (BP) symptoms are characterized by emotional instability, impulsivity, and interpersonal difficulties. Deficits in mentalization (ability to attribute mental states to self and others) and affective empathy (congruent and incongruent emotional reactions), may contribute to these social impairments. This study investigated whether, in a community sample, mentalization and affective empathy mediate the relationship between BP symptom severity and altered social decision-making in the Ultimatum Game (UG), where participants were asked to accept/reject fair and unfair monetary offers from Proposers displaying various emotional facial expressions (happy, angry, neutral).</p> Methods <p>A community sample of 111 participants completed a measure of BP symptom severity, a dispositional mentalization/affective empathy questionnaire, an emotion recognition task to assess behavioral mentalization, and played the role of Responders in the UG. Relations between BP symptom severity and mentalization and affective empathy measures were tested using hierarchical regressions. Social decision-making was analyzed using multilevel logistic regression models examining the effects of fairness, emotion, and BP symptom severity on acceptance decisions. The potential mediating role of mentalization and affective empathy was evaluated using multilevel mediation analyses.</p> Results <p>Higher BP symptom severity was significantly associated with poorer dispositional affective empathy (on both resonance and dissonance scores). Higher BP symptom severity was also associated with lower dispositional mentalization and impaired behavioral mentalization, especially for the recognition of happiness and anger. In the UG, BP symptom severity interacted with fairness of the offer: individuals with higher BP symptom severity were less likely to accept fair offers, whereas acceptance of unfair offers did not differ as a function of symptom severity. BP symptom severity was not associated with differential sensitivity to Proposers’ emotional expressions. Preliminary analyses indicated that mentalization and affective empathy were not associated with behavioral responses in the UG; therefore, mediation analyses were not conducted.</p> Conclusions <p>Findings highlight mentalization and affective empathy impairments as important elements of BP symptomatology. However, altered social decision-making associated with BP symptoms appears to be specific to responses to fairness rather than to emotional expressions per se. Further research in clinical populations is needed to clarify the mechanisms underlying these interpersonal decision-making patterns.</p>