Borderline personality disorder (BPD) and bipolar disorder (BD) have overlapping clinical presentations and symptoms – sources of persistent clinical confusion. Game-theory can characterize how social function might be sub-optimal in the two disorders and move the field beyond the anecdotal description of clinical history. Here, we tested the hypothesis that BPD and BD can be distinguished on the basis of diminished reciprocal altruism in iterated Prisoner's Dilemma (PD) games.

Twenty females with BPD, 20 females with euthymic BD and 20 healthy (non-clinical) females, matched for age and cognitive ability, were assessed for Axis-I and personality disorders, and completed psychometric measures of state affect, impulsivity and hostility. Participants completed two iterated PD games and a test of gaze-cueing.

In the PD games, BPD participants failed to show statistically stable preferences to cooperate with social partners (playing tit-for-tat) and made significantly fewer cooperative responses compared to BD or controls (ANOVA main effect p = 0.03, post-hoc Tukey p < 0.05 for both comparisons). BPD participants were also less likely to sustain cooperation following experiences involving mutual cooperation than the other groups. Neither BPD nor BD participants demonstrated impairments in shifting visual attention on the basis of other peoples’ gaze.

These data indicate that BPD is (selectively) associated with difficulties in establishing, and then maintaining, reciprocal cooperation, involving altruism. These difficulties are not seen in euthymic BD. Our data support the differentiation of BPD from BD and offer fresh insights into the social difficulties experienced by individuals with diagnoses of BPD.