Abstract
Background. Interpersonal difficulties are a core feature of borderline personality disorder (BPD). Saunders et al. (2015) reported that, in an iterated Prisoner’s Dilemma against a Tit-for-Tat partner, women with BPD cooperated less than both women with euthymic bipolar disorder and healthy controls, particularly after rounds with mutual cooperation. However, a Tit-for-Tat partner simply copies the participant's previous choice, so the game states a participant encounters are determined by her own behaviour: someone who always cooperates experiences only mutual cooperation, whereas someone who frequently defects rarely does. Cooperation rates conditional on the previous outcome are therefore potentially confounded with overall cooperativeness, and a group difference after mutual cooperation need not reflect a different response to that outcome.Methods. To test this possibility, we reanalysed the original data (n = 58; 19 with BPD, 39 controls [19 bipolar, 20 healthy]; all female). For each participant, we simulated the conditional cooperation rates expected if she cooperated with a fixed probability equal to her overall cooperation rate, irrespective of the previous outcome, given the transition structure imposed by Tit-for-Tat. We then compared observed with expected conditional rates in each game state.Results. Once overall cooperation rates were accounted for, conditional cooperation did not differ between groups: rather, observed rates matched expected rates in every state except after the exploitation of a cooperating partner, where all groups defected more than expected (in BPD only nominally significant, and not after false discovery rate correction). Further, the observed lower overall cooperation in BPD compared to both control groups was driven by consistent cooperators: a third of controls (13 of 39) but no participant with BPD (0 of 19) cooperated on at least 90% of rounds. Excluding these consistent cooperators reduced the group difference by about three-quarters.Conclusions. Women with BPD cooperated less overall, but this reflected an absence of consistently cooperating individuals rather than a specific failure to sustain mutual cooperation. More generally, analyses of iterated games against deterministic strategies must account for the state transitions induced by participants' own choices; otherwise, differences in overall cooperation can masquerade as differences in strategy.