Beyond Average Behaviour: Trust and Reciprocity Dynamics Reveal Distinct Behavioural Phenotypes

Cooperation depends not only on how much people trust and reciprocate, but on how it is maintained and repaired after rupture. We analysed a 10-round Trust Game in participants with borderline personality disorder (BPD), an affective-symptom sample (AD), and community controls (N = 1,300 participants contributing 1,310 games; BPD n = 468, AD n = 556, controls n = 286 games), using mixed-effects and hidden Markov models. Average behaviour was broadly similar across groups, although AD trustees returned more than controls. As investors, BPD participants adjusted less to feedback than controls. Among trustees, AD differed most clearly from BPD: AD trustees followed cooperative response patterns more often, resumed cooperation more readily after exploitative returns, and had higher model-implied long-run cooperation at low and moderate partner investments. The occupancy and repair differences persisted when hidden states were estimated without diagnostic information, but not when BPD was restricted to interview-confirmed cases. AD–control differences in cooperative patterns did not persist among participants tested online. BPD–control differences in maintaining and restoring cooperation were smaller and depended on investment and hidden-state specification. Clinical differences were more evident in interaction dynamics than in average behaviour.