  Cooperation and competition between human players in repeated microeconomic
games offer a powerful window onto social phenomena such as the establishment,
breakdown and repair of trust. This offers the prospect of particular insight
into populations of subjects suffering from socially-debilitating conditions
such as borderline personality disorder. However, although a suitable
foundation for the quantitative analysis of such games exists, namely the
Interactive Partially Observable Markov Decision Process (I-POMDP),
computational considerations have hitherto limited its application. Here, we
improve inference in I-POMDPs in a canonical trust game, and thereby highlight
and address two previously unmodelled phenomena: a form of social risk-aversion
exhibited by the player who is in control of the interaction in the game, and
irritation or anger exhibited by both players. Irritation arises when partners
apparently defect, and it causes a precipitate breakdown in cooperation.
Failing to model one's partner's propensity for it leads to substantial
economic inefficiency. We illustrate these behaviours using evidence drawn from
the play of large cohorts of healthy volunteers and patients.
