‘Reciprocity’ examines the effects of reciprocal behaviour. Reciprocity should only be possible in repeated games, and if this series of games has a finite end backwards induction should state that cooperation is not the optimal course of action. However, real world relationships are usually open ended, so cooperation becomes feasible. Folk theorem states that we do not need an external enforcement agency, but self-policing by players only works if transgressions are visible. Tit-for-tat is another popular strategy, but this only works in pairwise interactions — in larger groups others can inflict punishment. Game theory explains notions like authority and duty as emergent phenomena, and emotions as hardwired, quick fire responses.