Extortion - A voracious prosocial strategy

Recently Press and Dyson have dramatically changed our view on the Prisoner's Dilemma by proposing a new class of strategies that enforce a linear relationship between the two players' scores. Players adopting 'extortion' respond with cooperation to cooperation in most cases, defect in other rounds, but respond to defection with defection. In this way, extortion enforces full cooperation of the partner who accedes to it because he profits from doing so. This unbeatable strategy is nevertheless prosocial because it is mostly cooperative and induces cooperation even though it gains most itself. Experiments show that about 40% of humans choose to use extortion in competitive situations or when they have the power to exchange coplayers. On being punished in egalitarian situations, they use a generous strategy.