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Abstract

Cooperative strategies are predicted for repeated social interactions. The recently described Zero Determinant (ZD) strategies enforce the partner’s cooperation because the ‘generous’ ZD players help their cooperative partners while ‘extortionate’ ZD players exploit their partners’ cooperation. Partners may accede to extortion because it pays them to do so, but the partner can sabotage his own and his extortioner’s score by defecting to discipline the extortioner. Thus, extortion is predicted to turn into generous and disappear. Here, we show with human volunteers that an additional monetary incentive (bonus) paid to the finally competitively superior player maintains extortion. Unexpectedly, extortioners refused to become disciplined, thus forcing partners to accede. Occasional opposition reduced the extortioners’ gain so that using extortion paid off only because of the bonus. With no bonus incentive, players used the generous ZD strategy. Our findings suggest that extortion strategies can prevail when higher competitiveness is rewarded with extra gain.