The Dose of the Threat Makes the Resistance for Cooperation
  We propose to reformulate the payoff matrix structure of Prisoner's Dilemma Game, by introducing threat and greed factors, and show their effect on the co-evolution of memory and cooperation. Our findings are as follows. (i) Memory protects cooperation. (ii) To our surprise, greater memory size is unfavorable to evolutionary success when there is no threat. In the absence of threat, subsequent generations lose their memory and are consequently invaded by defectors. (iii) In contrast, the presence of an appropriate level of threat triggers the emergence of a self-protection mechanism for cooperation, which manifests itself as an increase in memory size within subsequent generations. On the evolutionary level, memory size acts like an immune response of the generations against aggressive defection. (iv) Even more extreme threat results again in defection. Our findings boil down to the following: The dose of the threat makes the resistance for cooperation. 
Uzay Cetin and Haluk O. Bingol
physics.soc-ph cs.GT