  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.
