  We have studied the effect of memory on evolution of the prisoner's dilemma
game using square lattice networks. Based on extensive simulations, we found
that the density of cooperators was enhanced by an increasing memory effect for
most parameters. However, we also observed that the density of cooperators
decreased with an increased memory effect in the case of a large memory and
moderate temptation. It is interesting to note that memory makes cooperators
immune from temptation. The strength of protection reaches its maximal value
only for the moderate memory effect.
