<P> Theory and simulations confirm that beyond a critical population size, ZD extortion loses out in evolutionary competition against more cooperative strategies, and as a result, the average payoff in the population increases when the population is bigger . In addition, there are some cases in which extortioners may even catalyze cooperation by helping to break out of a face - off between uniform defectors and win--stay, lose--switch agents . </P> <P> While extortionary ZD strategies are not stable in large populations, another ZD class called "generous" strategies is both stable and robust . In fact, when the population is not too small, these strategies can supplant any other ZD strategy and even perform well against a broad array of generic strategies for iterated prisoner's dilemma, including win--stay, lose--switch . This was proven specifically for the donation game by Alexander Stewart and Joshua Plotkin in 2013 . Generous strategies will cooperate with other cooperative players, and in the face of defection, the generous player loses more utility than its rival . Generous strategies are the intersection of ZD strategies and so - called "good" strategies, which were defined by Akin (2013) to be those for which the player responds to past mutual cooperation with future cooperation and splits expected payoffs equally if he receives at least the cooperative expected payoff . Among good strategies, the generous (ZD) subset performs well when the population is not too small . If the population is very small, defection strategies tend to dominate . </P> <P> Most work on the iterated prisoner's dilemma has focused on the discrete case, in which players either cooperate or defect, because this model is relatively simple to analyze . However, some researchers have looked at models of the continuous iterated prisoner's dilemma, in which players are able to make a variable contribution to the other player . Le and Boyd found that in such situations, cooperation is much harder to evolve than in the discrete iterated prisoner's dilemma . The basic intuition for this result is straightforward: in a continuous prisoner's dilemma, if a population starts off in a non-cooperative equilibrium, players who are only marginally more cooperative than non-cooperators get little benefit from assorting with one another . By contrast, in a discrete prisoner's dilemma, tit for tat cooperators get a big payoff boost from assorting with one another in a non-cooperative equilibrium, relative to non-cooperators . Since nature arguably offers more opportunities for variable cooperation rather than a strict dichotomy of cooperation or defection, the continuous prisoner's dilemma may help explain why real - life examples of tit for tat - like cooperation are extremely rare in nature (ex . Hammerstein) even though tit for tat seems robust in theoretical models . </P> <P> Players cannot seem to coordinate mutual cooperation, thus often get locked into the inferior yet stable strategy of defection . In this way, iterated rounds facilitate the evolution of stable strategies . Iterated rounds often produce novel strategies, which have implications to complex social interaction . One such strategy is win - stay lose - shift . This strategy outperforms a simple Tit - For - Tat strategy--that is, if you can get away with cheating, repeat that behavior, however if you get caught, switch . </P>

The classic prisoner's dilemma is a conflict example from which theoretical perspective