  For the iterated Prisoner's Dilemma, there exist Markov strategies which
solve the problem when we restrict attention to the long term average payoff.
When used by both players these assure the cooperative payoff for each of them.
Neither player can benefit by moving unilaterally any other strategy, i.e.
these are Nash equilibria. In addition, if a player uses instead an alternative
which decreases the opponent's payoff below the cooperative level, then his own
payoff is decreased as well. Thus, if we limit attention to the long term
payoff, these \emph{good strategies} effectively stabilize cooperative
behavior. We characterize these good strategies and analyze their role in
evolutionary dynamics.
