prisoner dilemma is a caricature of a real situation in which selfish and altruist tendencies compete. It has been the subject of study of game theory for the last 60 years [@rapo; @axel; @now1]. Its name and formal elaboration is attributed to A. Tucker, who mentioned it in a classroom in 1950, but it was not until 1952 that the first results about it were published [@floo].

The formulation of the prisoner dilemma as a game is rather simple. It is played by two players who must choose their moves between two strategies: to cooperate (C) or to defect (D). The reward, or [*payoff*]{}, obtained by each player after one round of the game is given by Table \[tabla1\]:

               [**C**]{}   [**D**]{}
  ----------- ----------- -----------
   [**C**]{}      $r$         $s$
   [**D**]{}      $t$         $p$

  : Payoff table for the prisoner’s dilemma: the strategy in each row gets the payoff given by the table when playing again the strategies in the columns.[]{data-label="tabla1"}

Each element in the payoff matrix represents the payoff of a player using the strategies in the rows, when confronting a player choosing the strategies in the columns. A defector ${\bf D}$ receives $t$, the temptation to defect, when its opponent is a 