The Selfish Gene

by Richard Dawkins

Now suppose Tit for Tat plays against a strategy called Naive Prober. Naive Prober wasn’t actually entered in Axelrod’s competition, but it is instructive nevertheless. It is basically identical to Tit for Tat except that, once in a while, say on a random one in ten moves, it throws in a gratuitous defection and claims the high Temptation score. Until Naive Prober tries one of its probing defections the players might as well be two Tit for Tats. A long and mutually profitable sequence of cooperation seems set to run its course, with a comfortable 100 per cent benchmark score for both players. But suddenly, without warning, say on the eighth move, Naive Prober defects. Tit for Tat, of course, has played COOPERATE on this move, and so is landed with the Sucker’s payoff of o points. Naive Prober appears to have done well, since it has obtained 5 points from that move. But in the next move Tit for Tat ‘retaliates’. It plays DEFECT, simply following its rule of imitating the opponent’s previous move. Naive Prober meanwhile, blindly following its own built-in copying rule, has copied its opponent’s COOPERATE move. So it now collects the Sucker’s payoff of o points, while Tit for Tat gets the high score of 5. In the next move, Naive Prober—rather unjustly one might think—‘retaliates’ against Tit for Tat’s defection. And so the alternation continues. During these alternating runs both players receive on average 2.5 points per move (the average of 5 and o). This is lower than the s...