HUMAN COOPERATION IN THE SIMULTANEOUS AND THE ALTERNATING PRISONERS-DILEMMA - PAVLOV VERSUS GENEROUS TIT-FOR-TAT

The iterated Prisoner's Dilemma has become the paradigm for the evolut ion of cooperation among egoists. Since Axelrod's classic computer tou rnaments and Nowak and Sigmund's extensive simulations of evolution, w e know that natural selection can favor cooperative strategies in the Prisoner's Dilemma. According to recent developments of theory the las t champion strategy of ''win-stay, lose-shift'' (''Pavlov'') is the wi nner only if the players act simultaneously, In the more natural situa tion of players alternating the roles of donor and recipient a strateg y of ''Generous Tit-for-Tat'' wins computer simulations of short-term memory strategies, We show here by experiments with humans that cooper ation dominated in both the simultaneous and the alternating Prisoner' s Dilemma. Subjects were consistent in their strategies: 30% adopted a Generous Tit-for-Tat-like strategy, whereas 70% used a Pavlovian stra tegy in both the alternating and the simultaneous game. As predicted f or unconditional strategies, Pavlovian players appeared to be more suc cessful in the simultaneous game whereas Generous Tit-for-Tat-like pla yers achieved higher payoffs in the alternating game. However, the Pav lovian players were smarter than predicted: they suffered less from de fectors and exploited cooperators more readily, Humans appear to coope rate either with a Generous Tit-for-Tat-like strategy or with a strate gy that appreciates Pavlov's advantages but minimizes its handicaps.