 Whereas the 1970s saw a broader realization of the advantages of improper models to yield results that were often good enough  (section 2.3),  the 1980s and 1990s witnessed a series of results involving improper models yielding results that were strictly better than what was prescribed by the corresponding proper model. In the early 1980s Robert Axelrod held a tournament to empirically test which among a collection of strategies for playing iterations of the prisoner’s dilemma performed best in a round-robin competition. The winner was a simple reciprocal altruism strategy called tit-for-tat (Rapoport &amp; Chammah 1965), which simply starts off each game cooperating then, on each successive round, copies the strategy the opposing player played in the previous round. So, if your opponent cooperated in this round, then you will cooperate on the next round; and if your opponent defected this round, then you will defect the next. Subsequent tournaments have shown that tit-for-tat is remarkably robust against much more sophisticated alternatives (Axelrod 1984). For example, even a rational utility maximizing player playing against an opponent who only plays tit-for-tat (i.e., will play tit-for-tat no matter whom he faces) must adapt and play tit-for-tat—or a strategy very close to it (Kreps, Milgrom, et al. 1982).