The importance of TIT FOR TAT in the evolution of co-operative behavior was discovered in a very unusual way, through a worldwide computer competition to find the winning strategy for the well known paradox ‘The Prisoner’s Dilemma’. In 1981 TIT FOR TAT won that competition, and ever since then it has grown in stature to where it now dominates our thinking about the evolution of co-operative behaviour in animal and human societies.
The prisoner's dilemma refers to an imaginary situation in which two individuals are imprisoned and are accused of having co-operated to perform some crime. The two prisoners are held separately, and attempts are made to induce each one to implicate the other. If neither one does, both are set free. This is the co-operative strategy available to both prisoners. In order to tempt one or both to defect, each is told that a confession implicating the other will lead to his or her release and, as an added incentive, to a small reward. If both confess, each one is imprisoned. But if one individual implicated the other and not vice versa, then the implicated partner receives a harsher sentence than if each had implicated the other.
From an analysis of the 3-million choices made in the second competition, four features of TIT FOR TAT emerged:
1. Never be the first to defect 2. Retaliate only after your partner has defected and then at 90%. 3. Be prepared to forgive after carrying out just one act of retaliation 4. Adopt this strategy only if the probability of meeting the same player again exceeds 2/3.
The studies of TIT FOR TAT answer these questions about initial viability, robustness and stability. Provided that the probability of future interaction between two individuals is sufficiently great, co-operation based on reciprocity can indeed get started in an asocial world, can flourish in a variegated environment and can defend itself once fully established.
According to Axelrod, TIT FOR TAT is a successful Evolutionary Stable Strategy because it is 'nice', 'provokable' and 'forgiving'. A nice strategy is one which is never first to defect. In a match between two nice strategies, both do well. A provokable strategy responds by defecting at once in response to defection. A forgiving strategy is one which readily returns to co-operation if its opponent does so; unforgiving strategies are likely to produce isolation and end co-operative encounters.
It is beginning to appear that the strategy of TIT FOR TAT is very bit as robust in real life as it is in computer competitions. Laboratory tests of TIT FOR TAT have become a growth industry as the theory gains in stature. We can expect new revelations about its worth as a theory to explain the evolution of co-operative behaviour. But whatever the outcome of this debate, one fact remains unchallenged. TIT FOR TAT is a major regulator of human behaviour. It may be a Culturally Stable Strategy (CSS) - one that humans just learned as a way of regulating our co-operative behaviour - or it may indeed be a very necessary, naturally selected co-operative Evolutionary Stable Strategy.