Tit For Tat - Overview

Overview

This strategy is determined by four conditions:

1. The agent will always cooperate, until provoked
2. If provoked, the agent will always retaliate
3. The agent is quick to forgive
4. The agent must have a good chance of competing against the opponent more than once.

In the last condition, the definition of "good chance" depends on the payoff matrix of the prisoner's dilemma. The important thing is that the competition continues long enough for repeated punishment and forgiveness to generate a long-term payoff higher than the possible loss from cooperating initially.

A fifth condition applies to make the competition meaningful: if an agent knows that the next play will be the last, it should naturally defect for a higher score. Similarly if it knows that the next two plays will be the last, it should defect twice, and so on. Therefore the number of competitions must not be known in advance to the agents.

Tit for tat was superior to a variety of alternative strategies, winning (getting a higher average payoff) in several annual iterated Prisoner's Dilemma tournaments for autonomous programs against (generally far more complex) strategies created by teams of computer scientists, economists, and psychologists. Some game theorists informally believe the strategy to be optimal, although no proof is presented.

In some competitions tit for tat was not the highest-scoring strategy. However, tit for tat would have been the most effective strategy if the average performance of each competing team were compared. The team which recently won over a pure tit for tat team outperformed it with some of their algorithms because they submitted multiple algorithms which would recognize each other and assume a master and slave relationship (one algorithm would "sacrifice" itself and obtain a very poor result for the other algorithm to be able to outperform tit for tat on an individual basis, but not as a pair or group). But this violates the basic assumptions of a Prisoner's Dilemma model, which requires that every agent have a rational preference for the outcome in which he defects and the opponent cooperates, over the outcome in which he cooperates and the opponent defects. However, that this winning solution does not work effectively against groups of agents running tit for tat illustrates the strengths of tit for tat when employed in a team (that the team does better overall, and all the agents on the team do well individually, when every agent cooperates).