Tit For Tat

Where it comes from

Submitted by Anatol Rapoport to Robert Axelrod's first computer tournament for the iterated Prisoner's Dilemma, and the winner of it. It was the shortest program entered.

How it plays

Tit For Tat is the most famous result in game theory that a non-specialist can hold in their head. It never defects first, it always answers a defection immediately, and it goes straight back to co-operating the moment the opponent does. Four lines of behaviour, and it beat every cleverer entry in Axelrod's tournament, including the ones written specifically to exploit nice strategies.

The reason it wins is worth being precise about, because it is often stated wrongly. Tit For Tat never beats its opponent in a single exchange. It cannot: it only ever matches. What it does is refuse to be exploited while still being available for the co-operative outcome, so it collects the high mutual pay-off against every other reciprocator and denies the free lunch to everyone else. Winning a tournament is about total pay-off, not about who came out ahead in each pairing.

In football terms this is a side that opens honestly, answers every dirty challenge inside the same move, and holds no grudge once the exchange is settled. Against another honest side you get an open, high-scoring game and both sides benefit. Against a side that presses relentlessly you get a fight, but a fight in which the ruthless side gets nothing free.

Its known weakness is escalation. Two strict copiers who fall into mutual retaliation have no mechanism to stop, because each is only ever answering the last thing that happened. A single accident can lock two well-behaved sides into an ugly ninety minutes, which is precisely the problem Generous Tit For Tat exists to solve.

In this league

Mirrowick Town of Mirrowick play Tit For Tat. That you never need to be the one who starts it in order to win. They have not played yet this season.

Watch: Tit For Tat explained