Lessons from the Prisoner's Dilemma

7: Post-match analysis (3)

Tit for Tat co-operates on its first move and from then on copies the other player's previous move. Once you discovered that, you probably realised that you gain nothing by defecting (until the last move). In this way, Tit for Tat trained you to co-operate for mutual benefit, unlike Massive Retaliatory Strike.

Once again, the highest-scoring strategy is to co-operate until the final move and then defect. If the game were going on indefinitely, then it would be in your interest to always co-operate.

We could go on considering improved strategies for playing Iterated Prisoner's Dilemma, but I want to stop now and draw a few conclusions.

• At the start, it seemed obvious that the best way to play Prisoner's Dilemma is to always defect.
• Then we saw that the highest-scoring strategy depends on who you are playing against. If the other player rewards your actions, then the best strategy might be to always co-operate.
• We assumed all along that the players were pure egotists, yet we have seen that under some circumstances co-operation is desirable and, in addition, forgiveness is desirable for such players.
• Tit for Tat is a good strategy in lots of situations, yet it never wins: it will always either draw or lose by 5 points.

Questions to think about

• How would Tit for Tat cope with imperfect communication, where co-operation might be mistaken for defection (Imagine two players using the TFT strategy on each other with occasional "accidental" defections)?
• Just as TFT never wins, there are strategies that never lose. For example, the strategy that defects on its first turn and thereafter copies the other player's previous move. What is the disadvantage of playing this strategy?
• How does the nature of the game change if your goal is to
  1. beat your opponent by the largest margin?
  2. maximise the total gain of you and the other player?
  What are the best strategies in each of these games?
• Are there real-life examples of Tit for Tat strategies? Of people co-operating out of self-interest?
• On the last move, you may as well defect: this applies in real life as well. If you rent a flat for a while, you and your landlord/landlady may as well be nice to each other, but on the last month, it is in your interest to stiff them for the last rent payment, and their interest to withhold your deposit. Can you think of other examples where behaviour changes on the "final turn"?

More depth on the Prisoner's Dilemma is available by consulting the recommended reading.

INDEX | Further Reading

This tutorial is licensed under a Creative Commons Attribution License.