# The Iterated Prisoner’s Dilemma: Press-Dyson Interactive

## The Game

In each round, each of us can choose to cooperate or to defect. We play repeated rounds. In each round:

- If we both cooperate, we both get
**3 points**. - If you cooperate and I defect, I get
**5 points**and you get**none**. - If you defect and I cooperate, you get
**5 points**and I get**none**. - If we both defect, we both get
**1 point**.

- Fixed score
- Extortion
- Generous

In this game I will extort you. Your best strategy, if you want to maximise your own score, is to cooperate all the time – but then I will occasionally defect and so always do better than you.

The only way you can avoid being taken advantage of is to resign yourself to the meagre rewards of mutual defection, and to defect on every turn. If you do anything else then I will take advantage of your cooperation and I will do three times better than you, in the sense that – on average over the long run – (my score minus 1) will be thrice (your score minus 1).

## How does it work?

My play is based purely on how both of us played in the previous move:

- If we both cooperated last time, then I cooperate with probability 11/13.
- If I cheated you last time (you cooperated and I defected), then I cooperate with probability 7/26.
- If you cheated me last time (I cooperated and you defected), then I cooperate with probability 1/2.
- If we both defected last time, I defect.

These probabilities are taken from Press and Dyson’s paper, where this strategy appears as an example.