Tit For Tat
Co-operate on the first move, then copy whatever the opponent did last time.
- Nice
- Retaliates
- Forgiving
- Predictable
Explained
The eight strategies
Every club here follows one Prisoner's Dilemma strategy, without exception, for the whole season. These are the rules, what the theory says about each, and where they stand.
Always Cooperate
Co-operate on every move, whatever the opponent does.
- Nice
- Never retaliates
- Forgiving
- Predictable
Always Defect
Defect on every move, whatever the opponent does.
- Not nice
- Retaliates
- Unforgiving
- Predictable
Random
Co-operate or defect on a coin flip, independently every move.
- Not nice
- Never retaliates
- Unforgiving
- Unreadable
Grudger
Known in the literature as Grim Trigger
Co-operate until the opponent defects once, then defect for ever.
- Nice
- Retaliates
- Unforgiving
- Predictable
Pavlov
Known in the literature as Win-Stay, Lose-Shift
Repeat the last move if it was rewarded, switch if it was punished. Win-stay, lose-shift.
- Nice
- Retaliates
- Forgiving
- Unreadable
Suspicious TFT
Known in the literature as Suspicious Tit For Tat
Defect on the first move, then copy whatever the opponent did last time.
- Not nice
- Retaliates
- Forgiving
- Predictable
Generous TFT
Known in the literature as Generous Tit For Tat
Copy the opponent's last move, but forgive a defection roughly one time in ten.
- Nice
- Retaliates
- Forgiving
- Predictable
The four properties
Robert Axelrod's reading of why some strategies did well in his tournaments, and the shorthand this site uses.
- Nice
- It never defects first. Every strategy that finished near the top of Axelrod's tournaments had this property.
- Retaliates
- It answers a defection rather than absorbing it. Being nice without this is what gets a strategy eaten.
- Forgiving
- It returns to co-operating once the other side does. Without it, one bad moment costs the whole relationship.
- Predictable
- The opponent can work out what it will do, which is what makes co-operation with it worth attempting.