Prisoner’s dilemma tournament
Multiple strategies compete in an iterated Prisoner’s Dilemma. Run round-robin tournaments, watch population dynamics shift over generations through evolutionary selection, and discover why nice strategies tend to win — eventually.
The prisoner's dilemma
Two players simultaneously choose to cooperate or defect. Mutual cooperation pays well (R=3 each). Mutual defection pays poorly (P=1 each). But if one defects while the other cooperates, the defector gets the highest payoff (T=5) and the cooperator gets the lowest (S=0). Rational self-interest says defect — but mutual defection leaves both worse off.
Iterated play and strategies
When the game repeats, strategies can condition on history. Tit-for-Tat cooperates first, then copies the opponent’s last move. It won Robert Axelrod’s famous 1980 tournament despite never “beating” any opponent. It is nice (never defects first), retaliatory (punishes defection), forgiving (returns to cooperation), and clear (easy to understand).
Evolutionary dynamics
In the evolutionary version, strategies that score well reproduce, and poor performers die out. Over generations, the population composition shifts. Defectors may dominate initially, but cooperative strategies often recover through mutual aid — the evolutionary basis for the emergence of cooperation.
Noise
With noise, players occasionally make errors — cooperating when they meant to defect, or vice versa. This small change has profound effects: Tit-for-Tat can get locked in mutual retaliation after a single error. Forgiving strategies like Generous Tit-for-Tat and Pavlov handle noise better.