The Prisoner's Dilemma
You and an opponent each choose: Cooperate or Defect. You don't know what they'll pick. Payoffs:
| They Cooperate | They Defect | |
|---|---|---|
| You Cooperate | Both get 3 | You get 0, they get 5 |
| You Defect | You get 5, they get 0 | Both get 1 |
Played across 10 rounds. Your score is the sum of your payoffs. After the match, you can play again — but with a different opponent.
Why is this interesting?
Mutual cooperation gives the best group outcome (3+3=6), but defecting when your opponent cooperates is even better for you personally (5 vs 0). The catch: if both players reason the same way, you both defect and end up with 1+1=2 — worse than cooperating.
The "dilemma" is that individual rationality leads to a collectively worse outcome. Iterated play (multiple rounds) opens the door to strategies like "Tit for Tat" — start cooperative, then mirror your opponent. Over many matches, that strategy often wins overall.
Built as a hobby project. Play.