Prisoner's dilemma

The prisoner's dilemma is a paradox about co-operation. It shows why a fully rational individual might not act in ways that seem in their best interests. It is studied in game theory.

Game structure

Setup

The police catch two criminals after they committed a crime. The police do not know which person committed the crime and which person just helped. They question the two in separate cells, so the two prisoners cannot talk to each other in any way. Each prisoner can either stay silent or betray the other by blaming the crime on them.

If both stay silent, they only go to jail for 2 years. If one betrays and the other stays silent, the one that stays silent goes to jail for 10 years and the other one does not go to jail at all. If they both betray each other, they each go to jail for 5 years. No matter what happens, the prisoners will never see each other again.

Both prisoners are very logical, and will choose the option that gives them the shortest prison sentence.

Strategies

If you are a prisoner in this situation and you want the shortest prison sentence, the ideal choice is to betray the other prisoner.

If the other prisoner stays silent, also staying silent will put you in jail for two years, while betraying them will set you free. If the other prisoner betrays you, staying silent will put you in jail for ten years, while betraying them back will only put you in jail for five.

The other prisoner realizes this too, and will also choose to betray you, giving you both a five year prison sentence. But this is not the best outcome; if both of you cooperated, you both could've gotten away with only two years. Betrayal benefits each prisoner individually, but harms the group as a whole.

Game theory was much studied during the Cold War period. In that case the "players" being studied were the United States and the Soviet Union.