Two prisoners are questioned separately. Each must decide whether to stay silent or accuse the other. Neither can know what the other will choose.
In the classic setup, each person does better by accusing, whatever the other decides. If the other stays silent, accusation brings a lighter sentence for you. If the other accuses, accusing still leaves you better off than remaining silent. So each has a reason to defect.
But if both follow that logic, both fare worse than if they had stayed silent together. The dilemma shows how individually sensible choices can lead to a result everyone would rather avoid. It relies on a carefully arranged payoff structure, not a claim that people are naturally selfish. Add trust, repeated encounters, or different incentives, and the situation may change.