Two suspects are questioned in separate rooms. If both stay silent, both get off lightly. If one talks, the talker walks free and the silent one pays dearly. If both talk, both pay heavily. Each reasons the same way: whatever the other does, talking serves me better. So both talk — and both lose.

The trap is that individual logic and group logic point in opposite directions. Neither player is stupid or wicked; the payoffs themselves reward defection, and the bad ending follows from everyone being sensible.

It is everywhere once you see it: two colleagues who could share credit but each grab it, two shops that could hold prices but discount each other into a war, two roommates who could both clean but each wait the other out.

The lesson is not that people are selfish. It is that some situations are built to punish trust — and the fix is to change the situation, not to lecture the players.