Suppose the police have arrested two people whom they know have committed an armed robbery together. Unfortunately, they lack enough admissible evidence to get a jury to convict. They do, however, have enough evidence to send each prisoner away for two years for theft of the getaway car.

The chief inspector now makes the following offer to each prisoner (held in isolation from each other): ‘If you will confess to the robbery, implicating your partner, and he does not also confess, then you’ll go free and he’ll get ten years. If you both confess, you’ll each get five years.