To return to our prisoners, suppose that, contrary to our assumptions, they do value each other’s well-being as well as their own. In that case, this must be reflected in their utility functions, and hence in their payoffs. If their payoff structures are changed so that, for example, they would feel so badly about contributing to inefficiency that they’d rather spend extra years in prison than endure the shame, then they will no longer be in a PD. But all this shows is that not every possible si