Whether or not to punish a defector.

Payoffs to punishers and non-punishers when with a defector in a group of three. Payoffs are shown for the row player only. p is the cost of carrying out one unit of punishment and s is the benefit an individual receives when any group member punishes. This benefit is assumed to arise from defectors switching to cooperating after being punished. Provided s>p punishment pays. In (a) each act of punishment has an additive effect on the benefits arising, whereas in (b) punishing when another individual is already punishing brings no additional benefit.