““The individuals in a cooperative social group cannot afford to tolerate repeated defections by selfish “free riders,” such as those who hoard food or shirk responsibility for the common defense. Any group too tolerant of defectors would be subsidizing them at its own expense, which would amount eventually to collective suicide. Organisms that temporarily forsake immediate personal advantage in the expectation of equivalent near-term reciprocation from nonkin (“reciprocal altruism”; Trivers 1971) or deferred and roundabout forms of longterm reciprocation through third parties (“indirect reciprocity”; Alexander 1987) must therefore evolve ways of reliably discriminating between a cooperator and a defector.””
