Richard Alexander has argued that moral systems derive from indirect reciprocity. We
analyse a simple case of a model of indirect reciprocity based on image scoring.
Discriminators provide help to those individuals who have provided help. Even if the help is
never returned by the beneficiary, or by individuals who in turn have been helped by the
beneficiary, discriminating altruism can be resistant against invasion by defectors.
Indiscriminate altruists can invade by random drift, however, setting up a complex dynamical …