Nowak and Sigmund conclude that cooperation may have evolved through indirect reciprocity by image scoring. Their simulations and analytical models predict long-term cyclical dynamics between cooperative and defector populations rather than an evolutionarily stable equilibrium. Here we add a realistic feature to their model: that there are always some individuals unable to cooperate owing to their poor phenotypic condition (we call these individuals 'phenotypic defectors'). The presence of phenotypic defectors paradoxically allows persistent discriminating cooperation under a much wider range of conditions than found by Nowak and Sigmund because there is selection against both defection and unconditional altruism. In real populations there will nearly always be some level of defection because phenotypic defectors (such as the young, sick or handicapped) may be unable to help even if they have a genetic predisposition to do so.
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