Can natural selection favour indiscriminate spite?

Spiteful behaviours occur when an actor harms its own fitness to inflict harm on the fitness of others. Several papers have predicted that spite can be favoured in sufficiently small populations, even when the harming behaviour is directed indiscriminately at others. However, it is not clear that truly spiteful behaviour could be favoured without the harm being directed at a subset of social partners with relatively low genetic similarity to the actor (kin discrimination). Using mathematical models, we show that: (1) the evolution of spite requires kin discrimination; (2) previous models suggesting indiscriminate spite involve scenarios where the actor gains a direct feedback benefit from harming others, and so the harming is selfish rather than spiteful; (3) extreme selfishness can be favoured in small populations (and in some cases small groups) because this is where the feedback benefit of harming is greatest.