University of Oxford, Department of Zoology. The Tinbergen Building, South Parks Road , Oxford, OX1 3PS, United Kingdom, email firstname.lastname@example.org
Spite, altruism's neglected ugly sister, is the most mysterious and controversial of the four social behaviours. How can an individual be favoured to harm itself and its social partners? Hamilton's rule, which was devised in order to explain altruistic behaviours, has a darker side that reveals when spite will be favoured. Specifically, it requires that the spiteful actor and its victim be negatively related. I develop theory for the evolution of spite in competitive environments, and show that increasingly strong local competition can favour spiteful behaviour. Application of the theory to chemical warfare in microbes and suicidal sibling rivalry in parasitoid wasps leads to novel predictions for parasite virulence and sex allocation theory. I discuss the semantics of spite and ambiguities in the standard classification of social behaviours.