Emergence of parochial altruism in well-mixed populations

• We institute an analytical model to study the evolution of parochial altruism.
• Conditions for parochial altruism to be favored by selection are specified.
• Intergroup conflict can promote the evolution of parochial altruism.
• High mutation rates are detrimental to the evolution of parochial altruism.

Evolution of parochial altruism is studied in a well-mixed population subdivided into two groups. Individuals' strategy is contingent on interacting partners' group property. Participants from the same group play prisoner's dilemma game, otherwise they play punishment game. We specify precisely the game parameters scope in which parochial altruism is favored for weak selection. For low mutation, we find that intergroup conflict can promote the evolution of parochial altruism. However, parochial altruism can never be favored for high mutation. We also present the conditions under which parochial altruism enjoys a higher frequency than neutral drift for intermediate mutation.