Friendliness and reciprocity in equilibrium selection

In this paper, the situation in which every player has an altruistic behavior, called friendly behavior, is incorporated into the general context of normal form games by introducing a new refinement concept (called friendliness equilibrium). Existence, properties and connections with another refinement concept, introduced by the authors in a previous paper and which captures an idea of reciprocal altruism, are investigated. Examples and counterexamples are given illustrating concepts, results and links with other classical refinement concepts.