1.5 More about the Scope of Game Theory 29 may make a costly donation to a common pool that provides a social benefit which is greater than the contribution. Because of the free rider problem, it is typically a dominant strategy not to contribute anything. Nevertheless, with as many as ten or more players, some players do contribute to the common pool. In this book, only games with selfish players are studied because this seems to be, in general, the most relevant assumption in wireless games. However, it is useful to bear in mind that this assumption is not necessary and there exists a literature dedicated to the problem of altruism in games (e.g., see Geanakoplos et al. (1989) and Rabin (1993)). 1.5.3 About Interaction A game corresponds to a situation where (autonomous) decision-makers interact. Is there interaction in wireless networks? The answer depends on the scenario. Often interaction does exist because terminals share common resources. For example, when users exploit the same band at the same time, in the same geographical area, some multiuser interference occurs. Decisions made by the players are inter-dependent, in the sense that their performance metric depends on what the other players do: this is the interaction through actions. It is useful to know that there can be inter- action between players even if the player's utility does not directly depend on the