Social Dilemma and related Definitions

Informally, a social dilemma is a collective action situation in which there is a conflict between individual and collective interest. It is a situation in which individuals could do better if they either changed their strategies or changed the rules of the game.

Some technical definitions:

Nash equilibrium
A set of strategies for the players in a game with the property that no player can benefit by deviating from his or her strategy, and thereby improve his or her payoffs while the strategies of other players are unchanged.
Pareto optimal
An outcome is Pareto optimal if there is no alternative outcome that makes some individuals better off and leaves all other individuals at least as well off.

More formally, a social dilemma is a collective action situation in which the Nash equilibrium results in outcomes below the Pareto optimal.

A dominating strategy is a strategy that yields the best outcome for an individual regardless of what anyone else does.

Definitions of Nash and Pareto are from Ostrom and Walker (eds.), Trust and reciprocity: interdisciplinary lessons from experimental research, Russell Sage Foundation, 2003.

Definition of Dominating Strategy from Kollock, P., "Social Dilemmas: The Anatomy of Cooperation", Annual Review of Sociology, 24: 183-214, 1998.