Game theory
Game theory is the study of how rational parties make decisions when each one's outcome depends on the choices of others.
When your best move depends on what someone else will do, and theirs on what you will do, ordinary decision-making is not enough. Game theory is the mathematics of that mutual dependence.
Game theory is the study of strategic decision-making, how rational parties choose when the outcome for each depends on the choices of all. It provides a formal framework for analysing situations of conflict and cooperation, from pricing wars to arms races to everyday bargaining.
Strategy under interdependence
What distinguishes a game from a simple decision is interdependence: your payoff depends not only on your action but on others' actions, and theirs on yours. This circularity, I must consider what you will do, knowing you are considering what I will do, is what ordinary optimisation cannot handle and what game theory was built to address. The players, their possible strategies, and the payoffs from each combination define the game.
Equilibrium as the central idea
The core analytical tool is the equilibrium, a combination of strategies from which no player wishes to deviate given what the others are doing. The most famous, the Nash equilibrium, identifies outcomes that are self-enforcing: once reached, no one can do better by changing course alone. Equilibrium analysis lets game theory predict where strategic interactions settle, even when those outcomes are collectively poor, as the prisoner's dilemma shows.
Reach across disciplines
Game theory's influence extends far beyond economics. It illuminates biology, where strategies evolve; political science, where it models conflict, voting, and deterrence; and business, where it informs competition, auctions, and negotiation. Its power is in stripping a complex strategic situation down to players, choices, and payoffs, exposing the underlying logic that drives behaviour, though that very abstraction can also omit the psychology and context that matter in practice.
Game theory reframes a vast range of human and natural interactions as games with a structure that can be analysed. It does not always predict behaviour perfectly, since real players are not perfectly rational, but it provides an indispensable language for thinking clearly about any situation where the best thing to do depends on what others choose.