Zero-sum game
A zero-sum game is one in which one party's gain is exactly matched by another's loss.
In some contests, one side's gain is exactly the other's loss, and there is no way for both to win. That is a zero-sum game, and treating the world as if everything were one is a common and costly error.
A zero-sum game is a situation in which one party's gain is exactly balanced by another's loss, so that the total of gains and losses is zero. Dividing a fixed pie, or a bet between two people, is zero-sum: whatever one wins, the other loses, and cooperation cannot make both better off.
Fixed pie, pure conflict
The defining feature is that the total is fixed, so the only question is how it is divided. This makes zero-sum games purely competitive: there is no scope for mutual gain, no cooperative solution that helps everyone, only the distribution of a constant total. Poker among friends, a tennis match, or two firms fighting over a fixed market with no growth are roughly zero-sum, contests over shares rather than opportunities to create value together.
The danger of zero-sum thinking
The greater significance of the concept is the error of treating non-zero-sum situations as if they were zero-sum. Many of the most important interactions, trade, negotiation, collaboration, are positive-sum: cooperation can enlarge the total, leaving everyone better off. Seeing these as zero-sum, where any gain for the other side must be a loss for yours, destroys value, poisons negotiations, and forgoes the gains from cooperation. The mistaken belief that one party can only win if another loses is among the most damaging in economics and politics.
Recognising the difference
The practical skill is distinguishing genuinely zero-sum situations, where the total really is fixed and competition is the only game, from positive-sum ones, where cooperation can grow the pie. Trade is the classic positive-sum case that is perennially misread as zero-sum, the fallacy that one country's gain from trade is another's loss. Most rich interactions contain both elements: a positive-sum dimension of creating value and a zero-sum dimension of dividing it, which is the tension at the heart of most negotiations.
The zero-sum game is a precise idea, pure, fixed-total conflict, that is valuable as much for what it is not as for what it is. Recognising when a situation is not zero-sum, when both sides can gain, is often the key to turning a fight over shares into a search for mutual advantage.