Oligopoly

Most important markets are dominated not by one firm or by many, but by a few. That middle ground, oligopoly, is where strategic interdependence becomes the whole game.

An oligopoly is a market structure dominated by a small number of large firms, whose decisions are interdependent: each must consider how the others will react to its actions. It is among the most common and most analytically interesting market structures, because the mutual dependence of the few makes behaviour strategic.

The defining interdependence

What sets oligopoly apart is interdependence. In perfect competition each firm is too small to affect others; in monopoly there are no others. In oligopoly, a handful of firms are each large enough that one firm's pricing, output, or product decisions materially affect the rest, and each knows it. So every firm must anticipate its rivals' reactions before acting, and the rivals must anticipate its anticipation. This strategic interdependence is why oligopoly is the natural home of game theory, which was largely developed to analyse exactly such situations.

Competition or collusion

Oligopolists face a recurring tension between competing and cooperating. They could compete fiercely, cutting prices to win share, but mutual price-cutting erodes everyone's profits, the prisoner's dilemma writ across an industry. Alternatively they could collude, openly or tacitly, to keep prices high and share the market, which is collectively profitable but illegal where it is explicit, and unstable, since each firm is tempted to cheat. Much oligopoly behaviour is an uneasy navigation between these poles, producing outcomes from vicious price wars to comfortable tacit coordination.

Why outcomes vary so widely

Because oligopoly is governed by strategic interaction rather than a single logic, its outcomes are hard to predict and vary enormously. The same basic structure, a few large firms, can produce intense competition that benefits consumers or cosy coordination that exploits them, depending on factors like the number of firms, the ease of detecting cheating, the similarity of products, and the height of entry barriers. This indeterminacy is why competition authorities watch concentrated industries so closely and why oligopoly resists simple characterisation.

Oligopoly is the structure of much of the modern economy, from airlines to supermarkets to technology platforms, and the one where strategy matters most, because the actions of a few interdependent firms shape the market. Its defining feature, that each firm's best move depends on the others', makes it the richest and least predictable of the market structures.