duopolies

Duopoly is a market structure in which two firms hold a dominant share of sales in a particular industry. It is a specific form of oligopoly characterized by interdependence: each firm's optimal output or pricing depends on the anticipated reaction of the other. Duopolies arise where entry is costly, technology or capital requirements are high, or product differentiation sustains two strong brands.

Two classic models are often used to study duopolies: Cournot competition, where firms choose quantities simultaneously, and Bertrand competition, where firms compete on price. In Stackelberg models, one firm acts as a leader and the other follows. Real markets may feature product differentiation, capacity constraints, or strategic alliances, which complicate the outcomes. With identical products and perfect information, a Bertrand duopoly can yield the lowest possible price (often near marginal cost), while quantity competition can produce higher prices and different revenue shares. In differentiated markets, equilibria depend on cross-price effects and cost structures.

Outcomes depend on barriers and expectations. High entry barriers, network effects, or switching costs can sustain profits and reduce welfare losses. Price wars may be short-lived, or firms may tacitly collude to keep prices higher. Regulators scrutinize potential anticompetitive practices in duopolies, given the potential for reduced consumer welfare.

Prominent examples include the commercial aircraft market, dominated by Boeing and Airbus, and the beverage industry in many countries, where Coca-Cola and PepsiCo have major shares. In some regions, two large telecom operators also dominate the market, illustrating a regional duopoly.