Oligopoly
An oligopoly is a market structure in which a small number of large firms supply most or all of an industry's output. Because each firm is big enough to affect the market, their pricing and output decisions are interdependent.
An oligopoly sits between monopoly (one seller) and monopolistic competition (many sellers with differentiated products). A handful of firms control the bulk of industry sales, and high barriers to entry — heavy capital requirements, regulation, patents, network effects, or control of a scarce input — keep new competitors out. The products may be nearly identical, as with steel or airline seats, or differentiated by brand, as with wireless carriers and soft drinks.
The defining feature is interdependence. In perfect competition, no single seller's decisions move the market. In an oligopoly, if one firm cuts its price, rivals notice immediately and usually respond, so each firm must anticipate competitors' reactions before acting. Economists often illustrate this with the kinked demand curve: rivals match price cuts (so a cut wins little market share) but ignore price increases (so a raise loses customers). The result is unusually rigid prices, with firms competing through advertising, features, and service instead.
Interdependence also creates pressure toward collusion. Firms can earn monopoly-like profits by coordinating on price or output, which is why explicit cartels are illegal under U.S. antitrust law and why tacit price leadership draws regulatory scrutiny. Collusive arrangements tend to be unstable, since each member has an incentive to quietly undercut the agreed price.
Market structure is a standard economics topic on the Series 65 and Series 66, and the broader relationship between industry concentration, pricing power, and profitability shows up in fundamental analysis questions on the SIE and Series 7. Know the four structures in order — perfect competition, monopolistic competition, oligopoly, monopoly — and the number of sellers, barriers to entry, and degree of price control that distinguish each.
Key takeaways
- An oligopoly is a market dominated by a small number of large sellers protected by high barriers to entry.
- Firms are interdependent: each must anticipate how rivals will respond to a price or output change.
- The kinked demand curve explains why oligopoly prices tend to be rigid and competition shifts to advertising and product features.
- Collusion is tempting but illegal under antitrust law, and cartels are inherently unstable.
- Securities exams such as the Series 65 test the four market structures and how to tell them apart.