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An oligopoly is a market structure dominated by a small number of large firms, each holding a significant share of total output. It is arguably the most important structure to master at A-Level because it describes so much of the modern economy — supermarkets, mobile networks, banking, petrol retailing, energy supply, cars and airlines. Its defining feature is interdependence: because firms are few, each one's pricing, output, advertising and investment decisions visibly affect its rivals, and each must therefore anticipate how rivals will react before it acts. This single feature makes oligopoly the richest structure for analysis and evaluation, because firm behaviour is indeterminate: the same industry can be fiercely competitive (a price war) or quietly collusive (stable high prices), and the economist's job is to explain which and why. This lesson develops the two core analytical tools the specification requires — the kinked demand curve (explaining price rigidity) and game theory / the prisoner's dilemma (explaining the tension between collusion and competition) — and applies them to collusion, price leadership, non-price competition and consumer welfare.
AQA A-Level Economics (7136) — this lesson sits within 4.1.5 The market mechanism, market failure and government intervention in markets: oligopoly, concentration, collusion and the kinked demand curve, plus an introduction to game-theoretic reasoning. It connects forwards to the price-discrimination and competition-policy lessons.
Assessment Objectives developed here:
| AO | Skill | In this lesson |
|---|---|---|
| AO1 | Knowledge | Define oligopoly, concentration ratio, HHI, interdependence; state the kinked-demand and prisoner's-dilemma models; define overt/tacit collusion |
| AO2 | Application | Apply concentration data and game payoffs to named UK industries |
| AO3 | Analysis | Chain interdependence → kinked demand → price rigidity; derive the dominant strategy and Nash equilibrium; explain cartel instability |
| AO4 | Evaluation | Judge whether oligopoly is competitive or collusive, and whether it benefits or harms consumers — conditional on circumstances |
| Characteristic | Explanation |
|---|---|
| Few dominant firms | A small number of firms supply the majority of output |
| High concentration | The combined share of the largest firms (CR4 or CR5) is high |
| Interdependence | Each firm must anticipate rivals' reactions to its decisions |
| High barriers to entry | Economies of scale, brand loyalty, legal barriers and sunk costs deter entry, allowing long-run supernormal profit |
| Product differentiation | Products may be differentiated (cars, smartphones) or near-homogeneous (oil, cement) |
| Non-price competition | Firms compete heavily on advertising, branding, quality and service |
| Price rigidity | Prices are often stable — firms fear that a price change will trigger a damaging reaction |
Interdependence is worth dwelling on, because it is what makes oligopoly qualitatively different from every other structure. A perfect competitor or a monopolistic competitor is so small that it can ignore the reaction of any single rival; a monopolist has no rivals to react. Only the oligopolist must play a strategic game in which the best move depends on what the others do — which is precisely why game theory, not a single equilibrium diagram, is the natural language of oligopoly.
| Measure | Definition | Illustrative use |
|---|---|---|
| n-firm concentration ratio (CRn) | Combined market share of the n largest firms | A CR4 well above half indicates an oligopoly; the UK grocery market, dominated by a handful of chains, is the standard example |
| Herfindahl–Hirschman Index (HHI) | Sum of the squared market shares of all firms | Squaring weights large firms heavily, so the HHI rises sharply as a market concentrates; the CMA uses HHI thresholds and the change in HHI to screen mergers |
Exam Tip: Use concentration evidence to justify the claim that a market is an oligopoly rather than asserting it. The HHI's squaring of shares is the examinable subtlety: a market with one 50%-share giant and many tiny firms has a far higher HHI than one with five equal 20% firms, even though the CR5 is identical — so the HHI captures dominance, not just fewness. Quote shares as approximate and well-established (e.g. "the largest four grocers supply roughly two-thirds of the market") rather than inventing precise figures.
Oligopoly is not an accident; it is sustained by high barriers to entry that both create the structure and protect the supernormal profit it can earn in the long run. The most important is economies of scale: where the minimum efficient scale is large relative to total market demand, only a handful of firms can each operate at low average cost, and a small-scale entrant would face crippling unit-cost disadvantages. To this are added brand loyalty built by years of advertising (a newcomer must out-spend established names to win attention), legal barriers such as patents and licences, control of essential inputs or distribution, and sunk costs — the irrecoverable spending on plant, marketing or specialist assets that makes entry risky and deters the "hit-and-run" entrant of contestable-market theory. The height of these barriers is decisive for the evaluation of oligopoly: where barriers are high, incumbents can collude or set high prices with little fear of new competition; where barriers are low and the market is contestable, even a concentrated industry must price keenly to keep potential entrants out. This is why a sophisticated answer never treats concentration alone as proof of consumer harm — it asks how entrenched the few firms really are.
The kinked demand curve, developed by Paul Sweezy (1939) and independently by Hall and Hitch (1939), is the classic explanation of price rigidity under oligopoly. It builds the firm's demand curve directly out of an assumption about how rivals react.
An oligopolist setting price P1 and output Q1 believes its rivals react asymmetrically:
Splicing these two segments together produces a demand (AR) curve with a kink at (Q1,P1): elastic above, inelastic below.
Because the two segments of AR have different slopes, the marginal revenue curve has a vertical discontinuity (a gap) directly beneath the kink. The profit-maximising firm produces where MC=MR; but as long as the MC curve passes through this vertical gap, the profit-maximising output stays at Q1 and the price stays at P1 — even if costs change. Marginal cost can rise or fall within the gap and the firm's optimal price does not move. This is the model's central result: it explains why oligopoly prices are often sticky, changing far less often than costs do.
| Strength | Weakness |
|---|---|
| Explains observed price rigidity and the reluctance to start price wars | Does not explain how P1 was set in the first place — it takes the current price as given |
| Intuitive and uses standard AR/MR tools | Relies on a specific asymmetric-reaction assumption that need not hold |
| Consistent with firms' fear of triggering a reaction | Empirical evidence is mixed — many oligopoly prices do move with costs, and price wars do break out |
Exam Tip: Treat the kinked demand curve as a useful partial model, never the final word. Its fatal limitation is that it assumes the starting price rather than deriving it, and it cannot explain price wars or coordinated price changes. The examiner rewards candidates who pair it with game theory — which can explain how prices arise and why firms oscillate between competition and collusion.
Game theory — pioneered by von Neumann and Morgenstern (1944) and given its central solution concept by John Nash (1950) — models the strategic interdependence at the heart of oligopoly. Rather than assuming a fixed reaction (as the kinked-demand model does), it asks what rational firms do when each one's best move depends on the others'.
The prisoner's dilemma is the canonical example, and it captures exactly why collusion is tempting yet fragile. Consider two firms each choosing a high or low price; the payoff matrix shows their profits (illustrative figures, £m per period):
| Firm B: High price | Firm B: Low price | |
|---|---|---|
| Firm A: High price | A £5m, B £5m | A £1m, B £8m |
| Firm A: Low price | A £8m, B £1m | A £3m, B £3m |
To find each firm's best move, hold the rival's choice fixed and compare:
Pricing low is therefore A's dominant strategy — best whatever B does — and by symmetry it is B's too. So both choose low and the outcome is (Low, Low), earning £3m each. This is the Nash equilibrium: given the other's choice, neither firm can do better by unilaterally switching. Yet it is collectively worse than (High, High), where both would earn £5m. The dilemma is that the jointly best outcome is individually irrational to sustain: each firm is tempted to undercut, so the cooperative high-price outcome unravels. This is the formal heart of why cartels are unstable — the agreed high price is (High, High), but every member has a private incentive to cheat towards (Low).
graph TD
A["Cartel agrees HIGH price (best joint payoff)"] --> B["Each member: 'If I secretly cut price I gain customers and profit'"]
B --> C["Dominant strategy = cut price (cheat)"]
C --> D["Both cheat -> Nash equilibrium (LOW, LOW)"]
D --> E["Collectively worse than cooperation -> collusion is fragile"]
The static one-shot dilemma predicts competition, yet real cartels sometimes endure. The resolution is that oligopoly is a repeated game played period after period. In a repeated setting, firms can sustain the high-price outcome by credible retaliation: a firm contemplating cheating knows that a price cut today will be detected and punished by a price war tomorrow, wiping out the short-term gain. Strategies such as "tit-for-tat" (match cooperation with cooperation, defection with defection) and "trigger" strategies (cooperate until anyone cheats, then revert to the low-price Nash outcome forever) can make sustained collusion a rational equilibrium — provided firms value future profits enough, can detect cheating quickly, and expect to interact indefinitely. This is why tacit collusion is most stable in markets that are transparent (cheating is spotted fast), concentrated (few firms to coordinate), and enduring (the future matters). Conversely, opaque pricing, many firms, or an uncertain future make the cooperative outcome collapse back to competition. Bringing in the repeated game is the single most effective way to lift a game-theory answer from description to genuine analysis.
| Concept | Explanation | Illustrative example |
|---|---|---|
| Dominant strategy | Best regardless of the rival's choice | Heavy advertising by major brands — each advertises even though all might save by mutual restraint |
| Nash equilibrium | No firm gains by changing strategy alone | Price-matching at petrol stations — once all match, none benefits from deviating |
| First-mover advantage | Acting first secures a strategic edge | An early, large investment in distribution or logistics that rivals struggle to replicate |
| Credible threat | A threat the rival believes will be carried out | A reputation for fierce price retaliation that deters entry — credible only if the firm has the cost base to follow through |
Because price competition erodes the supernormal profit that high barriers would otherwise protect, oligopolists have a powerful incentive to collude — to coordinate on price, output or market shares so as to behave, jointly, like a monopolist and split the monopoly profit.
| Type | Explanation | Illustrative example |
|---|---|---|
| Overt (formal) collusion | An explicit agreement on price, output or market sharing — a cartel | International commodity cartels that set production quotas among members |
| Tacit (informal) collusion | Coordination without an explicit agreement, via price leadership or parallel pricing | Fuel retailers whose pump prices move together without any meeting |
A cartel is an attempt to impose the (High, High) outcome on a prisoner's-dilemma game — so the very logic of the dilemma works to undermine it:
| Destabilising factor | Explanation |
|---|---|
| Incentive to cheat | Each member can lift its own profit by secretly undercutting the agreed price — the dominant-strategy temptation |
| Detection problems | Cheating is hard to spot where pricing or discounts are opaque, so cheats may go unpunished |
| Cost differences | Low-cost members want a lower price than high-cost members — agreement is hard to reach and to hold |
| New entry | High cartel prices and profits attract entrants (unless barriers are very high), diluting members' shares |
| Legal penalties | Cartels are illegal in the UK and most jurisdictions; participation risks heavy fines, director disqualification and even imprisonment, and exposes firms to the regulator's leniency-for-whistle-blowers incentive |
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