Define oligopoly; explain the role of interdependence and concentration ratios in characterising oligopolistic markets
Analyse the kinked demand curve model and explain why it predicts price rigidity in oligopoly
Apply game theory and the Prisoner's Dilemma to oligopoly behaviour, including Nash equilibrium and dominant strategies
Evaluate collusion, cartels, and non-price competition as oligopoly outcomes, with reference to UK/global examples and CMA enforcement
Features of Oligopoly
What Is Oligopoly?
Key Definition
OLIGOPOLY: a market dominated by a few large firms (high concentration). Key feature: INTERDEPENDENCE — each firm must consider rivals' reactions when making pricing or output decisions. Firms are price-makers but constrained by rivals. High barriers to entry (similar to monopoly) allow supernormal profit to persist in the long run.
CONCENTRATION RATIO
Measures market share of top N firms. 5-firm concentration ratio (CR5) = sum of top 5 firms' market shares. UK petrol retail: top 5 control ~80%. UK banking (current accounts): top 5 ~85%. UK supermarkets: CR4 ≈ 67% (Tesco, Sainsbury's, Asda, Morrisons). High CR → oligopolistic structure.
FEATURES
Few firms (typically 3–10); differentiated or homogeneous products; high barriers (economies of scale, brand loyalty, sunk costs); interdependence; tendency toward non-price competition; may collude or compete fiercely.
EXAMPLES
UK supermarkets (Tesco, Sainsbury's, Asda, Morrisons, Aldi); global airlines (BA, Ryanair, EasyJet); telecoms (BT, Sky, Virgin); smartphones (Apple, Samsung — ~80% of global market); UK banking (Lloyds, Barclays, HSBC, NatWest).
Kinked Demand Curve
The Kinked Demand Curve & Price Rigidity
Kinked demand curve
MR (with discontinuity gap)
MC₁ — both pass through gap
MC₂ — price stays at P*
THE KINK LOGIC
If one firm raises price above P*, rivals do NOT follow (they gain market share) → elastic demand above kink (large Q loss). If one firm cuts price below P*, rivals MATCH (to protect market share) → inelastic demand below kink (little Q gain). Result: the "best" response is to keep price at P* — price rigidity.
THE MR GAP
The kinked demand curve creates a discontinuity (vertical gap) in the MR curve. As long as MC passes through this gap, the profit-maximising output and price are unchanged even if costs rise or fall. This explains why oligopoly prices are often "sticky" despite input cost changes.
LIMITATIONS OF THE MODEL
Doesn't explain how P* is determined in the first place; assumes symmetric rivals but firms are often different sizes; doesn't account for collusion; Stigler (1947) found little empirical support for price rigidity. Better models include game theory.
Game Theory
Prisoner's Dilemma & Nash Equilibrium
Key Concept
GAME THEORY: the study of strategic interactions where the outcome for each player depends on the choices of others. The PRISONER'S DILEMMA shows how rational individual decisions can lead to collectively sub-optimal outcomes. In oligopoly: each firm independently chooses to compete (cut prices) or collude (keep prices high). Colluding together is best collectively; competing is individually rational.
THE PAYOFF MATRIX
Firm A vs Firm B — options: Compete or Collude.
Both collude: (£5m, £5m) ← best joint outcome
A competes, B colludes: (£8m, £2m)
A colludes, B competes: (£2m, £8m)
Both compete: (£3m, £3m) ← Nash equilibrium Dominant strategy for each: Compete (always better regardless of rival's choice).
DOMINANT STRATEGY
A strategy that gives the best payoff regardless of what the rival does. In PD, "Compete" is the dominant strategy for both. Result: Nash equilibrium is (Compete, Compete) even though both would be better off colluding. Explains why cartels are unstable — temptation to defect is always present.
NASH EQUILIBRIUM
A set of strategies where no player can improve their outcome by unilaterally changing their strategy, given the other's choice. Named after John Nash (Nobel 1994, "A Beautiful Mind"). In PD: (Compete, Compete) is Nash equilibrium even though it is not Pareto optimal.
Collusion & Cartels
Collusion, Cartels & CMA Enforcement
EXPLICIT COLLUSION (CARTEL)
Firms formally agree to fix prices and/or divide the market. Behave collectively like a monopoly — maximise joint profit. Illegal under UK competition law (CMA) and EU law (Article 101 TFEU). OPEC (oil cartel): formally restricts output to keep prices high — legal as sovereign states.
TACIT COLLUSION
Informal price coordination without explicit agreement. Price leadership (dominant firm raises price; others follow). "Conscious parallelism" — parallel pricing without communication. Hard for regulators to prove. UK petrol retailers: prices move in lock-step despite no formal agreement.
WHY CARTELS BREAK DOWN
Prisoner's dilemma incentive to cheat (secret price cuts gain market share). Detection and punishment (CMA fines up to 10% of global turnover; prison sentences for directors). Entry of new firms (cartel profits attract entrants). Changing economic conditions. OPEC: frequent defection by members on output quotas.
CMA ENFORCEMENT
Competition & Markets Authority investigates cartels. Recent cases: galvanised steel tanks (2021 — fines); pharmaceutical price fixing (Pfizer/Flynn, 2022). Leniency programme: first firm to self-report cartel gets immunity from fine → destabilises cartels. Affected firms can sue for cartel damages.
Non-Price Competition
Non-Price Competition in Oligopoly
WHY NON-PRICE?
In oligopoly, price cuts are quickly matched → no long-run advantage. Price wars destroy industry profit (race to bottom). Better strategy: compete on non-price dimensions where rivals cannot instantly replicate. Kinked demand curve predicts this outcome.
ADVERTISING & BRANDING
Creates product differentiation and brand loyalty. Coca-Cola vs Pepsi: near-identical product, vastly different brand perceptions. Supermarket own-brand vs Heinz. Advertising raises barriers to entry (high brand value requires heavy ad spend to match). Creates informational and psychological switching costs.
QUALITY, INNOVATION & SERVICE
Product features, design, after-sales service. Apple vs Samsung — both oligopolists competing heavily on design, ecosystem, camera quality rather than price. Dynamic efficiency argument: oligopolists invest in R&D (funded by supernormal profits) → more innovation than perfect competition.
LOYALTY SCHEMES & SWITCHING COSTS
Tesco Clubcard, Nectar, Boots Advantage Card — create switching costs (losing points if you move). Mobile phone contracts — locking consumers in for 24 months. Banking: complex switching (direct debits, salary) → low switching rates despite better deals → oligopolists retain customers without competing on price.
Evaluation
Evaluating Oligopoly: For & Against
Arguments For (dynamic efficiency case)
Oligopolists earn supernormal profit → fund large-scale R&D and innovation (Apple, Samsung, GSK — pharmaceutical R&D only viable at scale)
Economies of scale pass some cost savings to consumers via lower prices than monopoly
Tacit collusion may produce price stability (predictable markets benefit investment planning)
Arguments Against (allocative failure)
Price above MC → allocative inefficiency; consumer surplus is reduced vs perfect competition
Collusion and cartels exploit consumers via supracompetitive pricing (Pfizer/Flynn: epilepsy drug price rose 2,600%)
High barriers to entry protect incumbents, blocking new innovative firms from competing
Advertising may be purely persuasive (not informative) — raises costs and prices without increasing consumer welfare
Essay Tip: "The key evaluation is the trade-off between static and dynamic efficiency. Oligopoly may be statically inefficient (P > MC) but dynamically efficient (R&D, innovation). For AQA 25-mark essays: acknowledge game theory limitations, distinguish collusive from competitive oligopoly, and deploy a real-world example (OPEC, CMA enforcement, Apple vs Samsung). Avoid treating oligopoly as simply 'bad' — the outcome depends heavily on whether firms compete or collude."
Evaluation
Deeper Evaluation Points
Contestability argument
If markets are contestable (low sunk costs, easy entry/exit), even oligopolists face competitive pressure and price near MC
Threat of entry disciplines behaviour — oligopolists may limit prices to deter entrants (limit pricing)
Budget airlines (Ryanair) entered as contestable challenger — forced incumbent carriers to lower fares
Digital markets lower entry costs for some sectors — potential competition from tech disruptors (e.g. fintech vs banks)
Regulator limitations
Tacit collusion is almost impossible to prove legally — regulators can only act on explicit agreements
CMA fines (up to 10% of global turnover) may be insufficient deterrent for large multinationals
Regulatory capture: regulator may be influenced by the industry it oversees, reducing effectiveness
Global cartels (OPEC) are beyond UK/EU jurisdiction — national regulation has limits
AQA Command Words: "Analyse" → chains of reasoning with diagrams. "Evaluate" → weigh both sides with a justified conclusion. "Discuss" → consider multiple perspectives. Always end with a nuanced conclusion: e.g. "Whether oligopoly is harmful depends on the degree of contestability, regulatory effectiveness, and whether firms compete or collude."
Glossary
Key Terms
Oligopoly
A market structure dominated by a few large firms with high concentration ratios. Key feature: interdependence — each firm's decisions depend on expected rival reactions. Barriers to entry allow supernormal profit to persist.
Interdependence
The defining feature of oligopoly: each firm's optimal strategy depends on what rivals do. Firms must anticipate and react to rivals' pricing, output, and advertising decisions. Creates strategic uncertainty absent in other market structures.
Kinked Demand Curve
A model explaining price rigidity in oligopoly. Demand is elastic above P* (rivals don't follow price rises) and inelastic below P* (rivals match price cuts). The resulting MR discontinuity means MC changes don't affect profit-maximising price.
Game Theory / Nash Equilibrium
Game theory models strategic interaction. Nash equilibrium: a set of strategies where no firm can improve its outcome by changing strategy unilaterally. In the Prisoner's Dilemma, (Compete, Compete) is Nash equilibrium despite being collectively sub-optimal.
Collusion
Firms coordinating behaviour to avoid price competition. Explicit collusion (cartel) is illegal under UK/EU competition law. Tacit collusion (price leadership, conscious parallelism) is legal but hard to prove. Both reduce consumer surplus.
Non-Price Competition
Competing on dimensions other than price: advertising, branding, product quality, innovation, loyalty schemes, customer service. Preferred in oligopoly because price cuts are easily matched, destroying profit. Raises barriers to entry via brand value.
Question 1 of 8 · Oligopoly
Which feature most clearly distinguishes oligopoly from monopolistic competition?
A
Firms sell differentiated products
B
Firms are interdependent — each must consider rivals' reactions to its decisions
C
Firms face a downward-sloping demand curve
D
Firms earn supernormal profit in the short run
Answer · Question 1
Which feature most clearly distinguishes oligopoly from monopolistic competition?
A
Firms sell differentiated products
B
Firms are interdependent — each must consider rivals' reactions to its decisions
C
Firms face a downward-sloping demand curve
D
Firms earn supernormal profit in the short run
Correct: B. Interdependence is the defining feature of oligopoly. In monopolistic competition, there are many firms — each is too small to affect rivals, so they ignore competitors' reactions. In oligopoly, there are few large firms and each firm's pricing or output decision directly affects rivals' market share, forcing strategic consideration of rivals. Options A, C, and D are features shared with monopolistic competition and monopoly respectively.
Question 2 of 8 · Oligopoly
The kinked demand curve model of oligopoly predicts that:
A
Oligopolists constantly engage in price wars to gain market share
B
Prices in oligopoly are rigid — firms avoid price changes even when costs change
C
Oligopolists collude to set a single joint profit-maximising price
D
Each firm faces a perfectly elastic demand curve above the current price
Answer · Question 2
The kinked demand curve model of oligopoly predicts that:
A
Oligopolists constantly engage in price wars to gain market share
B
Prices in oligopoly are rigid — firms avoid price changes even when costs change
C
Oligopolists collude to set a single joint profit-maximising price
D
Each firm faces a perfectly elastic demand curve above the current price
Correct: B. The kinked demand curve creates a discontinuity (gap) in the MR curve. As long as MC remains within this gap — which it will for a range of cost increases or decreases — the profit-maximising output Q* and price P* are unchanged. This is why oligopoly prices are often "sticky": firms don't raise prices (rivals won't follow, so they lose customers) and don't cut prices (rivals match the cut, giving little benefit). The model explains price stability, not collusion.
Question 3 of 8 · Oligopoly
In a standard Prisoner's Dilemma payoff matrix for two oligopolists, the Nash equilibrium is:
A
Both firms collude — this maximises joint profit
B
One firm competes and one colludes — giving the highest individual payoff
C
Both firms compete — neither can improve by unilaterally switching to collude
D
Firms alternate between competing and colluding each period
Answer · Question 3
In a standard Prisoner's Dilemma payoff matrix for two oligopolists, the Nash equilibrium is:
A
Both firms collude — this maximises joint profit
B
One firm competes and one colludes — giving the highest individual payoff
C
Both firms compete — neither can improve by unilaterally switching to collude
D
Firms alternate between competing and colluding each period
Correct: C. Nash equilibrium is where no player can improve their payoff by unilaterally changing strategy. In the PD payoff matrix (e.g. both collude = £5m each; both compete = £3m each; defect while rival colludes = £8m), "Compete" is the dominant strategy for each firm regardless of the rival's choice. So both compete — earning £3m each — is Nash equilibrium, even though both would earn £5m if they colluded. This is the tragedy of the PD: rational individual choice produces a collectively sub-optimal result.
Question 4 of 8 · Oligopoly
A "dominant strategy" in game theory means:
A
The strategy chosen by the largest firm in the market
B
The strategy that produces the highest joint payoff for all players
C
A strategy that gives the best payoff regardless of what rivals choose
D
The strategy that emerges only when firms communicate and coordinate
Answer · Question 4
A "dominant strategy" in game theory means:
A
The strategy chosen by the largest firm in the market
B
The strategy that produces the highest joint payoff for all players
C
A strategy that gives the best payoff regardless of what rivals choose
D
The strategy that emerges only when firms communicate and coordinate
Correct: C. A dominant strategy is optimal regardless of what the other player does. In the Prisoner's Dilemma, "Compete" is dominant for both firms: if the rival colludes, you earn £8m by competing vs £5m by colluding; if the rival competes, you earn £3m by competing vs £2m by colluding. In both cases, competing is better — making it dominant. This is what drives the Nash equilibrium to (Compete, Compete) even without communication.
Question 5 of 8 · Oligopoly
Why do cartels tend to be unstable over time?
A
Cartel members always agree on price but disagree on output allocations
B
Each cartel member has an incentive to secretly undercut the agreed price to gain market share, even though this destroys the cartel if everyone does it
C
Regulators always detect cartels immediately, making them impossible to sustain
D
Cartel profits are always lower than competitive profits, removing the incentive to form them
Answer · Question 5
Why do cartels tend to be unstable over time?
A
Cartel members always agree on price but disagree on output allocations
B
Each cartel member has an incentive to secretly undercut the agreed price to gain market share, even though this destroys the cartel if everyone does it
C
Regulators always detect cartels immediately, making them impossible to sustain
D
Cartel profits are always lower than competitive profits, removing the incentive to form them
Correct: B. This is the Prisoner's Dilemma applied to cartels. Each firm earns more by secretly cheating on the agreed cartel price — undercutting rivals to steal market share while others maintain the high price. But when all firms cheat, the cartel collapses and all earn less. OPEC frequently faces this problem: member countries repeatedly exceed their output quotas, undermining the agreed price floor. The CMA leniency programme exploits this instability — the first firm to self-report gets immunity, incentivising defection.
Question 6 of 8 · Oligopoly
Tacit collusion differs from explicit collusion (a cartel) because:
A
Tacit collusion involves secret written agreements; explicit collusion is verbal only
B
Tacit collusion is always legal; explicit collusion is always illegal in all countries
C
Tacit collusion involves parallel pricing behaviour without formal agreement, making it harder for regulators to prove
D
Tacit collusion only occurs in monopoly; explicit collusion is unique to oligopoly
Answer · Question 6
Tacit collusion differs from explicit collusion (a cartel) because:
A
Tacit collusion involves secret written agreements; explicit collusion is verbal only
B
Tacit collusion is always legal; explicit collusion is always illegal in all countries
C
Tacit collusion involves parallel pricing behaviour without formal agreement, making it harder for regulators to prove
D
Tacit collusion only occurs in monopoly; explicit collusion is unique to oligopoly
Correct: C. Tacit collusion — also called "conscious parallelism" — occurs when firms independently follow a price leader or mirror each other's pricing without any explicit communication or agreement. UK petrol retailers moving prices in unison is a classic example. Regulators struggle to prosecute it because no agreement exists to produce as evidence. Explicit collusion (cartels) involves actual agreements — emails, meetings, phone calls — which CMA can seize as evidence. Note: OPEC is an explicit cartel that is legal because its members are sovereign states.
Question 7 of 8 · Oligopoly
Non-price competition is the preferred competitive strategy in oligopoly primarily because:
A
Competition law prohibits price competition between oligopolists
B
Price cuts are quickly matched by rivals, giving no lasting advantage — while non-price strategies are harder to replicate instantly
C
Consumers in oligopolistic markets are not responsive to price changes
Correct: B. This follows directly from the kinked demand curve: price cuts below P* are matched by rivals (inelastic section) — so the firm gains very little market share but triggers a price war. Brand investment, R&D, loyalty programmes, and advertising are harder and slower to replicate — they offer more durable competitive advantage. Apple's ecosystem lock-in took years to build; a rival cannot instantly copy it, unlike a 5% price cut. This is why UK supermarkets compete fiercely on loyalty cards, product ranges, and convenience rather than headline prices.
Question 8 of 8 · Oligopoly
The UK supermarket sector has a four-firm concentration ratio (CR4) of approximately:
A
35% — indicating a competitive market with many rivals
B
50% — indicating moderate concentration
C
67% — indicating a highly concentrated oligopolistic structure
D
95% — indicating near-monopoly dominance by one or two firms
Answer · Question 8
The UK supermarket sector has a four-firm concentration ratio (CR4) of approximately:
A
35% — indicating a competitive market with many rivals
B
50% — indicating moderate concentration
C
67% — indicating a highly concentrated oligopolistic structure
D
95% — indicating near-monopoly dominance by one or two firms
Correct: C. UK supermarkets (Tesco, Sainsbury's, Asda, Morrisons) together account for approximately 67% of grocery market share — a CR4 of ~67%. This is the textbook AQA example of an oligopoly: few large firms, high concentration, significant interdependence (each supermarket monitors rivals' pricing, promotions, and loyalty schemes). Despite the presence of discounters (Aldi ~10%, Lidl ~7%), the market remains oligopolistic. A CR4 above 60% is generally considered a strongly oligopolistic structure.
Exam Preparation
AQA Exam Technique: Oligopoly
DIAGRAM TECHNIQUE
Kinked demand curve: draw the kink clearly at P*, label elastic segment above and inelastic below. Show MR with a clear gap (discontinuity). Draw two MC curves both passing through the gap — this is the key mechanism. Label axes, P*, Q*, and the gap. Do not draw a smooth MR curve — the gap is the entire point of the model.
GAME THEORY QUESTIONS
If given a payoff matrix: (1) identify each firm's best response to each rival strategy, (2) find the cell where both are making their best response simultaneously — that is Nash equilibrium. State whether a dominant strategy exists. Always link back to oligopoly: why does the PD show cartels are unstable?
25-MARK ESSAY STRUCTURE
Para 1: Define oligopoly, state key feature (interdependence). Para 2: Kinked demand → price rigidity (with diagram). Para 3: Game theory → collusion incentive (with payoff matrix). Para 4: Non-price competition (examples). Para 5: Evaluation — collusion vs competition, static vs dynamic efficiency, contestability. Conclusion: conditional judgement.
HIGH-MARK TRIGGERS
Mention: Stigler's critique of the kinked demand model. CMA leniency programme destabilising cartels. OPEC as a legal cartel (sovereign states). Contestability theory as an alternative. Real examples: Pfizer/Flynn pharmaceutical cartel; Apple vs Samsung non-price competition; UK petrol tacit collusion.
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Lesson Complete
You've covered features of oligopoly, concentration ratios, the kinked demand curve, game theory and the Prisoner's Dilemma, collusion and cartel instability, CMA enforcement, and non-price competition — with real-world UK and global examples throughout.