Oligopoly

Interdependence, the kinked demand curve, game theory & the prisoner's dilemma, collusion, and why this is the highest-frequency essay topic on the entire A Level

SPEC 3.4 • MARKET STRUCTURES

Key Definitions

Oligopoly

A market structure dominated by a few large firms with high barriers to entry, interdependent decision-making and product differentiation.

Concentration Ratio

The combined market share of the largest firms in an industry. E.g. 5-firm CR = combined share of 5 largest firms.

Collusion

When firms agree to coordinate their pricing or output decisions to maximise joint profits — behaving as a collective monopoly.

Four Characteristics of Oligopoly

The Four Characteristics

Dominated by a few large firms — high concentration ratio (5-firm CR often >60%)

High barriers to entry and exit — economies of scale, brand loyalty, sunk costs, legal barriers

Interdependence — each firm's decision directly affects rivals; firms must anticipate rivals' responses (strategic behaviour)

Product differentiation — both homogeneous (petrol, steel) and differentiated (cars, phones) oligopolies exist

Concentration Ratio — Worked Example

UK grocery market shares: Tesco 27%, Sainsbury's 15%, Asda 14%, Morrisons 10%, Aldi 9%.5-firm CR = 27 + 15 + 14 + 10 + 9 = 75%A CR above 60% typically indicates oligopoly. Above 80% = tight oligopoly with strong interdependence.

Exam Tip

Interdependence is the single most important characteristic. It is what drives all oligopoly behaviour — kinked demand, game theory, collusion. If you only have time for one characteristic, this is it.

Interdependence — The Core Concept

In oligopoly, each firm's pricing and output decisions directly affect rivals. Unlike in perfect competition (where firms are too small to matter) or monopoly (where there are no rivals), oligopolists must think strategically.

Firm A cuts price

Firm B loses customers

Firm B must respond

Match the cut? Ignore? Retaliate harder?

This strategic uncertainty creates two possible outcomes:

Competitive Oligopoly

Firms compete aggressively on price or through non-price methods. Price wars, predatory pricing, heavy advertising. Consumers benefit from lower prices.

Collusive Oligopoly

Firms coordinate to avoid price wars. Set high prices jointly, restrict output, divide markets. Consumers harmed — higher prices, less choice.

The exam's key question: "Are oligopolists more likely to compete or collude?" — this is the highest-frequency essay on the course.

The Kinked Demand Curve Model

The kinked demand curve explains price rigidity — why oligopoly prices tend to be sticky even when costs change.

The Logic

Above P*: If a firm raises price, rivals don't match → firm loses many customers → demand is elastic

Below P*: If a firm cuts price, rivals immediately match → firm gains few extra customers → demand is inelastic

Result: a kink in the demand curve at the current price P*

The Consequence

The kink creates a vertical discontinuity in the MR curve at Q*

As long as MC passes through this gap, price and output stay the same

MC can shift up or down within the gap without changing Q* or P*

This explains price rigidity — sticky prices in oligopoly

Kinked Demand Curve — Diagram

Examiner Alert

The kinked demand model explains price rigidity, NOT how the original price was set. This is its key limitation — always state this in evaluation.

Game Theory — The Prisoner's Dilemma

Game theory models the strategic decisions of interdependent firms. The classic setup is a 2×2 payoff matrix — two firms, two strategies. This is a favourite 4-mark question format.

Nash Equilibrium

The outcome where no firm can improve its payoff by unilaterally changing strategy. Both firms are doing the best they can given what the other firm is doing.

Dominant Strategy

A strategy that gives the best outcome for a firm regardless of what the rival does.

Prisoner's Dilemma

A situation where rational individual behaviour leads to a collectively worse outcome — both firms would be better off cooperating, but each has an incentive to defect.

The Payoff Matrix — Step by Step

Firm A and Firm B each choose between High Price and Low Price. Payoffs are profits (£m).

Firm B

High PriceLow Price

Firm AHigh PriceA: £10m B: £10mA: £2m B: £14m

Low PriceA: £14m B: £2mA: £4m B: £4m ★

★ = Nash Equilibrium Shaded yellow = collusive outcome (both better off)

Reading the Matrix

Firm A's logic: If B plays High → A earns £14m (Low) vs £10m (High) → choose Low. If B plays Low → A earns £4m (Low) vs £2m (High) → choose Low. Low Price is dominant.

By symmetry: Low Price is also dominant for Firm B

Nash equilibrium: Both choose Low Price → both earn £4m

But: Both would earn £10m if they colluded on High Price — the prisoner's dilemma

Why Do Oligopolists Collude?

The prisoner's dilemma shows that competition leads to lower profits for all firms. This creates a powerful incentive to coordinate — if firms can agree to both set high prices, joint profits are maximised.

Interdependence

Price war = Nash equilibrium (low profit)

Incentive to collude

Joint profit maximisation

Behave as collective monopoly

Overt Collusion (Cartel)

Formal, explicit agreement to fix prices, restrict output, or divide markets. Illegal in UK and EU. Example: CMA can fine up to 10% of global turnover.

Tacit Collusion

Informal, unspoken coordination. Price leadership — dominant firm sets price, rivals follow. Legal but monitored. Common in petrol, banking.

What Makes Collusion Stable (or Unstable)?

This is the evaluation the examiner is looking for in any oligopoly essay. Collusion is inherently unstable — there's always an incentive to cheat.

FactorSupports CollusionUndermines Collusion

Number of firmsFewer firms → easier to coordinate and monitorMany firms → harder to agree, easier to cheat undetected

Cost structuresSimilar costs → easy to agree on priceDifferent costs → firms want different prices

Price transparencyHigh transparency → cheating detected fastLow transparency → easy to secretly undercut

Frequency of interactionRepeated games → tit-for-tat punishment worksOne-shot interactions → no retaliation threat

Legal deterrence—CMA fines up to 10% of global turnover → deters overt collusion

Competition in Oligopoly

When oligopolists compete rather than collude, they tend to prefer non-price competition — because price cuts are quickly matched (kinked demand) while non-price methods can build lasting advantage.

Price Competition

Price wars — mutually destructive, erode profits

Predatory pricing — pricing below cost to force rival out (illegal if abusing dominance)

Limit pricing — pricing just low enough to deter new entrants

Non-Price Competition

Advertising & branding

Product innovation

Loyalty schemes

After-sales service

Packaging & convenience

Exam Tip

Oligopolists prefer non-price competition because price cuts are quickly matched by rivals (kinked demand), while advertising can build brand loyalty and shift demand. This links the kinked demand model to competitive behaviour — the examiner rewards this connection.

Efficiency in Oligopoly

Efficiency TypeOligopoly?Explanation

Allocative✗P > MC — firms have price-setting power

ProductiveAmbiguousPossible through EoS, but X-inefficiency may offset

Dynamic✓ LikelySupernormal profits fund R&D. Non-price competition incentivises innovation.

X-inefficiencyPossibleDepends on competitive intensity. Collusion → more slack. Competition → less.

The Key Evaluation

Whether oligopoly is efficient depends on whether firms compete or collude. A competitive oligopoly with strong innovation may deliver better dynamic efficiency than perfect competition. A collusive oligopoly behaves like a monopoly — static welfare loss.

AO4 Evaluation Toolkit

Evaluative ArgumentDevelopment

Compete or collude?Depends on: number of firms, cost similarity, price transparency, repeated interaction, legal deterrence. The answer is industry-specific.

Dynamic efficiencySupernormal profits + non-price competition → strong R&D incentives. May outweigh static allocative inefficiency.

ContestabilityEven a tight oligopoly may price competitively if the market is contestable (low sunk costs). The threat of entry disciplines behaviour.

Kinked demand limitationsExplains price rigidity but NOT how the original price was set. Does not explain price wars when they do occur.

CMA enforcementFines of 10% global turnover deter overt collusion — but tacit collusion (price leadership) is legal and harder to detect.

Common Errors — From Examiner Reports

Examiner Alert — Game Theory

Payoff matrix not logically consistent — the marker checks: if one firm deviates, the other must be worse off. A matrix that doesn't demonstrate the prisoner's dilemma correctly scores maximum 3/4.

Not identifying the Nash equilibrium — you must state which cell is the Nash equilibrium AND explain why neither firm would deviate.

Confusing Nash equilibrium with the best outcome — Nash equilibrium is often the WORST joint outcome (that's the dilemma).

Examiner Alert — Kinked Demand

Forgetting the MR discontinuity — the vertical gap in MR at Q* is the whole point of the model

Saying oligopoly always has sticky prices — price wars DO happen; the model doesn't explain them

Not evaluating the model — always state it explains rigidity but not how the original price was determined

Practice Question

Question (25 marks — Paper 3 style): Evaluate whether oligopolistic firms are more likely to compete or collude. Use game theory in your answer.

1

Define oligopoly (few firms, high barriers, interdependence) and collusion (coordinating pricing/output to maximise joint profits).

2

KAA 1 — Case for collusion: Interdependence → price war = Nash equilibrium with low profits. Draw payoff matrix. Both firms earn more by colluding (High/High = £10m each vs Low/Low = £4m each). Tacit collusion via price leadership is legal and common. "As shown in the payoff matrix, the collusive outcome dominates the competitive equilibrium by £6m per firm."

3

KAA 2 — Case for competition: The prisoner's dilemma creates an incentive to cheat — if rival plays High, defecting to Low yields £14m vs £10m. Dominant strategy is to defect → collusion is inherently unstable. Non-price competition (branding, innovation) is preferred as price cuts are matched (kinked demand).

4

Evaluation: (i) Number of firms — fewer firms = more likely to collude (duopoly vs 6-firm oligopoly). (ii) Repeated interaction — firms that interact frequently use tit-for-tat → collusion is more stable. (iii) CMA enforcement — fines of 10% global turnover deter overt collusion but tacit collusion is harder to detect. (iv) Contestability — even a collusive oligopoly must keep prices below the level that would attract new entrants.

5

Conclusion: "The likelihood of collusion depends critically on market conditions. Tight oligopolies with similar costs and transparent pricing are more likely to sustain tacit collusion. However, even in these markets, the incentive to cheat and the threat of CMA intervention place limits on the duration and extent of collusion. The answer is therefore industry-specific — there is no universal prediction."

Summary

Oligopoly — Key Facts

ConceptDetail

CharacteristicsFew firms, high barriers, interdependence, differentiation

Kinked demandElastic above P*, inelastic below → MR discontinuity → price rigidity

Nash equilibriumNo firm can improve by unilaterally changing strategy

Prisoner's dilemmaRational self-interest → collectively worse outcome → incentive to collude

Overt collusionCartel — illegal. CMA fines up to 10% global turnover

Tacit collusionPrice leadership — legal but monitored

Collusion More Likely If

Few firms (tight oligopoly)

Similar cost structures

High price transparency

Repeated interaction

Competition More Likely If

Many firms

Different costs

Strong CMA enforcement

Market is contestable

The Golden Rule

Oligopoly is the highest-frequency essay topic on the A Level. Every oligopoly essay must include: (1) a payoff matrix or kinked demand diagram, (2) the concept of interdependence, and (3) evaluation of whether firms compete or collude — with conditions.