# Oligopoly

> CIE A-Level Economics · The Price System and the Microeconomy
> Source: https://www.owlsprep.com/study/cie-9708-u2-oligopoly/

This module covers core characteristics of oligopoly, key models of collusive and non-collusive firm behavior, game theory applications, and efficiency evaluation of oligopoly markets, aligned to CIE A-Level assessment objectives.

**Prerequisites:** [Perfect competition and monopoly market structures](https://www.owlsprep.com/study/cie-9708-u2-monopoly/); [Profit maximization and marginal analysis](https://www.owlsprep.com/study/cie-9708-u2-firm-costs-revenue/)

## Learning objectives

- Explain the core characteristics of oligopoly as a market structure
- Analyze the kinked demand curve model of non-collusive oligopoly
- Evaluate collusive oligopoly and price leadership models
- Apply game theory and the Prisoner's Dilemma to oligopoly strategic behavior
- Assess the efficiency outcomes of oligopoly markets

## Core Characteristics of Oligopoly

**Oligopoly** — A market structure dominated by a small number of large firms, with high barriers to entry and interdependence of firm decision-making.

*Example:* Examples include the global oil market, national supermarket sectors, and the global car manufacturing industry.

The defining feature of oligopoly that sets it apart from other market structures is **interdependence**: each firm’s pricing, output, and marketing decisions directly impact the profits of rival firms, so all firms must anticipate how rivals will respond to their choices.

- High barriers to entry (economies of scale, legal barriers, brand loyalty, sunk costs)
- Products can be homogeneous (e.g. steel, oil) or differentiated (e.g. soft drinks, cars)
- Non-price competition is common (advertising, product innovation, loyalty schemes)
- Imperfect information about rival costs and demand
- Strong incentive to collude to reduce uncertainty and raise profits

**Worked example:** A new coffee chain wants to enter the UK coffee market. Identify two barriers to entry it would face as a new oligopoly firm.

1. First, existing chains like Starbucks and Costa have strong brand loyalty built up from decades of advertising. Many customers will stick to the brands they know, so the new chain will need to spend large sums on advertising to attract customers, which is a major sunk cost barrier.
2. Second, existing chains benefit from internal economies of scale in purchasing, distribution, and store operations. They can offer lower prices than a new small entrant, making it hard for the new chain to compete on price.

> **Exam tip:** In any answer about oligopoly, always highlight interdependence first — it is the most commonly expected characteristic for marking points.

## Kinked Demand Curve Model

**Kinked Demand Curve** — A model of non-collusive oligopoly that assumes firms match price cuts by rivals but do not match price increases. This creates a kink in the firm's demand curve at the current market price, leading to sticky (stable) prices.

*Example:* Explains why petrol prices stay stable for weeks even when crude oil prices change slightly.

The kink creates two distinct sections of the demand curve: above the kink, demand is very price elastic (firms lose a lot of customers if they raise prices, because rivals do not follow). Below the kink, demand is very price inelastic (firms gain almost no new customers if they cut prices, because all rivals cut prices too to match).

This gap in the marginal revenue (MR) curve means that changes in marginal cost (MC) that fall within the gap will not change the profit-maximizing price or output.

**Worked example:** An oligopoly firm has a current equilibrium price of \textsterling 1.50 per litre of petrol, with a MR gap between \textsterling 0.30 and \textsterling 0.70. If marginal cost increases from \textsterling 0.40 to \textsterling 0.60, explain why the profit-maximizing price will not change.

1. Profit maximization for any firm occurs where $MR = MC$. The kink at the current price creates a vertical gap in the MR curve between \textsterling 0.30 and \textsterling 0.70.
2. Original MC of \textsterling 0.40 falls within the MR gap, so profit-maximizing output stays at the level corresponding to the kink, with price \textsterling 1.50.
3. After the cost increase, new MC of \textsterling 0.60 still falls within the MR gap. The profit-maximizing condition $MR > MC$ for all units up to the kink, and $MC > MR$ for all units beyond the kink, still holds.
4. So the firm does not change its output or price: prices remain sticky.

> **Exam tip:** Always label the gap in the marginal revenue curve in diagram questions — this is the most commonly missed marking point.

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## Collusive Oligopoly and Price Leadership

**Collusion** — An agreement between oligopoly firms to limit competition to increase joint profits. Formal collusion (a cartel) is a written, public agreement, while tacit collusion is an informal, unwritten agreement, common because formal collusion is illegal in most countries.

*Example:* OPEC is a well-known formal cartel that coordinates oil output levels to set global oil prices.

The most common form of tacit collusion is **price leadership**, where one dominant firm (usually with the largest market share) sets the market price, and all smaller firms follow the price. This avoids price wars and keeps prices high for all firms.

**Worked example:** A banking market has one large dominant bank with 65% market share, and 7 small banks with 5% each. Explain how price leadership works in this market.

1. The dominant bank calculates its profit-maximizing price based on its own marginal costs and residual demand (the demand left after all small banks supply their desired output at the chosen price).
2. Small banks have no incentive to set a lower price than the dominant bank: if they do, the dominant bank will cut its price further, triggering a price war that will drive the smaller banks out of the market.
3. Small banks also have no incentive to set a higher price: if they do, they will lose almost all their customers to the dominant bank, which offers the same service at a lower price.
4. All small firms follow the dominant bank's price, so the market has stable, high prices and all firms earn higher profits than they would under competition.

> **Exam tip:** For 15-mark essays, always evaluate cartel stability: members have a strong incentive to cheat on output quotas, so most cartels break down over time.

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## Game Theory and Prisoner's Dilemma

**Prisoner's Dilemma** — A game theory model that demonstrates why two rational, self-interested oligopoly firms will fail to cooperate (collude), even when cooperation would leave both firms better off than non-cooperation.

Game theory is used to model strategic interaction between oligopoly firms, since each firm's profit (payoff) depends on both its own choice and the choice of its rival. The Prisoner's Dilemma explains why collusion is hard to sustain even without legal barriers.

**Worked example:** Two competing supermarkets can choose to set a high price or low price for bread. Payoffs (annual profit from bread) are: both high = \textsterling 10m each; both low = \textsterling 5m each; one high one low = low price gets \textsterling 15m, high price gets \textsterling 2m. Find the Nash equilibrium.

1. Find the dominant strategy (best choice regardless of rival's choice) for Supermarket A:
2. If Supermarket B chooses high price: A gets \textsterling 10m from high, \textsterling 15m from low → A will choose low.
3. If Supermarket B chooses low price: A gets \textsterling 2m from high, \textsterling 5m from low → A still chooses low.
4. Do the same for Supermarket B: B's dominant strategy is also low price.
5. The Nash equilibrium is both firms choose low price, each earning \textsterling 5m, even though both would earn higher profit (\textsterling 10m each) if they both chose high price. This is the dilemma.

> **Exam tip:** Always label payoffs for each player when drawing a game matrix, and explicitly state both the dominant strategy and Nash equilibrium in exams.

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## Common pitfalls

- **Wrong:** Claiming oligopoly always has differentiated products.
  - Why it fails: Product differentiation is not a defining feature of oligopoly.
  - Correct: State that the defining feature of oligopoly is interdependence of firms, and that products can be homogeneous or differentiated.
- **Wrong:** Shifting the entire kinked demand curve when marginal costs change.
  - Why it fails: The kink is positioned at the current market price; the demand curve only shifts if overall market demand changes or the equilibrium price changes.
  - Correct: Leave the kinked demand curve and kink position unchanged if marginal cost changes within the marginal revenue gap.
- **Wrong:** Claiming all collusion is formal cartels.
  - Why it fails: Formal collusion is illegal in most countries, so the majority of collusion in oligopoly is tacit (informal, unwritten).
  - Correct: Explicitly distinguish between formal cartels and tacit collusion such as price leadership in your answers.
- **Wrong:** Claiming the Prisoner's Dilemma outcome is collusion between firms.
  - Why it fails: The whole point of the model is to show that rational self-interest leads to non-cooperation.
  - Correct: Explain that the Nash equilibrium is non-cooperation, where both firms are worse off than the cooperative collusive outcome.
- **Wrong:** Stating oligopoly is always as inefficient as monopoly.
  - Why it fails: Oligopoly has competitive pressure between firms that incentivizes innovation, unlike monopoly.
  - Correct: Evaluate oligopoly: it is less efficient than perfect competition but often more efficient and innovative than monopoly.

## Cheatsheet

| Concept | Key Point | Exam Tip |
| --- | --- | --- |
| Core Oligopoly Feature | Interdependence of firms | Always mention this first |
| Kinked Demand Curve | Prices are sticky if MC is in MR gap | Label the MR gap in diagrams |
| Formal Collusion | Cartel = joint profit maximization | OPEC is the standard example |
| Tacit Collusion | Price leadership by dominant firm | Most common form of collusion |
| Prisoner's Dilemma | Nash equilibrium = non-cooperation | Dominant strategy = cheat/undercut |
| Oligopoly Efficiency | Higher price, lower output than perfect competition; more innovation than monopoly | Always evaluate in essays |

## What's next

Oligopoly is the final core market structure in the Price System unit, and it builds on your understanding of imperfect competition and strategic firm behavior. The market power of oligopoly firms creates a range of market failures, from higher prices to reduced output, which is why government intervention to regulate oligopolies is a common follow-on topic. Game theory concepts from this module are also applied to other advanced microeconomics topics like contestable markets and behavioral economics. Mastery of oligopoly is essential for both multiple choice and essay questions in Paper 1 and Paper 2.

- [Monopolistic Competition](https://www.owlsprep.com/study/cie-9708-u2-monopolistic-competition/)
- [Resource Allocation in Different Market Structures](https://www.owlsprep.com/study/cie-9708-u2-resource-allocation-in-different-market/)
- [Government Microeconomic Intervention](https://www.owlsprep.com/study/cie-9708-u3-overview/)

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