Different Market Structures

1. Why this topic matters

Market structure influences how much control a firm has over price, how easily rivals can enter, what demand curve the firm faces, whether supernormal profit can persist and how efficiently resources are used. The models are not boxes into which every real industry fits perfectly. They are analytical benchmarks.

The core discipline is to separate three stages:

1. Structure — the conditions in the market: number and size of firms, product differentiation, entry and exit, and information.
2. Conduct — how firms behave: output choice, pricing, advertising, innovation, collusion and strategic responses.
3. Performance — the resulting price, output, profit, efficiency, quality and consumer welfare.

A market with one large firm is not automatically uncompetitive. A market with many firms is not automatically competitive. Potential entry, sunk costs, product differentiation, buyer power, regulation and access to information can all alter behaviour.

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2. The market-structure spectrum

2.1 Market structure

A market structure describes the competitive conditions in a market. Cambridge requires comparison using:

• the number of buyers and sellers;
• product differentiation;
• the degree of freedom of entry and exit; and
• the availability of information.

Additional useful indicators include market concentration, economies of scale, sunk costs and the extent of strategic interdependence.

2.2 Perfect and imperfect competition

Perfect competition is the benchmark model with many buyers and sellers, a homogeneous product, free entry and exit, and complete information. Each firm is a price taker.

Imperfect competition covers markets in which at least one condition of perfect competition is absent. Cambridge names:

• monopoly;
• monopolistic competition;
• oligopoly; and
• natural monopoly.

Imperfect competition normally gives firms some market power, but the degree varies. A monopolistically competitive firm may have only limited power because many close substitutes exist. A monopoly may have substantial power if barriers are high and demand is inelastic.

2.3 Structure comparison

These are model predictions, not descriptions that must hold exactly in every observed market.

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3. Revenue curves and the profit-maximising rule

3.1 Demand and average revenue

For a firm, the demand curve is also its average revenue (AR) curve because:

AR = TR / Q = P

3.2 Perfect competition

A perfectly competitive firm can sell any feasible quantity at the market price but nothing at a higher price. Its firm-level demand curve is horizontal:

P = AR = MR

The market demand curve is still downward sloping. Do not confuse the horizontal demand curve facing one firm with the demand curve for the whole industry.

3.3 Imperfect competition

A price-making firm faces a downward-sloping AR curve. To sell more, it normally has to reduce price. For a single-price firm, the price cut applies to previous units as well as the extra unit, so:

MR lies below AR

The firm chooses the profit-maximising output where MC = MR, provided MC is rising through MR. It then reads the price from the AR curve at that output.

MC = MR identifies the output, not the price. Using the MR intersection as the selling price is a common exam error.

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4. Perfect competition

4.1 Assumptions

Perfect competition assumes:

• many small firms and buyers;
• a homogeneous product;
• complete information;
• free entry and exit;
• no individual firm able to influence market price; and
• firms seeking to maximise profit.

The model is valuable mainly as a benchmark. Real markets may approach some conditions without satisfying all of them.

4.2 Firm and industry

The industry determines the market price through market demand and market supply. Each individual firm takes that price as given. The firm produces where:

MC = MR = P, subject to the shutdown rule.

4.3 Short-run profit positions

In the short run, a competitive firm can make:

• supernormal profit if P > ATC at the profit-maximising output;
• normal profit if P = ATC;
• subnormal profit if AVC ≤ P < ATC; or
• shut down if P < AVC at every positive output.

A firm making a short-run loss may continue because revenue covers all variable costs and contributes something towards fixed costs.

4.4 Long-run adjustment

If firms earn supernormal profit, entry increases market supply and pushes market price down. If firms make persistent losses, exit reduces market supply and pushes price up. Under the standard assumptions, long-run equilibrium occurs where the representative firm earns normal profit.

For identical firms in long-run competitive equilibrium:

P = MR = MC = minimum ATC

This gives:

• allocative efficiency because P = MC; and
• productive efficiency because output is produced at minimum ATC.

The conclusion depends on the model assumptions. Externalities, imperfect information or heterogeneous costs can prevent the private-market outcome from being socially efficient.

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5. Shutdown, exit and the competitive firm’s supply curve

5.1 Short-run shutdown

The firm compares revenue with avoidable variable cost.

• If P > AVC, production contributes towards fixed cost.
• If P = minimum AVC, the firm is at the short-run shutdown price and is indifferent between producing at the relevant output and shutting down.
• If P < minimum AVC, the firm should shut down in the short run because producing adds more to variable cost than to revenue.

Shutting down means producing zero temporarily. It does not necessarily mean leaving the industry permanently.

5.2 Long-run exit

In the long run all costs are avoidable. The firm must cover total economic cost, including normal profit.

• The long-run break-even or exit threshold is minimum ATC in the standard model.
• If expected price remains below minimum ATC, the firm exits.
• If price equals minimum ATC, it earns normal profit and can remain.

It is clearer to call minimum AVC the short-run shutdown price and minimum ATC the long-run break-even/exit price.

5.3 Deriving supply

For each possible market price, a competitive firm chooses the output where P = MC, provided price is at least minimum AVC. Therefore:

• the firm's short-run supply curve is the rising part of MC above minimum AVC;
• the firm's long-run supply response is associated with MC above the long-run break-even condition, while industry entry and exit also change the number of firms.

The whole MC curve is not the short-run supply curve because the segment below minimum AVC would tell the firm to produce when shutdown is cheaper.

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6. Monopolistic competition

6.1 Structure

Monopolistic competition combines:

• many firms;
• product differentiation;
• relatively low barriers to entry and exit; and
• some information imperfections.

Examples may include local restaurants, salons or specialised retailers, but the classification depends on the market definition.

6.2 Firm demand

Each firm faces a downward-sloping demand/AR curve because its product is differentiated. Demand is often relatively elastic because many substitutes exist. Successful branding may shift demand right or reduce its elasticity.

6.3 Short run

The firm chooses MC = MR and can earn supernormal, normal or subnormal profit. Product differentiation does not guarantee profit.

6.4 Long run

Supernormal profit attracts entry. New substitutes reduce the demand facing each incumbent firm until normal profit remains in the standard long-run model. The long-run AR curve is tangent to ATC at the output where MC = MR.

Long-run monopolistic competition is normally:

• not productively efficient, because output is below the output at minimum ATC, creating excess capacity;
• not allocatively efficient, because P > MC; but
• potentially beneficial through variety, convenience, service and innovation.

Product differentiation may raise costs, but consumers may value the extra choice. Efficiency evaluation should not treat all differentiation or advertising as waste.

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7. Monopoly

7.1 Structure

A pure monopoly has one seller in the relevant market, no close substitute and high barriers to entry. The firm and industry are the same.

Real-world legal definitions of monopoly vary by country and purpose. For syllabus analysis, focus on market power and barriers rather than memorising a particular statutory market-share threshold.

7.2 Output, price and profit

The monopolist faces the market demand curve, so AR slopes downward and MR lies below AR. A profit-maximising monopolist:

1. chooses output where MC = MR;
2. charges the price shown on AR at that output; and
3. compares AR with ATC to determine profit or loss.

Monopoly does not guarantee supernormal profit. Weak demand or high cost can produce normal profit or loss. Strong barriers merely allow supernormal profit to persist if it exists.

7.3 Performance

Compared with a perfectly competitive benchmark using the same cost conditions, a profit-maximising monopoly commonly has:

• higher price;
• lower output;
• P > MC and allocative inefficiency;
• possible productive inefficiency;
• possible supernormal profit; and
• a deadweight welfare loss.

However, the comparison is conditional. A monopolist may achieve economies of scale, finance research and development, provide network infrastructure or innovate. Market power may also reduce pressure to control cost and create X-inefficiency.

7.4 Monopoly and dynamic efficiency

Supernormal profit can fund innovation, but it does not ensure innovation. The incentive depends on contestability, the threat of technological replacement, patent protection, management objectives and the ability to appropriate returns.

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8. Natural monopoly

8.1 Meaning

A natural monopoly exists where one firm can supply the relevant market demand at a lower average cost than two or more firms. It normally arises when:

• fixed and sunk infrastructure costs are very high;
• marginal costs are relatively low; and
• LRAC continues to fall across the relevant range of market demand.

Duplicating networks such as pipes, rails or local distribution infrastructure may therefore raise total cost.

Natural monopoly is a cost condition, not simply a firm protected by law and not simply any very large firm.

8.2 Pricing problem

If LRAC is falling, MC may lie below AC.

• Marginal-cost pricing sets P = MC and can achieve allocative efficiency, but the firm may make a loss because P < AC.
• Average-cost pricing sets P = AC, allowing normal profit, but gives a higher price and lower output than marginal-cost pricing.
• Unregulated profit maximisation sets MC = MR and may produce an even higher price and lower output.

The detailed evaluation of regulation belongs mainly to Topic 8.1, but this cost-price tension is essential to understanding natural monopoly.

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9. Oligopoly

9.1 Structure

An oligopoly is dominated by a few large firms. Products may be homogeneous or differentiated. Barriers are usually significant and concentration is high.

9.2 Interdependence

Interdependence is the defining behavioural feature. A firm's profit depends not only on its own decision but also on rivals' reactions. A price cut may win market share, trigger matching cuts or start a price war. An advertising campaign may provoke rival spending.

There is no single universal oligopoly equilibrium model. Outcomes range from aggressive rivalry to tacit coordination or explicit collusion.

9.3 Price rigidity

Prices may be relatively stable when firms fear rivals will match price cuts but not price rises. This is sometimes represented by a kinked-demand model, but the current 7.6 requirement focuses more broadly on interdependence, price and non-price competition, collusion and the Prisoner's Dilemma. The kinked curve is not a complete theory of how the original price is chosen.

9.4 Oligopoly performance

Oligopolies may:

• achieve economies of scale;
• invest heavily in research, design and advertising;
• provide wide product choice;
• earn persistent supernormal profit;
• create barriers strategically;
• charge prices above marginal cost; or
• suffer X-inefficiency.

The outcome depends on rivalry, entry conditions and regulation.

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10. Barriers to entry and exit

Cambridge groups barriers as legal, market, cost and physical barriers. One barrier may fit more than one category, so explain the mechanism rather than merely naming a label.

10.1 Legal barriers

Examples include:

• patents and copyright;
• exclusive licences;
• statutory monopoly rights;
• planning restrictions; and
• regulatory approval requirements.

Legal rules may protect innovation or safety, but they can also restrict entry.

10.2 Market barriers

Examples include:

• strong brand loyalty;
• control of distribution channels;
• network effects;
• customer switching costs;
• exclusive contracts; and
• strategic product proliferation.

The barrier arises from access to customers or demand rather than purely from production cost.

10.3 Cost barriers

Examples include:

• economies of scale enjoyed by incumbents;
• large research and development spending;
• expensive advertising needed to establish a brand;
• lower finance costs for established firms; and
• sunk setup expenditure.

A new entrant may have to enter at large scale to match incumbent unit costs, increasing the loss if entry fails.

10.4 Physical barriers

Examples include:

• control of scarce land, raw materials or infrastructure;
• limited airport slots, spectrum or retail sites;
• geographical isolation; and
• technically indivisible networks.

10.5 Barriers to exit

Barriers to exit can include:

• sunk costs;
• long contracts;
• redundancy liabilities;
• environmental cleanup obligations;
• specialised assets with little resale value; and
• reputational or strategic costs.

High exit barriers can deter entry because firms anticipate difficulty recovering resources if the venture fails.

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11. Short-run and long-run performance across structures

The short run and long run still matter in monopoly and oligopoly because costs and capacity can change over time. Barriers mean entry does not automatically remove supernormal profit; they do not abolish the distinction between time periods.

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12. Efficiency and X-inefficiency

12.1 Productive efficiency

A firm is productively efficient when it produces at the minimum attainable average cost for the relevant output and technology. In the standard long-run perfect-competition model, firms operate at minimum ATC.

Monopolistic competition usually has excess capacity. Monopoly and oligopoly may or may not operate at minimum ATC.

12.2 Allocative efficiency

Allocative efficiency requires P = MC where price and marginal cost correctly represent marginal social benefit and marginal social cost. Perfect competition reaches P = MC in the model. Imperfectly competitive firms normally charge P > MC.

12.3 Dynamic efficiency

Market power can generate resources for innovation, while rivalry can generate pressure to innovate. Dynamic efficiency cannot be inferred simply from the current profit level or number of firms.

12.4 X-inefficiency

X-inefficiency occurs when a firm operates above the minimum cost achievable for its chosen output because internal slack, weak incentives, poor management or organisational problems raise cost.

It is not the same as:

• allocative inefficiency;
• diseconomies of scale; or
• producing at an output different from minimum ATC because demand is limited.

Competitive pressure may reduce X-inefficiency, but even a monopolist can control costs if regulation, management incentives or potential entry are strong.

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13. Contestable markets

13.1 Meaning

A contestable market is one in which entry and exit are sufficiently easy that potential competition constrains incumbent firms. Perfect contestability assumes:

• no or very low barriers to entry and exit;
• no significant sunk costs;
• entrants can access the same technology and input prices as incumbents; and
• entry can occur rapidly before incumbents can retaliate successfully.

13.2 Hit-and-run entry

If incumbents charge a price that yields supernormal profit, an entrant may enter, serve the market and leave without unrecoverable cost. The threat can push even a single incumbent towards lower prices and efficient costs.

13.3 Implications

Contestability suggests that the threat of entry may matter more than the current number of firms. A concentrated market can behave competitively if entry is credible. Conversely, a market with several firms may be weakly contestable if sunk costs and customer lock-in are high.

13.4 Limits

Real markets are rarely perfectly contestable. Incumbents may have brand loyalty, capacity advantages, data, network effects, long-term contracts or the ability to retaliate before entry costs are recovered.

Contestability is a set of conditions, not a separate market structure in the same sense as monopoly or oligopoly.

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14. Price and non-price competition

14.1 Price competition

Price competition includes:

• direct price cuts;
• temporary promotions;
• discounts;
• lower subscription fees; and
• more favourable credit terms.

It may raise consumer surplus and market share but can reduce margins or trigger a price war.

14.2 Non-price competition

Non-price competition includes:

• product quality and design;
• branding and advertising;
• customer service;
• location and convenience;
• delivery speed;
• warranties;
• loyalty programmes; and
• innovation.

It is especially important where products are differentiated and firms fear that price cuts will be copied.

14.3 Evaluation

Non-price competition can provide valued quality and information, but persuasive advertising or artificial differentiation may raise cost and barriers. Price competition can benefit consumers, but predatory conduct and unsustainable price wars can reduce future competition. Pricing policies such as limit pricing, predatory pricing and price leadership are treated in Topic 7.8.

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15. Collusion and the Prisoner's Dilemma

15.1 Collusion

Collusion occurs when firms coordinate rather than compete independently. It may be:

• explicit/formal; or
• tacit, based on mutual understanding without a written agreement.

Coordination may concern price, output, market sharing or bidding. Collusion tends to raise joint profit by restricting competition, but it can harm consumers through higher prices and lower output.

15.2 Why collusion is unstable

Each member may have an incentive to cheat by cutting price or expanding output while others continue to cooperate. Collusion is more stable when:

• there are few firms;
• products and costs are similar;
• demand is stable;
• behaviour is easy to observe;
• interaction is repeated; and
• entry barriers are high.

15.3 A two-player payoff matrix

Suppose two firms choose High price or Low price. Payoffs are profits:

For each firm, Low gives a higher payoff whatever the rival chooses:

• if the rival chooses High, Low gives 11 rather than 8;
• if the rival chooses Low, Low gives 4 rather than 2.

Therefore Low is each firm's dominant strategy and the non-cooperative outcome is Low, Low with payoffs 4,4. Both would be better off jointly at High, High with 8,8, but each has an individual incentive to undercut.

This is the Prisoner's Dilemma: individually rational actions can produce a jointly inferior outcome.

15.4 Reading payoff matrices

Always:

1. identify whose payoff is written first;
2. compare one player's payoffs while holding the rival's action fixed;
3. identify dominant strategies if they exist;
4. locate the outcome produced by those strategies; and
5. compare it with the joint-profit maximum.

Do not add the two payoffs until individual incentives have been analysed.

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16. Concentration ratios

16.1 Definition

An n-firm concentration ratio is the combined market share of the largest n firms:

n-firm concentration ratio = sum of market shares of the largest n firms

If the four largest firms have shares of 28%, 22%, 14% and 10%:

CR4 = 28 + 22 + 14 + 10 = 74%

16.2 Interpretation

A higher concentration ratio suggests that a larger share of the market is controlled by leading firms. It may indicate oligopoly or monopoly power, but concentration is not identical to competitiveness.

16.3 Limitations

A concentration ratio can mislead because it depends on:

• how the product market is defined;
• whether the market is local, national or global;
• whether imports are included;
• whether shares are measured by units, revenue or capacity;
• the distribution of shares within the top n firms;
• buyer power;
• entry conditions; and
• changes over time.

A CR4 of 80% could describe four firms with 20% each or one firm with 70% and three with small shares. The strategic implications may differ.

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17. Integrated comparison and evaluation

A strong Paper 4 answer does not rank market structures mechanically. It asks:

• How strong are entry and exit barriers?
• Are economies of scale substantial?
• Is demand price elastic?
• Are products differentiated?
• Is the market contestable?
• Is innovation important?
• Are there externalities or information failures?
• Do firms compete or collude?
• How is the market defined?
• What happens in the short run compared with the long run?

17.1 Competition versus scale

More firms may increase competitive pressure but prevent the exploitation of scale economies. A natural monopoly may minimise production cost with one network, yet require regulation to protect consumers.

17.2 Static versus dynamic efficiency

Perfect competition performs strongly in the standard static model. Markets with some power may finance innovation, but protected profits can also reduce effort. The best judgement depends on evidence about actual innovation incentives and contestability.

17.3 Profit versus welfare

Supernormal profit is not itself proof of exploitation. It may reflect innovation, superior efficiency or temporary success. Persistent profit combined with high barriers, P well above MC, weak quality and little innovation provides stronger evidence of market power harming welfare.

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18. Exam traps

1. Using MC = MR to find price. It finds output; price is read from AR.
2. Treating monopoly profit as guaranteed. Barriers allow persistence, but demand and cost determine whether profit exists.
3. Calling the whole MC curve a competitive supply curve. The short-run supply segment is MC above minimum AVC.
4. Confusing shutdown with exit. Shutdown is a short-run zero-output decision; exit is long-run departure.
5. Saying monopolistic competition is productively efficient in the long run because it earns normal profit. Normal profit does not imply minimum ATC.
6. Equating concentration with collusion. High concentration can facilitate coordination but does not prove it.
7. Treating a natural monopoly as any legally protected monopoly. Natural monopoly is a cost condition.
8. Assuming many firms means contestability. Contestability concerns entry, exit and sunk cost.
9. Adding payoff-matrix profits before checking individual incentives. Analyse each player first.
10. Assuming advertising is always wasteful. It can inform, differentiate and intensify competition, though it may also create barriers.

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19. Summary

• Market structure is analysed through sellers/buyers, differentiation, entry/exit and information.
• Perfect competition gives a horizontal firm demand curve and long-run normal profit under free entry and exit.
• Monopolistic competition combines many firms with differentiated products and long-run excess capacity.
• Monopoly and oligopoly can sustain market power when barriers are high, but profit and innovation outcomes are not automatic.
• Natural monopoly is defined by cost conditions over the relevant market demand.
• Short-run shutdown depends on AVC; long-run exit depends on total cost.
• The competitive firm\'s short-run supply curve is rising MC above minimum AVC.
• Contestability emphasises potential entry and low sunk cost.
• Oligopoly requires strategic analysis, including price/non-price competition, collusion and the Prisoner's Dilemma.
• Concentration ratios are useful indicators but require careful market definition and interpretation.