Chapter 4 — Firms, Market Power, and Market Failure

Core question

When does decentralised exchange coordinate resources well, and when does the outcome leave unrealised gains or unfairly assigned costs?

Learning outcomes

You will be able to:

• connect marginal cost and revenue to a firm's output decision;
• compare market structures without treating them as rigid labels;
• diagnose market power, externalities, public goods, common resources, and information problems;
• match a failure to a policy mechanism and a government-failure risk.

1. Begin with the firm's decision

An output increase is worthwhile while marginal revenue exceeds marginal cost. The interior profit-maximising rule is:

High revenue does not imply high profit; a low marginal cost does not erase a large fixed cost.

2. Market structure is a diagnostic, not a ranking

A monopoly cannot charge any price it wants: demand constrains sales. A competitive firm can earn accounting profit while earning zero economic profit after all opportunity costs in long-run equilibrium.

3. Worked example: monopoly output

Suppose inverse demand and marginal cost are:

Total revenue is TR=(10-Q)Q, so:

Set MR=MC:

Under the simple competitive benchmark P=MC, quantity would be 8 and price 2. Monopoly restricts quantity, raises price, transfers some surplus, and removes some mutually beneficial trades.

This comparison assumes the same cost and product. Patents or scale may also create innovation or cost benefits; policy must examine both static loss and dynamic incentives.

4. Oligopoly adds strategic interaction

Two delivery platforms can keep a high fee or cut it:

Each may cut to avoid losing users, even though both would earn more if both kept fees high. Repeated interaction, capacity, switching costs, and credible entry change the game.

The table is a model of incentives, not evidence that firms colluded.

5. Digital platforms do not fit by seller count alone

Platforms may combine:

• network effects: value rises with participation;
• economies of scale and scope;
• zero-price services on one side and fees on another;
• data advantages and learning effects;
• switching costs, defaults, and ecosystem lock-in;
• a gatekeeper role while competing with business users.

Market definition, multi-sided pricing, entry, interoperability, and control of complementary services matter more than simply counting apps.

OECD analysis of G7 digital markets distinguishes conventional case-by-case competition enforcement from newer ex-ante rules for large platforms. The coexistence of both approaches reflects a diagnosis that some conduct may be difficult to remedy only after harm occurs (OECD, 2024).

6. Five common market failures

Do not prescribe “government intervention” before naming the missing price, right, information, or competitive constraint.

7. Externality worked example

Suppose each sugary drink creates a private marginal cost of £1 and an expected external health-system cost of £0.30 not reflected in the buyer–seller decision.

A £0.30 corrective tax can align the private price with marginal social cost in the textbook model. In practice, the correct rate, pass-through, substitution, administration, distribution, and health mechanism must be estimated.

The evidence in Chapter 3 shows observed prices and sales changed after real taxes, but does not by itself prove the socially optimal tax rate.

8. Match policy to diagnosis

Use this chain:

failure → behavioural mechanism → policy lever → predicted response
→ distribution → implementation → evidence → government-failure risk

Example: campus delivery platform

Problem: restaurants depend on one platform and cannot take customer data elsewhere.

Possible diagnoses:

• market power from network effects;
• switching and multi-homing costs;
• information imbalance over ranking;
• congestion externality from delivery traffic.

One fee cap cannot solve all four. Interoperability targets switching; ranking disclosure targets information; congestion pricing targets traffic; competition enforcement targets exclusionary conduct.

9. Government can fail too

Policy may suffer from:

• weak information about costs and behaviour;
• capture by regulated firms;
• inflexible rules as technology changes;
• administrative and compliance cost;
• unintended substitution or evasion;
• distributional harm hidden by an average benefit.

The relevant comparison is imperfect market versus feasible policy—not ideal government versus real market.

Practice: a policy hearing

Choose one case: social media, rented housing, antibiotics, urban traffic, or fisheries.

1. Define the market and affected non-participants.
2. Diagnose one primary failure.
3. Draw or state the private equilibrium mechanism.
4. Propose one policy that targets the failure directly.
5. Predict one behavioural response and one distributional effect.
6. Name one government-failure risk and evidence for evaluation.

Quick check

• Market power is constrained by demand and potential entry.
• MR=MC selects monopoly output; demand determines its price.
• Seller count alone is insufficient for digital markets.
• Every market failure has a specific missing mechanism.
• Policy quality depends on targeting, behaviour, implementation, and evidence.

Next: move from individual markets to national accounts.