Cambridge A Level Economics 9708
Market failure and externalities
Market failure is when a free market, left to itself, fails to allocate resources efficiently — producing too much of some goods, too little of others, or none at all. The causes on the Cambridge 9708 syllabus are externalities, public goods, merit and demerit goods, information failure and market power, and each has a matching set of government interventions with their own risk of government failure. The benchmark throughout is allocative efficiency: output where marginal social benefit equals marginal social cost.
Sections 7 and 8 of the A2 syllabus — where market failure and its correction live — account for 52% of the Paper 3 questions across the 21 sittings Quanta has mapped (329 of 630). This page gives the concepts, the externality diagram the questions are drawn from, the interventions, and original questions in Paper 3’s shape.
Updated 15 September 2026
The benchmark: MSB = MSC
A market is allocatively efficient when the last unit produced is worth to society exactly what it costs society: marginal social benefit equals marginal social cost. In a market with no externalities, private and social values coincide — MPB = MSB and MPC = MSC — so the demand and supply curves are the social curves and the equilibrium is efficient. Market failure is a gap between private and social, or a missing market altogether.
Externalities and the diagram
An externality is a cost or benefit of production or consumption that falls on a third party outside the transaction, and is therefore not in the price. Four cases:
- Negative production (pollution from a factory): MSC above MPC. The market over-produces.
- Negative consumption (smoking, congestion): MSB below MPB. The market over-consumes.
- Positive production (a firm training workers who move to other firms): MSC below MPC. Under-produced.
- Positive consumption (vaccination, education): MSB above MPB. Under-consumed.
The vertical distance between MSC and MPC is the marginal external cost. The market produces Qm at price Pm; the social optimum is the smaller Q* at the higher P*. Every unit between Q* and Qm has a social cost (on MSC) above its social benefit (on D), and the triangle between the two curves over that range is the welfare loss. A tax equal to the marginal external cost at Q* shifts MPC up to MSC and removes it — the tax internalises the externality.
Every other case is the same picture with a different curve moved. For a positive consumption externality, MSB sits above MPB, the market under-consumes, and the welfare loss is the triangle between MSB and MSC over the range of units not consumed; a subsidy equal to the marginal external benefit corrects it.
Public goods
A public good is non-excludable — once provided, nobody can be prevented from using it — and non-rival — one person’s use does not reduce another’s. Street lighting, national defence, flood defences. Non-excludability creates the free-rider problem: no one will pay for what they can use without paying, so a private firm cannot charge and the market provides nothing. This is the missing market — the purest market failure — and the standard remedy is government provision funded by taxation.
The exam distinguishes public goods from quasi-public goods (roads: non-rival until congested, excludable by tolling) and from merit goods, which are excludable and rival but under-consumed. A good being provided by the government does not make it a public good; the test is the two characteristics.
Merit, demerit goods and information failure
A merit good is under-consumed because consumers undervalue its benefits — education, health screening — often because the benefits are long-term or poorly understood. A demerit good is over-consumed because consumers underestimate its costs — tobacco, gambling. Both are cases of information failure: the private benefit consumers perceive differs from the true private benefit, before any externality is counted. They frequently carry externalities as well, which is why they are the standard example of both.
Two further sources the syllabus expects: asymmetric information, where one party to a transaction knows more than the other (a used-car seller, an insurance applicant), and factor immobility and market power, which prevent resources moving to their most valued use.
Government intervention
| Instrument | Corrects | Limit |
|---|---|---|
| Indirect tax | Negative externalities, demerit goods — raises MPC to MSC. | Needs the external cost valued; regressive; inelastic demand blunts it. |
| Subsidy | Positive externalities, merit goods — lowers cost, raises consumption. | Cost to taxpayer; may be captured by producers. |
| Regulation and bans | Where a quantity limit is clearer than a price — emissions standards, age limits. | Enforcement cost; no incentive to go beyond the standard. |
| Tradable permits | Pollution — caps total quantity and lets the market price it. | Setting the cap; permits given free hand profit to polluters. |
| Direct provision | Public goods, merit goods — the state supplies. | No price signal; risk of over- or under-supply. |
| Information provision | Information failure — labels, campaigns, disclosure rules. | Slow; consumers may not respond. |
| Property rights | Externalities where ownership is undefined — the parties bargain. | Transaction costs; many affected parties. |
| Price controls | Maximum prices for essentials; minimum prices for demerit goods. | Shortages or surpluses; black markets. |
The questions pair an instrument with a failure and ask whether the match is right — a subsidy does not correct a negative externality; a tax cannot supply a public good — or give a diagram and ask for the size of the corrective tax, which is the marginal external cost at the optimum output, not at the market output.
Government failure
Intervention can leave the allocation worse than the market did. Governments lack the information to value an externality precisely, so a tax may be set too high or too low; policies have administrative and enforcement costs; regulations create unintended incentives; and decisions may serve political rather than economic ends. A question that asks whether intervention is justified expects both the failure and the failure of the fix.
How Paper 3 tests it
Three questions in the paper’s shape — Quanta’s own:
1A chemical plant discharges waste into a river used by a fishing business downstream. Which statement is correct?
- AMarginal social cost of chemical production exceeds marginal private cost ✓
- BMarginal private cost of chemical production exceeds marginal social cost
- CMarginal social benefit of chemicals exceeds marginal private benefit
- DThe market will produce less than the socially optimal output
The fishing business bears a cost not paid by the plant: a negative production externality. MSC lies above MPC and the market over-produces, so D is the opposite of what happens.
2Which of the following is a public good?
- Aa toll motorway
- Ba state-funded hospital
- Ca lighthouse ✓
- Da school
Only the lighthouse is both non-excludable (any ship can see it) and non-rival (one ship's use leaves as much for others). Hospitals and schools are merit goods that governments often provide; a tolled road is excludable.
3To correct a negative production externality using an indirect tax, the tax per unit should equal
- Athe marginal external cost at the market output
- Bthe marginal external cost at the socially optimal output ✓
- Cthe difference between market price and the socially optimal price
- Dthe total external cost divided by market output
The tax must raise MPC to MSC at the optimum, Q*, so it equals the vertical gap between the curves there. At the market output the gap is generally different, and the price difference (C) is not the same as the cost gap.
All 9708 Paper 3 sittings → — each with its split across the five A2 sections.
Common mistakes
1.Moving the wrong curve
Production externalities move the cost curve (MSC vs MPC); consumption externalities move the benefit curve (MSB vs MPB). Decide which activity generates the effect before drawing.
2.The welfare loss triangle in the wrong place
It always lies between the social curves, over the range of units between the market output and the optimum. For over-production that is to the right of Q*; for under-consumption, to the left.
3.Public good means 'provided by the government'
The test is non-excludability and non-rivalry. State schools are not public goods; a privately owned lighthouse would be.
4.The corrective tax measured at market output
It equals the marginal external cost at the socially optimal output. Read the gap between MSC and MPC above Q*, not above Qm.
5.Intervention assumed to work
Every instrument has an information or incentive problem — government failure. Questions about whether to intervene test that you know it.
Common questions
What is a negative externality?
A cost imposed on a third party by production or consumption that is not paid by the producer or consumer — pollution, congestion, passive smoking. It puts marginal social cost above marginal private cost (or social benefit below private benefit) so the market produces or consumes too much.
What is the difference between a public good and a merit good?
A public good is non-excludable and non-rival, so the market cannot charge for it and provides none. A merit good is excludable and rival — it can be sold — but is under-consumed because its benefits are undervalued. The government often provides both, for different reasons.
How does a tax correct an externality?
A per-unit tax equal to the marginal external cost at the optimum raises the producer’s marginal private cost to the marginal social cost. Output falls to the social optimum and the welfare loss is eliminated — the externality is internalised into the price.
What is government failure?
Intervention that leaves resource allocation less efficient than it was — because the external cost was mis-valued, the policy was costly to administer, it created perverse incentives, or it was chosen for political reasons.
Practise market failure and externalities against real mark schemes
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