Market failure is when the free market produces too much or too little of something, so resources are misallocated and welfare is lost. Externalities are the case every board examines first: a cost or a benefit that lands on someone who was not part of the deal. This page covers market failure (A Level Economics, all boards): the six cost and benefit terms, the two diagrams the exam wants with the welfare triangle in the right place, the four policy fixes and how to evaluate them, a worked example in pounds and three exam-style questions with the answers derived.
Spec map: where market failure sits on your board
| Board | Where it sits | Papers that test it |
|---|---|---|
| AQA 7136 | 4.1.8.2 the meaning of market failure, 4.1.8.4 positive and negative externalities in consumption and production, 4.1.8.9 government intervention in markets, 4.1.8.10 government failure | Paper 1, Paper 3 |
| Edexcel 9EC0 | Theme 1: 1.3.1 types of market failure, 1.3.2 externalities (both diagrams, welfare loss and welfare gain areas named), 1.4.1 government intervention, 1.4.2 government failure | Paper 1, Paper 3 |
| OCR H460 | Component 01: 2.8 market failure and externalities (“explain, with the aid of a diagram”), 2.11 government intervention and government failure | Component 01, Component 03 |
| Cambridge International 9708 | A Level 7.4 (externalities, deadweight welfare loss) and 8.1 (policies to correct market failure) | Papers 3 and 4 |
The numbers come from the current documents, read in September 2026: AQA v1.3, Edexcel Issue 2, OCR v3.0 and the Cambridge syllabus for 2026-2028. Price controls have their own page, maximum and minimum prices. Information gaps, merit and demerit goods are covered separately too.
What you must be able to do
- Define market failure and name the three main types: externalities, public goods and information gaps.
- Distinguish private, external and social costs, and private, external and social benefits.
- Draw the negative production externality diagram with MSC above MPC and the welfare-loss triangle.
- Draw the positive consumption externality diagram with MSB above MPB and the welfare-gain triangle.
- Identify the market equilibrium and the social optimum on both, and explain why they differ.
- Explain how an indirect tax, a subsidy, regulation and tradable permits move the market towards the social optimum.
- Evaluate each policy, and explain government failure.
Private, external and social: the six terms
Every mark scheme uses these six terms, and the diagram is just the six of them drawn as curves. Learn the two identities and the rest follows.
| Term | Meaning | Curve on the diagram | An exam example |
|---|---|---|---|
| Marginal private cost (MPC) | What the producer pays to make one more unit | The supply curve | Wages, materials, energy at a paper mill |
| Marginal external cost (MEC) | The cost of one more unit that falls on third parties | The gap between MSC and MPC | Chemicals in the river below the mill |
| Marginal social cost (MSC) | MPC + MEC: the full cost to society | A supply curve above MPC by the external cost | What the mill costs everyone |
| Marginal private benefit (MPB) | What the buyer gains from one more unit | The demand curve | Not getting flu this winter |
| Marginal external benefit (MEB) | The benefit of one more unit that falls on third parties | The gap between MSB and MPB | The people you would have infected |
| Marginal social benefit (MSB) | MPB + MEB: the full benefit to society | A demand curve above MPB by the external benefit | The whole town’s winter |
With no externality, MSC = MPC and MSB = MPB, so the market clears at the social optimum. An externality breaks one of the two identities, and the market clears in the wrong place.
The negative production externality diagram
Use it for pollution, congestion, noise, anything where making something hurts a third party. Start with an ordinary supply and demand diagram. Supply is MPC. Demand is MPB, and because the externality is in production, demand is also MSB. Now add the external cost on top of supply. That gives MSC, a second supply curve parallel to the first and above it by the external cost per unit.
The market settles at A, where MPB meets MPC. Society would prefer B, where MSB meets MSC. Between them the market over-produces by 5 units. For every one of those units the cost to society is above the benefit, and the triangle between MSC and demand from 20 to 25 units adds up that excess. That triangle is the welfare loss, also called the deadweight loss.
Mark scheme: For the diagram marks the examiner wants five things. Both axes labelled. MSC drawn above MPC with the gap named “external cost”. The free-market equilibrium where MPB = MPC. The social optimum where MSB = MSC. And the welfare-loss triangle identified between the two quantities. Shading is optional. Labelling is not.
Common mistake: Shading the whole gap between MSC and MPC as the welfare loss. It is not. The welfare loss is only the triangle between Q₂ and Q₁. The rest of the gap is external cost that society would still bear at the optimum, because the optimum is not zero output.
The positive consumption externality diagram
Use it for vaccines, education, public transport, anything where one person’s consumption helps someone else. This time supply is fine: MPC = MSC. The externality is on the demand side. Demand is MPB, and MSB sits above it by the external benefit per unit.
The market settles at A, where MPB meets MPC, at 20 units. Society wants B, where MSB meets MSC, at 25 units. The market under-consumes by 5 units. The triangle between MSB and MSC from 20 to 25 units is the welfare gain the market leaves on the table, and the gain a subsidy could capture.
Exam tip: Two rules decide which curve moves. A production externality moves the cost side: MSC away from MPC. A consumption externality moves the benefit side: MSB away from MPB. Negative means the social curve is above on the cost side or below on the benefit side. Positive means the opposite. Say which one you are drawing in the first line of the answer.
Worked example — a factory on a river
A paper mill sells its output in a competitive market and dumps waste in the river. Small round numbers, in pounds.
- Demand, which is MPB and MSB: P = 60 − Q.
- Supply, which is MPC: P = 10 + Q.
- The external cost is £10 a unit, so MSC: P = 20 + Q.
- Free market: 60 − Q = 10 + Q, so Q₁ = 25 and P₁ = £35. That is point A.
- Social optimum: 60 − Q = 20 + Q, so Q₂ = 20 and P₂ = £40. That is point B.
- Welfare loss: a triangle with base 25 − 20 = 5 units and height £10, so ½ × £10 × 5 = £25.
- Now fix it. A tax of £10 a unit lifts the mill’s supply curve onto MSC. The market moves from A to B on its own: 20 units at £40. The tax raises £10 × 20 = £200, and the £25 welfare loss disappears.
Change any number in step 1 to 3 and the whole chain moves. That is why examiners like a numerical version: it shows you understand the mechanism, not just the picture.
Fixing it: tax, subsidy, regulation and permits
Every policy tries the same trick: make the private curve equal the social one, so the market clears at B by itself. The evaluation is about what goes wrong on the way.
| Policy | How it works on the diagram | Works best when | Evaluation |
|---|---|---|---|
| Indirect tax (a negative externality) | Shifts MPC up by the tax. Set the tax equal to the external cost and the market lands on the social optimum. Raises revenue | The external cost per unit can be measured and demand is not too inelastic | If demand is inelastic the quantity barely falls and the burden lands on consumers. The true external cost is a guess. Regressive if the good is a big share of low-income budgets |
| Subsidy (a positive externality) | Shifts MPC down by the subsidy. Set it equal to the external benefit and consumption rises to the optimum. Consumers pay less, producers receive more | The external benefit is large and the good is under-consumed because of price | Costs the taxpayer subsidy times quantity. Some goes to people who would have bought anyway. If demand is inelastic, producers capture most of it |
| Regulation | A limit, a standard or a ban, so the quantity is forced to the optimum or below | The harm is severe enough that a price signal is too slow, or the cost cannot be measured | No revenue. Enforcement costs. A fixed limit ignores that firms face different abatement costs, so it is not the cheapest fix |
| Tradable pollution permits | A cap on the total externality, with permits firms can trade. The permit price is a market-made tax | Many firms with different costs of cutting pollution, such as carbon | Cheap abaters sell permits, expensive ones buy, so the cut happens where it is cheapest. Too many permits and the price collapses, as in the early EU scheme |
Key terms: Government failure is intervention that leaves welfare lower than before. The exam causes are information gaps, when the tax or cap is set at the wrong level; unintended consequences, such as a landfill tax that increases fly-tipping; administrative and enforcement costs; and distorted price signals.
Exam-style questions
Q1: Using the numbers in the worked example, calculate the tax per unit that would move the market to the social optimum, the new equilibrium price and quantity, and the tax revenue. (4 marks)
A: The tax must equal the external cost, £10 a unit (1). Supply becomes P = 20 + Q, so 60 − Q = 20 + Q and Q = 20 at P = £40 (2). Revenue is £10 × 20 = £200 (1).
Q2: Suppose the external cost is £20 a unit instead of £10. Calculate the new social optimum and the welfare loss at the free-market output. (4 marks)
A: MSC is now P = 30 + Q. Social optimum: 60 − Q = 30 + Q, so Q = 15 at P = £45 (2). The free market still produces 25 units, because the externality does not change MPC or demand (1). Welfare loss = ½ × £20 × (25 − 15) = £100, four times the original, because both the height and the base of the triangle doubled (1).
Q3: Flu jabs have MPB P = 60 − Q, an external benefit of £10 a jab, and supply P = 20 + Q. Calculate the subsidy per jab that would achieve the social optimum, the price consumers pay, and the cost to the government. Evaluate whether a subsidy is the best policy. (10 marks)
A: Start with the definitions, carry the numbers through, then judge the policy.
- Knowledge and application: the free market gives 60 − Q = 20 + Q, so 20 jabs at £40. The social optimum is where MSB = MSC: 70 − Q = 20 + Q, so 25 jabs at £45. A subsidy equal to the external benefit, £10 a jab, shifts supply to P = 10 + Q. Then 60 − Q = 10 + Q gives 25 jabs. Consumers pay £35, producers receive £45, and the government pays £10 × 25 = £250.
- Analysis: the subsidy internalises the external benefit, so consumption rises from 20 to 25 and the £25 welfare gain is captured. Consumers save £5 a jab and producers gain £5, because demand and supply here have equal slopes.
- Evaluation: £250 buys a £25 welfare gain, and £200 of it goes to the 20 people who would have paid £40 anyway. Free NHS provision, or information that shifts MPB up towards MSB, may reach the extra 5 people more cheaply. The judgement depends on how many people the subsidy tips, which is the elasticity of demand.
Examiner’s checklist: the mistakes that lose market failure marks
- The whole gap shaded. The welfare loss is the triangle between the two quantities, nothing else.
- The wrong curve moved. A production externality moves MSC. A consumption externality moves MSB. Muddling them puts the triangle on the wrong side.
- The optimum at zero. The social optimum is B, not the axis. Some pollution is worth it because the output is worth more.
- Demerit good used as a synonym for negative externality. OCR’s 2023 report says this “continues to be an issue”. A demerit good is about the buyer’s own misjudgement. An externality is about third parties.
- A diagram with no words. Draw it, then refer to it: “at Q₁, MSC is above MPB, so the last unit costs society more than it is worth”. AQA’s report calls an unexplained diagram application and an explained one analysis.
- Policy without evaluation. Every fix has a cost, a measurement problem and a chance of government failure. One of the three per policy.
Key takeaways
- Market failure is the wrong quantity, not the wrong price. The market clears, but at A instead of B.
- MSC = MPC + MEC and MSB = MPB + MEB. An externality opens a gap in one of the two.
- Negative production: MSC above MPC, over-production, welfare loss between Q₂ and Q₁. Positive consumption: MSB above MPB, under-consumption, welfare gain between Q₁ and Q₂.
- A tax equal to the external cost, or a subsidy equal to the external benefit, moves the market to B on its own.
- Every policy needs one evaluation point: the cost, the information problem or the unintended consequence.
FAQ
Q: What is market failure in A Level economics?
A: Any situation where the free market fails to allocate resources efficiently, so society loses welfare. The three types every board names are externalities, public goods and information gaps. Externalities are the ones with the diagrams.
Q: How do I draw a negative externality diagram?
A: Draw demand and supply, label supply MPC and demand MPB = MSB. Draw MSC parallel to and above MPC, label the gap “external cost”. Mark A where demand meets MPC and B where demand meets MSC. Shade the triangle between them and label it “welfare loss”. Project both quantities to the axis.
Q: What is the difference between the market failure diagram for production and for consumption?
A: A production externality moves the cost curves: MSC sits above MPC for a negative one. A consumption externality moves the benefit curves: MSB sits above MPB for a positive one. Both diagrams are on the A Level economics diagrams page.
Q: How are externalities A Level economics questions marked?
A: The diagram earns 3 to 4 marks inside a 9, 10 or 12-mark question for correct labels and the two equilibria. The rest is the explanation of the diagram, a chain of reasoning about the policy, and evaluation. In a 25-marker: two policies, each evaluated, then a judgement.
Q: Is a demerit good the same as a negative externality?
A: No. A demerit good is over-consumed because buyers under-estimate the harm to themselves. A negative externality is a cost to third parties. Cigarettes are both, so the exam often asks you to separate the two.
Q: Which policy is best for a negative externality?
A: It depends on what can be measured. Known external cost and price-sensitive demand: a tax. Severe, immediate harm: regulation. Many firms with different abatement costs: tradable permits. Say which condition holds and the judgement writes itself. The revision hub lists the other topics that use the same policy table.
Turn the triangle into marks
If the welfare-loss triangle still ends up in the wrong place under exam pressure, one lesson fixes that. Tell me which board you are on and how far off the exam is. A reply with a first slot comes within 24 hours, and the A Level economics tutor page explains the rate and how sessions work.
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