Business and Technology · Microeconomic factors

Market Failure and Externalities for ACCA BT

Updated 11 October 2026 · Fact-checked

Market failure happens when a free market does not allocate resources efficiently or fairly. Main causes are externalities, public goods, monopoly power and information gaps. To solve a question, name the cause, say who bears the cost or benefit, then pick the government response that fits, such as tax, subsidy, regulation or direct provision.

Understand Market Failure and Externalities

In theory, a free market uses the price mechanism to match what people want with what firms produce. Prices rise when goods are scarce and fall when they are plentiful. Resources then go where they are valued most. Market failure is when this does not happen and the outcome is inefficient or unfair.

The first cause is externalities. An externality is a cost or benefit that falls on a third party who is not part of the transaction. A negative externality imposes a cost on others, for example a factory polluting a river. A positive externality gives a benefit to others, for example a firm training staff who later move to other employers. The market price ignores these effects. So negative externalities lead to too much output, and positive externalities lead to too little.

The second cause is public goods. A pure public good is non-excludable (you cannot stop people using it) and non-rival (one person's use does not reduce what is left for others). Street lighting and national defence are standard examples. People can free-ride, using the good without paying, so private firms will not supply it. A private good is both excludable and rival, such as a coffee or a laptop.

The third cause is monopoly power. A firm with little competition can restrict output and charge higher prices. The fourth is information gaps, where buyers or sellers lack key facts. Consumers may not know the risks of a product. There are also merit goods (such as education), which people undervalue and under-consume, and demerit goods (such as tobacco), which people over-consume.

Governments respond with taxes, subsidies, laws and regulation, direct provision, and competition policy. Each fits a particular failure. Government action can also fail, for example through poor information or high cost.

Key formulas to remember

Social cost: Social cost = private cost + external cost; Negative externality: the market ignores external cost, so output is too high.
Social benefit: Social benefit = private benefit + external benefit; Positive externality: the market ignores external benefit, so output is too low.
Public good test: Public good = non-excludable AND non-rival; Both features must hold for a pure public good. A private good is excludable and rival.
Matching the fix: Negative externality → tax or regulation; positive externality or merit good → subsidy or provision; public good → state provision; monopoly → competition policy; information gap → disclosure rules; Typical matches. Other tools can also be valid if you justify them.

How to solve Market Failure and Externalities questions

Use this method for any market failure question, whether it asks you to identify a cause, define a term or suggest a government response.

1Read the scenario and find who is affected apart from the buyer and seller.
2Decide the type of failure: externality, public good, monopoly power or information gap.
3For an externality, decide if it is a cost (negative) or a benefit (positive).
4For a public good, test both features: can people be excluded, and does one person's use reduce another's?
5Check the direction of the market error: too much output for negative externalities and demerit goods, too little for positive externalities and merit goods.
6Choose the government response that matches the cause: tax, subsidy, regulation, provision or competition policy.
7Check the exact wording of the question, such as 'select two' or 'which is NOT', before choosing your answer.

Quickest way: Cause, direction, fix

When to use it: Use it in Section A multiple choice questions when you have about a minute per mark pair.

Underline the key phrase: third party, free rider, sole supplier or lack of information.
Match it to the cause: third party means externality, free rider means public good, sole supplier means monopoly, lack of information means information gap.
Pick the fix that targets that cause.
Eliminate options that describe a different failure or a fix that does not fit.

Common mistakes in Market Failure and Externalities

Treating any cost to the producer as a negative externality.
Students see the word 'cost' and stop thinking.
Fix: An externality must fall on a third party outside the transaction. Costs the firm pays itself are private costs.
Calling a good a public good because the government provides it.
Education and healthcare are state-funded, so students assume they are public goods.
Fix: Use the test: non-excludable and non-rival. Education is usually a merit good, not a pure public good.
Mixing up non-excludable and non-rival.
The terms sound similar.
Fix: Excludable means you can stop people using it. Rival means one person's use leaves less for others. Remember the lighthouse: nobody can be stopped and use does not reduce it.
Choosing a subsidy for a negative externality.
Students remember that governments can both tax and subsidise and mix them up.
Fix: Tax things that harm others, subsidise things that benefit others.
Ignoring that government intervention can also fail.
Students assume state action always solves the problem.
Fix: Mention limits where asked: taxes are hard to set at the right level, regulation costs money to enforce, and governments may lack information.
Listing only externalities as market failure.
Externalities are the most discussed example.
Fix: Also remember public goods, monopoly power, information gaps and merit and demerit goods.

Worked examples

Example 1

A chemical company discharges waste into a river. Local fishermen lose income as fish die. The company's selling price does not reflect this. (a) Identify the market failure. (b) Name two government responses.

Show the solution

The fishermen are third parties who are not part of the sale of chemicals.
Their lost income is a cost imposed on them by the company, so it is a negative externality.
The price covers only the company's private costs, so social cost is higher than private cost and the market produces too much.
Responses: a pollution tax raises the company's costs, so it pays for the harm; regulation can limit or ban the discharge with fines for breaking the rule.

Answer: (a) A negative externality: the external cost falls on the fishermen and the market produces too much. (b) A pollution tax and regulation limiting discharges.

Example 2

Which ONE of the following is a pure public good? A. A toll motorway B. Street lighting in a town C. A state-funded school D. A cinema ticket

Show the solution

Apply the test: non-excludable and non-rival.
A toll motorway can exclude non-payers with barriers, so it is excludable. It is not pure.
Street lighting cannot practically exclude anyone, and one person's use does not reduce another's. It passes both tests.
A school has limited places, so it is rival and excludable. It is a merit good. A cinema ticket is excludable and rival, so it is private.

Answer: B. Street lighting is non-excludable and non-rival.

Exam tips

Learn the two tests for public goods in exact words. Many questions are just this test applied to an example.
In multiple response questions, select exactly the number stated. Check each option against the cause before you choose.
Match each government tool to the failure it fixes. Distractors often offer a real tool for the wrong problem.
Watch for merit and demerit goods. They are linked to information gaps and undervaluing, not to third-party effects.
If a question asks for a limitation of intervention, think of cost, poor information and unintended effects.

Practice questions from Microeconomic factors

Market Failure and Externalities in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Market Failure and Externalities: frequently asked questions

What are the main causes of market failure?

The main causes are externalities, public goods, monopoly power and information gaps. Merit and demerit goods are also linked to market failure. In each case the market does not give the best outcome for society.

What is the difference between public goods and private goods?

A pure public good is non-excludable and non-rival, so people can free-ride and private firms will not supply it. A private good is excludable and rival, so sellers can charge for it. Street lighting is public; a laptop is private.

What is the difference between positive and negative externalities?

A negative externality is a cost imposed on a third party, such as pollution. A positive externality is a benefit to a third party, such as a trained worker moving to another firm. Markets produce too much of the first and too little of the second.

How can a government correct market failure?

It can tax harmful activities, subsidise beneficial ones, regulate or ban behaviour, provide public goods directly, and use competition policy against monopoly power. It can also require information disclosure. The best tool depends on the cause of the failure.