Business Economics · Relationship between the government and the individual firm

Market Failure and Externalities Explained for CB2

Updated 11 October 2026 · Fact-checked

Market failure happens when a free market does not allocate resources efficiently, so price and output differ from the socially best level. Main causes are externalities, public goods, monopoly power and information asymmetry. To solve a question, name the failure, compare private and social costs or benefits, then suggest a correction.

Understand Market Failure and Externalities

A market is efficient when the price people pay reflects the full cost to society and the full benefit to society. In that case resources go to their best use. Market failure means this does not happen. Output is too high, too low, or the good is not supplied at all.

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, such as factory pollution. Social cost is then higher than private cost, so the market produces too much. A positive externality gives a benefit to others, such as vaccination or training that staff carry to other employers. Social benefit is then higher than private benefit, so the market produces too little.

A public good is non-excludable (you cannot stop people using it) and non-rival (one person's use does not reduce another's). Street lighting and national defence are examples. Because people can use it without paying, they have an incentive to be a free rider. Private firms cannot earn revenue, so the market supplies too little or none.

Two other causes matter for firms. Monopoly power lets a firm restrict output and raise price above marginal cost, which loses allocative efficiency. Information asymmetry means one side knows more than the other. In insurance this leads to adverse selection (high-risk people buy cover more readily) and moral hazard (people take more risk once insured). Both can make markets shrink or misprice.

Governments respond with taxes, subsidies, regulation, tradable permits, direct provision, competition policy and disclosure rules. These help firms by fixing the market, but they also add costs. Government action can itself fail, so always weigh the correction against its side effects.

Key rules to remember

Social cost: Marginal social cost (MSC) = marginal private cost (MPC) + marginal external cost (MEC); Use for negative production externalities. The efficient output is where MSC = MSB.
Social benefit: Marginal social benefit (MSB) = marginal private benefit (MPB) + marginal external benefit (MEB); Use for positive externalities. Market output is where MPB = MPC, which is below the social optimum.
Efficiency condition: Socially efficient output: MSB = MSC; Market failure exists when the market output does not satisfy this.
Corrective (Pigouvian) tax: Tax per unit = marginal external cost at the efficient output; This makes the firm face the full social cost. A subsidy per unit equal to marginal external benefit is the matching fix for positive externalities.
Public good test: Non-excludable AND non-rival; Both conditions are needed for a pure public good. Goods failing one test are quasi-public goods.

How to solve Market Failure and Externalities questions

Use this order for any market failure question, whether it is MCQ or written.

1Identify the source of failure: externality, public good, monopoly power or information asymmetry.
2State who bears the cost or receives the benefit outside the transaction, or who lacks information.
3Compare private and social positions: MPC with MSC, or MPB with MSB. Say whether the market produces too much or too little.
4State the effect on output, price and welfare. Name the deadweight or welfare loss if relevant.
5Propose a remedy that fits the cause: tax, subsidy, regulation, permits, direct provision, competition policy or disclosure.
6Evaluate the remedy: difficulty measuring the external cost, cost of enforcement, effect on firms, and risk of government failure.
7Give a one-line conclusion that answers the exact question asked.

Quickest way: Cause, direction, fix

When to use it: Use for MCQs and for the opening lines of a written answer when time is short.

Ask: is a third party affected? If yes, it is an externality. If no one can be excluded and use is non-rival, it is a public good.
Cost on others means too much output. Benefit to others means too little.
Match the fix: tax or regulation for negative externalities, subsidy for positive, provision or tax funding for public goods, competition rules for monopoly, disclosure for information gaps.
Check the wording for a trap such as private versus social, or excludable versus rival.

Common mistakes in Market Failure and Externalities

Treating any harmful effect as a negative externality.
Students forget the effect must fall on a third party outside the transaction.
Fix: Ask who pays. If the buyer and seller bear the cost themselves, it is not an externality.
Saying a public good is one provided by the government.
The two ideas are mixed up because governments often supply public goods.
Fix: Define it by non-excludability and non-rivalry. Government provision is a response, not the definition.
Mixing up the direction of over and under production.
Students do not compare private and social curves.
Fix: Negative externality: MSC above MPC, market overproduces. Positive externality: MSB above MPB, market underproduces.
Naming a remedy without linking it to the cause.
Students memorise a list of policies.
Fix: Pair each failure with a fitting remedy and explain how it changes the firm's incentives.
Ignoring government failure when evaluating.
Students assume intervention always works.
Fix: Add one point on measurement difficulty, cost, lobbying or unintended effects.
Confusing adverse selection with moral hazard.
Both come from information asymmetry.
Fix: Adverse selection occurs before the contract, since risky types join. Moral hazard occurs after it, since behaviour changes.

Worked examples

Example 1

A chemical plant has marginal private cost of ₹40 per unit. Each unit causes ₹15 of pollution damage to nearby residents. Market price is ₹60 and the plant is a price taker. State the marginal social cost and the per-unit corrective tax, and explain the effect on the firm's cost.

Show the solution

The pollution cost falls on residents, a third party, so this is a negative externality.
MEC = ₹15 per unit.
MSC = MPC + MEC = ₹40 + ₹15 = ₹55 per unit.
The efficient tax equals the marginal external cost at the efficient output, which here is ₹15 per unit if MEC is constant.
With the tax, the firm's marginal cost becomes ₹40 + ₹15 = ₹55, matching MSC, so it considers the full social cost when choosing output.
Output falls compared with the untaxed market.

Answer: MSC is ₹55 per unit. A tax of ₹15 per unit makes the firm bear the external cost, raising its marginal cost to ₹55 and reducing output toward the efficient level.

Example 2

A private health insurer finds that people who buy its policies are more likely to claim than the general population. Explain the market failure, its effect on the insurer, and two ways to reduce it.

Show the solution

Buyers know more about their own health than the insurer. This is information asymmetry.
Because higher-risk people are more likely to buy, the insured pool is riskier than average. This is adverse selection.
If the insurer prices for the average population, it makes losses on the riskier pool. If it raises premiums, low-risk people leave and the pool worsens further.
Remedy 1: underwriting, such as medical questionnaires and checks, to reduce the insurer's information gap.
Remedy 2: compulsory or widely mandated cover, which brings in low-risk people and spreads risk.
Evaluation: underwriting adds cost and may exclude some people, and mandation needs government action and enforcement.

Answer: The failure is information asymmetry causing adverse selection. The insurer faces a riskier pool than expected. It can reduce this by underwriting and by broader or compulsory cover, each with costs.

Exam tips

Define terms precisely in one line first. Examiners award marks for correct definitions of externality, public good and free rider.
Use the MPC, MSC, MPB and MSB language in written answers and say which curve is above which.
Always pair the failure with a matching remedy, then add one evaluation point on its limits.
In MCQs, test each option against the definition. Watch for private versus social and excludable versus rival.
Use insurance examples for information asymmetry. Adverse selection and moral hazard link directly to actuarial work.

Practice questions from Relationship between the government and the individual firm

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 is the difference between positive and negative externalities?

A negative externality imposes a cost on a third party, so the market produces too much. A positive externality gives a benefit to a third party, so the market produces too little. The test is whether the effect falls outside the transaction.

What is the free rider problem?

A free rider uses a good without paying for it. This happens with public goods because people cannot be excluded. Firms cannot earn enough revenue, so the market supplies too little or nothing.

How do governments correct market failure?

They use taxes on negative externalities, subsidies for positive ones, regulation, tradable permits, direct provision of public goods, competition policy and information disclosure rules. The choice depends on the cause of the failure.

Is monopoly power a market failure?

Yes. A monopoly can restrict output and charge a price above marginal cost, which loses allocative efficiency. Competition policy and regulation are common responses.

Is market failure the same as government failure?

No. Market failure is an inefficient outcome in a free market. Government failure is when intervention makes outcomes worse or less efficient than the market.