Business Economics · Government intervention in a market

Externalities and Public Goods: Government Intervention in Markets

Updated 11 October 2026 · Fact-checked

An externality is a cost or benefit of a transaction that falls on a third party and is not in the price. A public good is non-excludable and non-rival, so people free-ride and markets under-supply it. Governments respond with taxes, subsidies, regulation, tradable permits or direct provision.

Understand Externalities and Public Goods

A market works well when the price reflects all costs and benefits. Buyers pay for what they get. Sellers cover what it costs them. The market outcome is then efficient.

An externality breaks this link. It is a cost or benefit that reaches someone who is not part of the deal. A negative externality imposes a cost on others, such as pollution from a factory. A positive externality gives a benefit to others, such as vaccination, which protects people who did not pay for it.

The key idea is private versus social. Marginal social cost (MSC) = marginal private cost (MPC) + marginal external cost (MEC). With a negative externality, the firm only counts MPC, so it produces too much. With a positive externality, marginal social benefit (MSB) = marginal private benefit (MPB) + marginal external benefit (MEB). Buyers only count MPB, so the market produces too little. Either way, the market outcome (where private cost meets private benefit) is not the socially best output, where MSB = MSC. This gap is a market failure and creates a welfare loss.

Public goods fail for a different reason. A 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. A private good is both excludable and rival, like a packet of biscuits. Because people can use a public good without paying, each person has a reason to free-ride and let others pay. Private firms cannot earn revenue, so they under-supply it or do not supply it at all.

Government remedies follow the cause. For a negative externality, a Pigouvian tax set equal to the marginal external cost at the efficient output makes the producer pay the full social cost. Other tools are regulation (limits or bans), tradable permits and information campaigns. For positive externalities, use subsidies or provide the good directly. For public goods, the state usually provides them and pays for them through general taxation. Remedies have limits: the government may not know the true size of the external cost, and intervention can itself fail.

Key rules to remember

Marginal social cost: MSC = MPC + MEC; Used for negative externalities in production. MEC is the extra cost borne by third parties per unit.
Marginal social benefit: MSB = MPB + MEB; Used for positive externalities in consumption. MEB is the extra benefit to third parties per unit.
Socially efficient output: MSB = MSC; The market outcome is where MPB = MPC. With an externality these two points differ.
Pigouvian tax: Tax per unit = MEC at the socially efficient output; Shifts MPC up to MSC. If MEC is constant, the tax equals that constant.
Pigouvian subsidy: Subsidy per unit = MEB at the socially efficient output; Shifts MPB up to MSB for a positive externality.
Public good test: Public good = non-excludable and non-rival; Both conditions must hold. A good with only one is a quasi-public good, a club good or a common resource.

How to solve Externalities and Public Goods questions

Use this method for any question on externalities, public goods or remedies.

1Identify who is affected outside the transaction. If a third party bears a cost, it is a negative externality. If a third party gains, it is positive.
2Say whether it arises in production or consumption. This tells you which curve moves: MPC to MSC, or MPB to MSB.
3Write the relationship: MSC = MPC + MEC, or MSB = MPB + MEB. Insert the numbers if given.
4Find the market output where MPB = MPC. Then find the efficient output where MSB = MSC. Compare them and state whether the market over- or under-produces.
5For a public good question, test it for excludability and rivalry. Name the free-rider problem and explain why private supply fails.
6Choose a remedy that fits the cause: Pigouvian tax, regulation, permits, subsidy or state provision. Compute the tax or subsidy if asked.
7Evaluate. Mention difficulty of measuring MEC, effects on prices and jobs, and the risk of government failure.
8State a clear conclusion in one sentence.

Quickest way: Four-line check for externality questions

When to use it: Use it in MCQs and short written parts when time is tight.

Ask: who outside the deal gains or loses? That fixes the type of externality.
Match the direction: negative means over-production, positive means under-production.
For a calculation, set equation MSB = MSC and solve for the quantity. The tax is MEC at that quantity.
For public goods, tick two boxes: non-excludable and non-rival. If both are ticked, expect free riding and state provision.

Common mistakes in Externalities and Public Goods

Calling any good that the government provides a public good.
Students link public with state ownership.
Fix: Use the definition only. Education and healthcare are often state-provided but are excludable and rival, so they are not pure public goods.
Setting the Pigouvian tax equal to the market price or to total external cost.
The tax is confused with the full cost or with revenue.
Fix: The tax per unit equals the marginal external cost at the efficient output. Check it by seeing that MPC plus tax equals MSC there.
Drawing MSC below MPC for a negative externality.
Students mix up which curve carries the extra cost.
Fix: With a negative externality, MSC is above MPC by the MEC. With a positive externality, MSB is above MPB by the MEB.
Saying free riding means the good is not valued.
Non-payment is read as no demand.
Fix: People do value the good. They simply hope others pay. Free riding hides true demand from the market.
Claiming a tax removes the externality entirely.
Students treat the efficient output as zero pollution.
Fix: The aim is the efficient level, where MSB = MSC. Some output with some pollution remains at that point.
Giving a remedy with no evaluation.
Students stop once they name the tool.
Fix: Add one or two limits, such as the difficulty of measuring MEC, possible job losses and lobbying, or the cost of enforcing regulation.

Worked examples

Example 1

A chemical plant faces demand (MPB) P = 100 − 2Q and marginal private cost MPC = 20 + 2Q (₹ per unit). Each unit causes a constant external cost of ₹16. Find the market output, the efficient output and the Pigouvian tax.

Show the solution

Market output: set MPB = MPC. 100 − 2Q = 20 + 2Q, so 80 = 4Q and Q = 20.
MSC = MPC + MEC = 20 + 2Q + 16 = 36 + 2Q.
Efficient output: set MSB = MSC. There is no external benefit, so MSB = MPB. 100 − 2Q = 36 + 2Q, so 64 = 4Q and Q = 16.
The market produces 20 units, which is 4 more than the efficient 16, so there is over-production.
Pigouvian tax = MEC = ₹16 per unit. Check: with the tax, MPC + 16 = 36 + 2Q, which equals MSC, so the producer chooses Q = 16.

Answer: Market output is 20 units, efficient output is 16 units, and the Pigouvian tax is ₹16 per unit.

Example 2

Explain why a street light is a public good, describe the free-rider problem, and suggest how the government should respond.

Show the solution

Non-excludable: once a street is lit, it is not practical to stop any passer-by from benefiting.
Non-rival: one person using the light does not reduce the light available to others.
Because of this, a pure public good meets both conditions.
Free-rider problem: each resident gains from the light whether or not they pay, so each has an incentive to let others pay. A private firm cannot collect revenue, so it will under-supply or not supply the light, even though residents value it.
Response: the government (or local body) provides the light and funds it through general taxation, so everyone who benefits contributes.
Evaluation: the government must estimate the right amount to supply, since true demand is hidden, and tax funding may not match individual benefit.

Answer: A street light is non-excludable and non-rival, so free riding stops private supply. The government should provide it and fund it through taxation, while judging the right level of provision.

Exam tips

In MCQs, read the definition carefully. Check both tests, non-excludable and non-rival, before choosing public good.
In diagrams, label MPC, MSC, MPB and MSB, mark the market and efficient quantities, and shade the welfare loss.
For tax questions, show the MSB = MSC step and then the tax as the vertical gap, MEC, at the efficient output.
Always match the remedy to the failure and add an evaluation point. Written marks are often for the evaluation.
Use an Indian example where it fits, such as air pollution from industry or public lighting and roads, but keep the economics precise.

Practice questions from Government intervention in a market

Externalities and Public Goods: frequently asked questions

What is a Pigouvian tax?

A Pigouvian tax is a per-unit tax on a good that creates a negative externality. It is set equal to the marginal external cost at the efficient output. It makes the producer bear the full social cost, so output falls to the efficient level.

What is the difference between public goods and private goods?

A private good is excludable and rival, so sellers can charge and one person's use leaves less for others. A public good is non-excludable and non-rival. Because of this, people can use it without paying and markets under-supply it.

Can you give a free rider problem example?

Street lighting is a standard one. Everyone on the street benefits once it is installed, so each resident prefers that others pay. If all think this way, nobody pays and the light is not provided privately.

How can the government correct a negative externality?

It can impose a Pigouvian tax, set regulations such as emission limits, or issue tradable permits. Each aims to move output to where MSB = MSC. The choice depends on how well the government can measure the external cost and enforce the rule.