Difficulty: Introductory to Intermediate | Prerequisites: Supply and demand, consumer and producer surplus, deadweight loss (see Price Controls study notes).
Source: Intro to Microeconomics, University of Texas at Austin
Tags: public goods, externalities, free rider, Pigovian tax, Coase theorem, marginal social cost, marginal social benefit, cap-and-trade, tragedy of the commons, market failure, non-excludable, non-rival, ECO 101
The previous set of notes covered markets that work well on their own but are distorted by government controls. This topic flips the question: when do markets fail on their own? Public goods, externalities and common resources are all cases where private incentives diverge from what is best for society. Understanding these failures is the economic rationale for government intervention, and it connects consumer and producer surplus analysis to real policy debates about pollution, education and shared resources.
Markets underprovide public goods (because of free riders) and overproduce goods with negative externalities (because firms ignore external costs). Government tools such as Pigovian taxes, subsidies, property rights and cap-and-trade can move the market toward the socially optimal outcome. The Coase theorem says private bargaining can also work, but only when transaction costs are low.
Non-excludable
A good is non-excludable when you cannot prevent someone from using it, even if they have not paid. Think of a lighthouse: every passing ship benefits whether or not its captain contributed.
Non-rival
A good is non-rival when one person's use does not reduce the amount available to others. A radio broadcast is a good example: your listening does not stop anyone else from listening.
Public good
A good that is both non-excludable and non-rival. National defence and street lighting are classic examples. Private markets tend to underprovide them.
Private good
A good that is both rival and excludable. Food, clothing and most things you buy in a shop fall into this category.
Club good
A good that is excludable but non-rival (up to a point of congestion). Cable television and private parks are examples.
Common good (common-pool resource)
A good that is rival but non-excludable. Fish in the ocean, public grazing land and clean air are common goods. They are vulnerable to overuse.
Free rider
Someone who benefits from a good without paying for it. Free riding is the central problem with public goods: if everyone waits for someone else to pay, the good is not provided.
Externality
A cost or benefit of an economic activity that falls on a third party who is not involved in the transaction. In simple terms, it is a side effect the market price does not capture.
Negative externality
An external cost imposed on third parties. Pollution from a factory is the standard example.
Positive externality
An external benefit enjoyed by third parties. Education benefits not just the student but society at large through higher productivity and lower crime.
Private cost (MC)
The cost borne directly by the producer. This is what the firm considers when deciding how much to produce.
External cost
The cost imposed on society that the producer does not bear. Pollution damage is the usual example.
Marginal social cost (MSC)
The full cost to society of producing one more unit: MSC = MC + marginal external cost. When there is a negative externality, MSC exceeds private MC.
Marginal social benefit (MSB)
The full benefit to society of consuming one more unit. For public goods, MSB is the vertical sum of individual willingness-to-pay curves (since everyone consumes the same unit).
Pigovian tax
A tax set equal to the marginal external cost. It forces producers to "internalise" the externality by making the private cost match the social cost.
Cap-and-trade
A system where the government sets a total emissions cap and issues tradable permits. Firms that can reduce pollution cheaply sell permits to firms that cannot, so the cap is met at the lowest total cost.
Coase theorem
If property rights are clearly defined and transaction costs are low, private bargaining will reach an efficient outcome regardless of which party initially holds the rights.
Tragedy of the commons
The tendency for common goods to be overused and depleted because no individual has an incentive to conserve them. Each user captures the full benefit of extra use but shares the cost of depletion with everyone.
Category | Rival? | Excludable? | Examples |
|---|---|---|---|
Private goods | Yes | Yes | Food, clothing |
Club goods | No | Yes | Cable TV, private parks |
Public goods | No | No | National defence, public radio, street lighting |
Common goods | Yes | No | Fish stocks, forests, clean air |
This two-by-two grid is the foundation. Every good fits somewhere, and where it sits determines whether markets provide it well.
Because public goods are non-excludable, people can enjoy them without paying.
Rational individuals wait for others to fund the good, so private markets tend to underprovide it. This is market failure.
The government steps in by financing public goods through taxation (national defence, street lighting) or by providing them directly.
For private goods, the market demand curve is the horizontal sum of individual demand curves (each person buys their own units).
For public goods, everyone consumes the same unit, so the MSB curve is the vertical sum of individual willingness-to-pay curves.
Efficient provision occurs where MSB = MSC.
Education generates positive externalities (higher productivity, lower crime). Markets underprovide it, so governments subsidise or provide it directly.
Healthcare is marked by information asymmetries between doctors and patients, and by external benefits (vaccination protects others). Government intervenes through regulation, subsidies and direct provision.
A negative externality (e.g. factory pollution) means the social cost of production exceeds the private cost. The market overproduces because firms ignore the external cost.
A positive externality (e.g. education, neighbourhood beautification) means the social benefit exceeds the private benefit. The market underproduces because individuals ignore the external benefit.
In both cases, the market outcome differs from the socially optimal outcome.
MSC = MC + marginal external cost. With a negative externality, MSC > MC, so the true cost of production is higher than what firms pay.
MSB = marginal private benefit + marginal external benefit. With a positive externality, MSB > private demand.
The socially efficient quantity is where MSB = MSC, not where private supply meets private demand.
A coal plant's private cost understates the true social cost because it does not include health and environmental damage.
The market produces more than the socially optimal quantity.
Solutions:
Pigovian taxes: set the tax equal to the marginal external cost so the firm faces the full social cost.
Property rights: if clearly assigned, affected parties can negotiate (Coase theorem).
Regulation: mandate pollution controls or clean technology.
Cap-and-trade: set a total emissions cap, issue permits, let firms trade them.
Property rights: assigning rights to clean air (or to pollute) creates incentives to negotiate.
Mandated clean technology: direct regulation requiring firms to install pollution controls.
Pigovian taxation: a tax of, say, $5 per MWh aligns private cost with social cost and moves output to the efficient level.
Cap-and-trade: the cap ensures the total quantity of pollution is controlled; trading ensures it is reduced where it is cheapest to do so.
If property rights are clearly defined and transaction costs are low, private bargaining can resolve externalities without government intervention.
The initial assignment of rights does not affect the final efficient outcome. It only affects who pays whom.
Limitation: in practice, transaction costs are often high (many affected parties, legal costs, information problems), so the Coase theorem is more of a theoretical benchmark than a practical solution for most pollution problems.
Common goods are rival and non-excludable. Each user captures the benefit of extra use but shares the cost of depletion.
Result: overconsumption and resource depletion (overfishing, overgrazing, deforestation).
Solutions: government regulation (quotas, fishing licences), privatisation (assigning property rights), or community-managed agreements.
When consumption benefits others (education, vaccinations), the market underproduces because individuals consider only their own benefit.
Solutions: government provision, subsidies or vouchers to bring consumption up to the socially optimal level.
Negative externalities: taxes, regulation, property rights, cap-and-trade.
Positive externalities: subsidies, public provision, mandates.
Marginal social cost: MSC = MC + marginal external cost
Marginal social benefit (for public goods): MSB = vertical sum of individual willingness-to-pay curves
Efficient outcome: MSB = MSC
Pigovian tax rate: Tax per unit = marginal external cost at the socially optimal quantity
Example: if the external cost of pollution is $5 per MWh, a Pigovian tax of $5/MWh makes the firm's private cost equal to the social cost.
Carbon taxes (used in Canada, Sweden and others) are Pigovian taxes on CO2 emissions. The EU Emissions Trading System is the world's largest cap-and-trade programme. Overfishing in international waters is a live example of the tragedy of the commons; fishing quotas and international agreements attempt to manage it. Government-funded education (from primary school to subsidised university tuition) reflects the positive externality logic: society benefits when more people are educated, so the government pushes consumption above the level the market would deliver on its own.
Students often confuse public goods with government-provided goods. A good is public because of its economic properties (non-rival, non-excludable), not because the government supplies it. A private company could, in theory, provide a public good.
A frequent error is summing individual demand curves horizontally for public goods. For public goods, the MSB is the vertical sum (everyone consumes the same unit, so you add up what each person would pay for that unit).
Students sometimes think the Coase theorem means government intervention is never needed. In practice, high transaction costs, many affected parties and information problems often make private bargaining impractical.
Common goods and public goods are not the same. Common goods are rival (my fish is not your fish); public goods are non-rival. This distinction drives the tragedy of the commons.
⚠️ Be able to classify any good into the two-by-two rivalry/excludability grid. This is a very common exam question.
⚠️ Know the difference between horizontal summation (private goods, market demand) and vertical summation (public goods, MSB).
⚠️ Be ready to draw a diagram showing a negative externality: the MSC curve above the supply (MC) curve, the market equilibrium to the right of the social optimum, and the DWL triangle.
⚠️ Understand the Coase theorem and, critically, its limitations. Exam questions often test whether you know when it does and does not apply.
⚠️ Know the four government tools for negative externalities (property rights, regulation, Pigovian tax, cap-and-trade) and the tools for positive externalities (subsidies, public provision, mandates).
True or false: a public good is any good provided by the government.
Fill in the blank: for public goods, MSB is the ____ sum of individual demand curves.
True or false: a negative externality causes the market to underproduce relative to the social optimum.
Fill in the blank: MSC = MC + ____.
True or false: the Coase theorem requires low transaction costs to work.
Answers: 1. False (public goods are defined by non-rivalry and non-excludability). 2. Vertical. 3. False (it causes overproduction). 4. Marginal external cost. 5. True.
Q: Classify the following goods: (a) a public park bench, (b) national defence, (c) a Netflix subscription, (d) fish in the Atlantic Ocean.
A: (a) Common good (rival, since the bench can be full; non-excludable in an open park). (b) Public good (non-rival, non-excludable). (c) Club good (non-rival up to server capacity; excludable by subscription). (d) Common good (rival, since fish caught by one boat are unavailable to others; non-excludable in open waters).
Q: A factory emits pollution. The marginal external cost is $5 per unit. Explain how a Pigovian tax corrects the externality.
A: A Pigovian tax of $5 per unit is added to the firm's private cost, raising the supply curve to match the MSC curve. The firm now faces the full social cost, reduces output to the socially optimal quantity, and the deadweight loss from overproduction is eliminated.
Q: Why does the Coase theorem often fail in practice for pollution problems?
A: Pollution typically affects many people (high transaction costs for negotiation), property rights over clean air are often poorly defined, and affected parties may have unequal bargaining power or incomplete information about the damage.
Q: Explain the tragedy of the commons using overfishing as an example.
A: Each fishing boat captures the full value of the fish it catches but shares the cost of depleting the stock with every other boat. No individual boat has an incentive to reduce its catch, so the fleet collectively overfishes until the stock collapses or regulation intervenes.
Externalities build directly on the surplus analysis from the price controls topic: deadweight loss here comes from the market itself (not from a government control), and government intervention aims to reduce it rather than create it. The public goods framework connects to political economy and voting theory (majority rule as an allocation mechanism). The Coase theorem links to property rights and contract law. If you go on to study environmental economics or health economics, these concepts are the starting point.
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