Difficulty: Introductory | Prerequisites: Supply and demand, consumer and producer surplus (Docs 1 and 2 in this series).
Up to this point in the course, markets have been treated as largely self-correcting: supply meets demand, surplus is maximised, and everyone is better off. This set of notes is about the cases where that story breaks down. When goods are non-excludable or non-rival, private markets under-provide them. When production or consumption imposes costs (or confers benefits) on bystanders, the market quantity diverges from what is best for society. Understanding these failures, and the policy tools used to address them, is the bridge from pure market theory to real-world policy analysis.
Goods can be classified by whether they are rival (one person's use reduces availability) and excludable (non-payers can be kept out). Public goods are neither, which leads to free-riding and under-provision. Externalities are costs or benefits that spill over to third parties: negative externalities (e.g., pollution) cause over-production, positive externalities (e.g., education) cause under-production. Governments can correct these failures with taxes, subsidies, regulation, property rights, or cap-and-trade systems.
Rivalry (rival good)
A good is rival if one person's consumption reduces the amount available for others. In simple terms, if you eat the sandwich, nobody else can eat it.
Excludability (excludable good)
A good is excludable if non-payers can be prevented from using it. Think of it as whether you can put a fence around it.
Private good
Both rival and excludable (food, clothing). Standard market analysis applies.
Club good
Excludable but non-rival (cable TV, a gym membership). Providers can charge, but one person's use does not reduce availability for others.
Public good
Neither rival nor excludable (national defence, street lighting). Because non-payers cannot be excluded, private markets under-provide these goods.
Common good (common-pool resource)
Rival but non-excludable (ocean fish stocks, public parks). Because no one can be excluded, individuals tend to overuse the resource.
Externality
A cost or benefit of an activity that falls on a third party who is not involved in the transaction. In simple terms, it is a side effect that hits bystanders.
Negative externality
An external cost imposed on third parties (pollution, noise). The market over-produces goods with negative externalities because the producer does not bear the full cost.
Positive externality
An external benefit enjoyed by third parties (education, vaccinations). The market under-produces goods with positive externalities because the producer does not capture the full benefit.
Private cost
The cost borne by the producer or consumer in a transaction.
External cost
The cost imposed on third parties outside the transaction.
Social cost
Private cost plus external cost. This is the true cost to society.
Marginal social cost (MSC)
The cost to society of producing one more unit, equal to marginal private cost plus marginal external cost.
Marginal social benefit (MSB)
The benefit to society of consuming one more unit, equal to marginal private benefit plus marginal external benefit.
Pigovian tax
A tax set equal to the marginal external cost, designed to make the producer internalise the externality and produce at the socially optimal quantity.
Cap-and-trade
A system where the government sets a total quantity limit (cap) on an activity (e.g., pollution) and issues tradeable permits. Firms that can reduce the activity cheaply sell permits to firms for which reduction is expensive.
Coase theorem
If property rights are well-defined and transaction costs are low, private bargaining between parties will reach an efficient outcome regardless of who holds the initial rights.
Free-rider problem
The tendency for individuals to benefit from a public good without contributing to its cost, since they cannot be excluded.
Tragedy of the commons
The over-exploitation of a common resource because individuals acting in self-interest have no incentive to conserve it.
The two-by-two matrix of rivalry and excludability is the organising framework for this entire topic.
Excludable | Non-excludable | |
|---|---|---|
Rival | Private goods (food, clothing) | Common goods (ocean fish, forests) |
Non-rival | Club goods (cable TV, gyms) | Public goods (national defence, street lighting) |
The market works well for private goods. The other three categories each present a distinct problem: club goods can be privately provided but may be under-consumed; common goods are over-consumed (tragedy of the commons); public goods are under-provided (free-rider problem).
Because public goods are non-excludable, people can enjoy them without paying. Rational individuals have an incentive to let others fund the good while they benefit for free. The result is that private markets under-provide public goods relative to the socially optimal quantity.
The socially optimal quantity is where MSB = MSC. For a public good, MSB is found by vertically summing individual demand curves (everyone benefits simultaneously, so their marginal benefits stack), which differs from private goods (where demand curves are summed horizontally).
Government typically steps in to provide or finance public goods through taxation, since private firms cannot profitably supply a good that people can consume without paying.
When production imposes costs on third parties (e.g., a coal plant emitting pollution), the private cost understates the true social cost. The supply curve based on private cost alone sits too low, so the market produces more than the socially optimal quantity.
MSC = MC_{\text{private}} + MC_{\text{external}}The socially optimal quantity is where MSB = MSC, which is lower than the market equilibrium quantity. The gap between the two quantities represents over-production and generates deadweight loss.
Example: if a power plant's external cost of pollution is $5 per MWh, the MSC curve sits $5 above the private supply curve. The efficient output is where this higher MSC intersects demand.
When consumption generates benefits for third parties (e.g., education raising societal productivity, vaccinations reducing disease spread), the private benefit understates the true social benefit. The demand curve based on private benefit alone sits too low, so the market produces less than the socially optimal quantity.
MSB = MB_{\text{private}} + MB_{\text{external}}The socially optimal quantity is where MSB = MSC, which is higher than the market equilibrium quantity. The gap represents under-production.
Private costs/benefits are borne by or accrue to the buyer and seller directly.
External costs/benefits fall on third parties.
Social costs/benefits are the sum of private and external components and represent the true impact on society.
The Coase theorem states that if property rights are clearly assigned and transaction costs are low, private parties can negotiate an efficient solution to an externality on their own, regardless of who initially holds the rights.
In practice, transaction costs are often high (many parties, information asymmetry, legal fees), which limits the theorem's applicability. It remains important as a benchmark for understanding when government intervention is and is not needed.
A Pigovian tax is set equal to the marginal external cost. It raises the producer's private cost to match the social cost, shifting the supply curve upward and reducing output to the socially optimal level. The tax internalises the externality.
The government sets a total cap on an externality (e.g., total tonnes of CO2) and issues tradeable permits. Firms that can reduce emissions cheaply do so and sell their spare permits; firms facing high reduction costs buy permits instead. The market for permits ensures the cap is met at the lowest total cost.
Direct regulation mandates or prohibits specific activities (e.g., requiring scrubbers on smokestacks, banning certain chemicals). It is simple to enforce but can be blunt and may not achieve the lowest-cost reduction.
When a good generates positive externalities, subsidies lower the effective price and encourage greater consumption or production, moving the market closer to the socially optimal quantity. Public provision (the government directly supplying the good, as with public education or vaccinations) is another approach.
Common resources (rival but non-excludable) are prone to over-use because no individual bears the full social cost of their consumption. Solutions include regulation (catch limits, usage fees), privatisation (assigning property rights so the owner has an incentive to conserve), and community management (local rules and monitoring).
Carbon taxes and emissions trading schemes (such as the EU Emissions Trading System) are direct applications of Pigovian taxation and cap-and-trade. Public education and childhood vaccination programmes are government responses to positive externalities. Overfishing in international waters is a textbook tragedy of the commons, and fishing quotas (individual transferable quotas) are the cap-and-trade equivalent applied to common resources.
Students often think all externalities are negative. Positive externalities (education, vaccinations, research) are equally important and lead to under-production rather than over-production.
The Coase theorem is sometimes taken to mean government intervention is never needed. The theorem holds only when transaction costs are low and property rights are clear, conditions that rarely hold in practice for large-scale externalities like air pollution.
Students sometimes confuse public goods with goods provided by the government. "Public good" is an economic classification (non-rival, non-excludable), not a description of who provides it. A government can provide private goods (e.g., public housing), and private firms can provide club goods.
Common goods and public goods are frequently mixed up. The critical difference is rivalry: fish in the sea are rival (one person's catch reduces what is left), while national defence is non-rival.
The rivalry/excludability matrix is a staple multiple-choice question. Be able to classify any example into one of the four categories.
Expect graph-based questions: given a supply and demand diagram with an external cost or benefit, identify the market quantity, the socially optimal quantity, and the deadweight loss triangle.
Know the direction of the inefficiency: negative externalities cause over-production, positive externalities cause under-production.
Be ready to explain the Coase theorem and state its two key conditions (well-defined property rights, low transaction costs).
Questions on policy tools often ask you to compare the efficiency of a Pigovian tax vs command-and-control regulation.
True or false: a public good is rival and excludable. (False: a public good is non-rival and non-excludable.)
Fill in the blank: a negative production externality causes the market to ______ the good relative to the social optimum. (over-produce)
True or false: the Coase theorem requires low transaction costs. (True.)
Fill in the blank: social cost equals private cost plus ______ cost. (external)
True or false: the tragedy of the commons applies to public goods. (False: it applies to common goods, which are rival but non-excludable.)
Q: A factory emits pollution that imposes a marginal external cost of $3 per unit. The market equilibrium quantity is 1,000 units. Describe how a Pigovian tax would correct this externality.
A: The government sets a tax of $3 per unit, equal to the marginal external cost. This raises the firm's effective marginal cost by $3 at every quantity, shifting the supply curve upward. The new equilibrium quantity is lower (where the shifted supply intersects demand), matching the socially optimal output where MSC = MSB. The tax internalises the externality.
Q: Why does the free-rider problem lead to under-provision of public goods?
A: Because a public good is non-excludable, each individual has an incentive to let others pay for it while still enjoying the benefit. If everyone free-rides, no one contributes, and the good is not provided, or it is provided at a quantity well below the socially optimal level.
Q: Classify the following: a public park that is heavily congested. Is it a public good or a common good?
A: A congested park is rival (one person's use reduces the enjoyment available to others) but non-excludable (anyone can enter). That makes it a common good, not a public good. It is subject to the tragedy of the commons.
Q: Under what conditions does the Coase theorem hold, and why does it often fail in practice for pollution?
A: The Coase theorem holds when property rights are well-defined and transaction costs are low. For pollution, these conditions rarely hold: millions of people may be affected, individual damages are small and hard to quantify, and coordinating negotiations among so many parties is prohibitively expensive. Government intervention (taxes, regulation, cap-and-trade) is typically needed instead.
The surplus framework from Doc 1 (consumer and producer surplus, deadweight loss) is the tool used here to measure the size of market failures caused by externalities. Consumer choice theory from Doc 2 explains why individuals free-ride on public goods: each person optimises their own utility given prices and income. This material also leads into environmental economics, public finance (how governments raise revenue to fund public goods), and the broader debate about when government intervention improves or worsens market outcomes.
Rivalry, excludability, private good, club good, public good, common good, common-pool resource, externality, negative externality, positive externality, private cost, external cost, social cost, marginal social cost, MSC, marginal social benefit, MSB, Pigovian tax, cap-and-trade, emissions trading, Coase theorem, property rights, transaction costs, free-rider problem, tragedy of the commons, market failure, government intervention, regulation, subsidies, public provision, overfishing, pollution, deadweight loss, allocative efficiency, welfare economics