Why do free markets under-provide goods that no one can be excluded from, and how does the free-rider problem arise?
Topic 6.3 Public and Private Goods: classify goods by rivalry and excludability, explain the free-rider problem for public goods, and explain why markets under-provide public goods and how government provision responds.
A focused answer to AP Microeconomics Topic 6.3, covering the classification of goods by rivalry and excludability, public goods, common resources and the tragedy of the commons, the free-rider problem, and why markets under-provide public goods, with worked exam-style questions.
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What this topic is asking
Topic 6.3 explains a second source of market failure: public goods. The College Board wants you to classify goods by rivalry and excludability, define a public good, explain the free-rider problem, and explain why markets under-provide public goods so that government provision is often needed. The classification scheme also covers private goods, common resources, and club goods.
Classifying goods: rivalry and excludability
The two properties give four categories:
- Private good (rival and excludable): food, clothing, most market goods, well handled by markets.
- Public good (non-rival and non-excludable): national defense, a lighthouse, clean air, under-provided by markets.
- Common resource (rival but non-excludable): ocean fisheries, public grazing land, prone to overuse, the tragedy of the commons.
- Club good (non-rival but excludable): cable television, a toll road with spare capacity.
The market handles private goods well, but the non-excludable categories (public goods and common resources) cause market failure, which is what this topic and the externality topic between them explain.
The free-rider problem and under-provision
The deeper point links back to Topic 6.1: a public good with large social benefits would be worth providing (its marginal social benefit exceeds its marginal social cost), but the market fails to provide it because no firm can profit from a good it cannot charge for. Government provision, financed by taxes that everyone must pay, is the usual remedy; for common resources, the remedy is often to assign property rights or regulate use to prevent overuse.
Try this
Q1. State the two characteristics that define a pure public good. [2 points]
- Cue. It is non-rival (one person's use does not reduce another's) and non-excludable (people cannot be prevented from using it).
Q2. Explain why a private firm is unlikely to provide a public good. [2 points]
- Cue. Because the good is non-excludable, free riders consume without paying, so the firm cannot earn enough revenue to cover its costs, leading to under-provision.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)1 marksMultiple choice. A pure public good is (A) rival and excludable. (B) non-rival and non-excludable. (C) rival and non-excludable. (D) non-rival and excludable. (E) provided only by private firms.Show worked answer →
The answer is (B). A pure public good is non-rival (one person's use does not reduce another's) and non-excludable (people cannot be prevented from using it), such as national defense.
(A) is a private good. (C) is a common resource. (D) is a club good. (E) is false; public goods are typically provided by government because the market under-provides them.
AP 2021 (style)3 marksFree response (short). (a) Define a public good using its two characteristics. (b) Explain the free-rider problem. (c) Explain why the free market under-provides public goods.Show worked answer →
A three-point short FRQ.
(a) (1 point): a public good is non-rival (one person's consumption does not reduce the amount available to others) and non-excludable (people cannot be prevented from consuming it).
(b) (1 point): the free-rider problem is that, because people cannot be excluded, they have an incentive to consume the good without paying, hoping others will fund it.
(c) (1 point): because free riders will not pay, firms cannot capture enough revenue to cover costs, so the market produces too little (or none) of the good, which is why government often provides it.
Related dot points
- Topic 6.1 Socially Efficient and Inefficient Market Outcomes: define allocative efficiency as marginal social benefit equal to marginal social cost, identify deadweight loss, and explain what causes market failure.
A focused answer to AP Microeconomics Topic 6.1, covering allocative efficiency as marginal social benefit equal to marginal social cost, total surplus, deadweight loss, and the main sources of market failure, with worked exam-style questions.
- Topic 6.2 Externalities: distinguish negative from positive externalities, show the divergence of private and social cost or benefit, identify the resulting overproduction or underproduction and deadweight loss, and explain corrective taxes and subsidies.
A focused answer to AP Microeconomics Topic 6.2, covering negative and positive externalities, the divergence between private and social cost or benefit, the overproduction and underproduction they cause, the deadweight loss, and corrective taxes, subsidies, regulation, and property rights, with worked exam-style questions.
- Topic 6.4 The Effects of Government Intervention in Different Market Structures: analyze antitrust policy, the regulation of a natural monopoly through marginal-cost and average-cost pricing, and how intervention can reduce deadweight loss when a market failure exists.
A focused answer to AP Microeconomics Topic 6.4, covering antitrust policy against market power, the regulation of a natural monopoly through marginal-cost and average-cost (fair-return) pricing, and how well-targeted intervention can reduce deadweight loss when a market failure exists, with worked exam-style questions.
- Topic 6.5 Income and Wealth Inequality: distinguish income from wealth, measure inequality with the Lorenz curve and Gini coefficient, identify the sources of inequality, and explain how progressive taxes and transfers redistribute income.
A focused answer to AP Microeconomics Topic 6.5, covering the difference between income and wealth, the Lorenz curve and Gini coefficient, the main sources of inequality, and how progressive, proportional, and regressive taxes and transfer programs redistribute income, with worked exam-style questions.
- Topic 2.6 Market Equilibrium and Consumer and Producer Surplus: find equilibrium price and quantity, identify consumer and producer surplus on a graph, and explain why the competitive equilibrium maximizes total surplus (allocative efficiency).
A focused answer to AP Microeconomics Topic 2.6, covering how supply and demand determine equilibrium price and quantity, the measurement of consumer and producer surplus, total surplus, and why the competitive equilibrium is allocatively efficient, with worked exam-style questions.
Sources & how we know this
- AP Microeconomics Course and Exam Description — College Board (2023)