AP Microeconomics, AP exams

Unit 6: Market Failure and the Role of Government

Unit 6 covers externalities, public goods, and inequality, along with the policy tools intended to address them. Socially optimal versus market outcomes and the resulting deadweight loss are the core graphical analysis. The unit closes the course by asking when government intervention improves efficiency and when it does not.

Position: 6 of 6 units
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Microeconomics. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Efficient and inefficient market outcomes: Allocative efficiency where marginal social benefit equals marginal social cost, and the deadweight loss when it is not.
Negative externalities: Costs imposed on third parties, overproduction relative to the social optimum, and corrective taxation.
Positive externalities: Benefits accruing to third parties, underproduction relative to the social optimum, and subsidies as a remedy.
Public goods and common resources: Non-rivalry and non-excludability, free riding, and the tragedy of the commons.
Government intervention across market structures: Antitrust policy, regulation of natural monopoly, and the trade-offs of intervening in imperfect markets.
Income and wealth inequality: The Lorenz curve and Gini coefficient, sources of inequality, and redistributive policy tools.

Study tips

How to practice Market Failure.

Section-specific guidance that follows from how this material is usually tested.

Draw externality graphs with both private and social curves and label the socially optimal quantity every time.
Practice explaining why the market fails rather than only identifying that it does.
Contrast public goods with common resources using excludability and rivalry as the two axes.

Where this fits

Unit 6: Market Failure and the Role of Government is 6 of 6 units in the AP Microeconomics guide. Use the full guide to balance it against everything else you have to review.

Back to the AP Microeconomics guide Official exam information

Sample Market Failure questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
Which pair of characteristics defines a pure public good such as national defense?
1. ANon-rival and non-excludable
2. BRival and excludable
3. CRival and non-excludable
4. DNon-rival and excludable
Show answer
Answer: A. Non-rival and non-excludable
One person's consumption of a public good does not reduce what is available to anyone else, and no one can be prevented from benefiting, which together create the free-rider problem that keeps private markets from supplying enough. Rival and non-excludable is the tempting distractor, but that combination defines a common resource such as an ocean fishery, which suffers from overuse rather than undersupply.
Key insight: Rivalry and excludability form a two-by-two grid: public goods sit in the non-rival, non-excludable corner and common resources in the rival, non-excludable one.
Question 2Foundational
A factory's production imposes pollution costs on nearby residents who are not part of the transaction. How does the unregulated market outcome compare with the socially optimal one?
1. AThe market produces more than the socially optimal quantity
2. BThe market produces less than the socially optimal quantity
3. CThe market produces exactly the socially optimal quantity
4. DThe market cannot operate at all under these conditions
Show answer
Answer: A. The market produces more than the socially optimal quantity
The firm bases its supply decision on its private costs and ignores the pollution costs borne by others, so the good looks cheaper to produce than it truly is and too much of it is made. Underproduction is the tempting distractor, but that is the signature of a positive externality, where third parties gain benefits the market does not reward.
Key insight: Negative externalities produce overproduction and positive externalities produce underproduction, so the corrective tool is a tax in one case and a subsidy in the other.
Question 3Intermediate
Which policy does standard economic analysis recommend for correcting a positive externality, such as the wider benefits of vaccination?
1. AA per-unit subsidy equal to the external benefit
2. BA per-unit tax equal to the external benefit
3. CA binding price ceiling on the good
4. DA quota restricting how much may be produced
Show answer
Answer: A. A per-unit subsidy equal to the external benefit
A subsidy set to the size of the spillover benefit lowers the effective price to buyers and raises output to the point where marginal social benefit equals marginal social cost. A tax is the tempting distractor because corrective taxes are the familiar externality remedy, but taxing here would shrink an activity society already gets too little of.
Key insight: Match the tool to the direction of the failure: tax what the market overproduces and subsidize what it underproduces.
Question 4Intermediate
What does a Lorenz curve show, and what does it mean when the curve bows farther away from the line of perfect equality?
1. AIt plots cumulative income share against cumulative population share, and a greater bow means more inequality
2. BIt plots cumulative income share against cumulative population share, and a greater bow means less inequality
3. CIt plots price against quantity demanded, and a greater bow means more elastic demand
4. DIt plots inflation against unemployment, and a greater bow means a worse policy trade-off
Show answer
Answer: A. It plots cumulative income share against cumulative population share, and a greater bow means more inequality
Perfect equality would place every point on the 45 degree line, so the farther the actual curve sags below it, the larger the share of income concentrated among top earners. Reading a greater bow as less inequality is the tempting distractor, but it reverses the geometry, and the Gini coefficient formalizes exactly this distance as a number between zero and one.
Key insight: The Gini coefficient is the area between the Lorenz curve and the equality line as a share of the whole triangle, so a bigger gap means a bigger Gini.
Question 5Advanced
At the market equilibrium quantity in an industry that generates a negative production externality, what is the relationship between marginal social cost and marginal social benefit?
1. AMarginal social cost exceeds marginal social benefit, so the last units produced destroy value
2. BMarginal social benefit exceeds marginal social cost, so more should be produced
3. CMarginal social cost equals marginal social benefit, so the outcome is efficient
4. DMarginal private cost exceeds marginal social cost, so the firm overpays
Show answer
Answer: A. Marginal social cost exceeds marginal social benefit, so the last units produced destroy value
The market clears where marginal private cost meets demand, but the external damage means marginal social cost lies above marginal private cost, so at that quantity society is paying more for the final units than they are worth and the excess is deadweight loss. The claim that marginal private cost exceeds marginal social cost is the tempting distractor, but with a negative externality the ordering runs the other way, since social cost adds the third-party harm to private cost.
Key insight: Allocative efficiency requires marginal social benefit to equal marginal social cost, and externalities are simply cases where private and social curves diverge.

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Unit 6: Market Failure and the Role of Government