Market Failure Practice Questions

8 representative multiple-choice questions on market failure for AP Microeconomics, drawn from our 15-question bank for this module. Work through each one, then open “Show answer” for the correct choice and an explanation. For scored, timed practice across the full bank, take a full practice test.

1. 1. A chemical plant dumps waste into a river, harming downstream fisheries. This is an example of:

   • A. A positive externality, because the chemical plant creates jobs
   • B. A negative externality, because third parties bear costs not reflected in the market price
   • C. A public good, because the river is available to everyone
   • D. Moral hazard, because the plant changed its behavior after entering the market
   Show answer

   Correct answer: B. A negative externality, because third parties bear costs not reflected in the market price

   The downstream fisheries bear real costs (damaged catches, contaminated water) from production they had no part in. Textbook negative externality: social cost exceeds private cost and the market overproduces. (A) is wrong because jobs go to the plant's own workers; they're a private outcome, not a spillover benefit to unrelated third parties. (C) confuses the river as a common resource with the concept of a public good, and the real issue here is the pollution cost, not how to classify the river.

2. 2. When a positive externality exists in a market, the socially optimal quantity is:

   • A. Less than the market equilibrium quantity
   • B. Equal to the market equilibrium quantity
   • C. Greater than the market equilibrium quantity
   • D. Impossible to determine without knowing the demand curve
   Show answer

   Correct answer: C. Greater than the market equilibrium quantity

   With positive externalities, the social benefit exceeds what private buyers account for. Consumers only weigh their own gain, so the market underproduces relative to what society needs. The socially optimal quantity is therefore higher than the market equilibrium. (A) describes negative externalities, where overproduction is the problem. (B) would only be true if there were no externality at all.

3. 3. In Akerlof's "market for lemons" model, what causes the market to break down?

   • A. Buyers offer too much for low-quality goods
   • B. The government sets a price ceiling on used cars
   • C. Sellers of high-quality goods exit because buyers cannot distinguish quality, driving average quality down
   • D. Too many buyers enter the market, creating a shortage of used cars
   Show answer

   Correct answer: C. Sellers of high-quality goods exit because buyers cannot distinguish quality, driving average quality down

   Buyers can't distinguish peaches from lemons, so they offer an average price. Owners of good cars find that average insulting compared to what their car is worth and pull out. The remaining pool skews toward lemons. Buyers adjust downward, more good-car owners leave, and the spiral of adverse selection can collapse the market. (A) reverses the mechanism; the problem is buyers paying too little for good cars, not too much for bad ones. (B) is wrong because the breakdown comes from information asymmetry, not price controls.

4. 4. A Pigouvian tax on pollution is designed to:

   • A. Raise government revenue to fund public schools
   • B. Punish firms for unethical behavior
   • C. Close the gap between private cost and social cost so the market produces the efficient quantity
   • D. Eliminate all pollution by making production unprofitable
   Show answer

   Correct answer: C. Close the gap between private cost and social cost so the market produces the efficient quantity

   The tax equals the marginal external cost per unit, which lifts the firm's private cost to match social cost. The market then naturally adjusts to the socially optimal output. (D) misunderstands the goal; it's not zero pollution but the efficient level, where the marginal social benefit of one more unit equals the marginal social cost. Some pollution is worth tolerating when the goods being produced are valuable enough. (B) frames it as moral punishment, but a Pigouvian tax is a corrective efficiency tool, not an ethical judgment.

5. 5. Which of the following is an example of moral hazard?

   • A. A used car seller hides a known defect from the buyer
   • B. A person with health insurance visits the doctor more frequently because visits are covered
   • C. A firm with market power raises prices above the competitive level
   • D. Residents free-ride on a neighbor's pest control service
   Show answer

   Correct answer: B. A person with health insurance visits the doctor more frequently because visits are covered

   Moral hazard is changed behavior after a deal is struck because the other party can't fully observe you. Someone with insurance uses more medical care because the insurer picks up most of the tab. (A) is adverse selection, meaning hidden information before the transaction, not changed behavior after it. That adverse selection vs. moral hazard distinction is one of AP Micro's most commonly tested points in the market failure unit. (D) is the free-rider problem, related to public goods and externalities, not post-contract behavior change.

6. 6. A Pigouvian tax is set at $30 per unit to correct a negative externality. Compared to a command-and-control regulation that mandates the same output level, the Pigouvian tax is generally preferred by economists because:

   • A. It generates no deadweight loss while the regulation does
   • B. It allows firms with lower abatement costs to reduce pollution more, achieving the target at lower total cost
   • C. It completely eliminates all pollution while regulation only reduces it
   • D. It does not require the government to have any information about external costs
   Show answer

   Correct answer: B. It allows firms with lower abatement costs to reduce pollution more, achieving the target at lower total cost

   The tax puts a price on the externality and lets each firm decide how to respond based on its own cost structure. Firms that can cut pollution cheaply do so and avoid the tax; firms where cleanup is expensive pay the tax instead. Same pollution reduction, lowest total cost to the economy. A blanket regulation that forces uniform cuts ignores the fact that abatement costs vary wildly across firms. (A) is misleading because both can eliminate the externality-related DWL; the tax's real advantage is cost-effectiveness. (C) is wrong because neither aims for zero pollution. (D) is wrong because setting the right Pigouvian tax requires knowing the marginal external cost.

7. 7. National defense is classified as a public good because it is:

   • A. Produced by the government and funded through taxes
   • B. Non-rival and non-excludable: one person's protection does not reduce another's, and no citizen can be excluded
   • C. Expensive to produce, making it impractical for private firms
   • D. Consumed equally by all citizens regardless of income
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   Correct answer: B. Non-rival and non-excludable: one person's protection does not reduce another's, and no citizen can be excluded

   The two defining features: non-rivalry (defending one person doesn't reduce defense for others) and non-excludability (you can't selectively leave certain residents unprotected). (A) confuses how the good is provided with what makes it a public good. Government provision is the response to the public goods problem, not the definition. (C) just identifies a practical difficulty, but lots of expensive goods are perfectly private. (D) describes a consequence of non-rivalry and non-excludability rather than the classification criteria themselves.

8. 8. Which of the following government interventions is most appropriate for correcting the underproduction of a good with a positive externality?

   • A. A per-unit tax equal to the marginal external benefit
   • B. A per-unit subsidy equal to the marginal external benefit
   • C. A price ceiling set below the market equilibrium price
   • D. A quota that limits production to the current market level
   Show answer

   Correct answer: B. A per-unit subsidy equal to the marginal external benefit

   Positive externality means MSB > MPB and the market underproduces. A per-unit subsidy equal to the marginal external benefit shifts effective demand up to align with the social benefit curve, pushing consumption to the socially optimal level. (A) would make things worse because you don't tax a good that's already being underproduced. (C) creates a shortage and could further reduce quantity exchanged. (D) locks in the current inefficient output instead of correcting it.