AP Microeconomics / Market Failure and the Role of Government
Lesson
Externalities and missing prices
When a trade spills costs or benefits onto bystanders, the market quantity is the wrong one.
Learning goals
- Show a negative externality as a gap between private and social cost.
- Show a positive externality as a gap between private and social benefit.
Explanation
A negative externality, such as pollution, means part of the cost lands on people who are not in the deal. Private supply understates social cost, so the market quantity is too high. A tax equal to the external cost can move quantity toward the social optimum.
A positive externality, such as a neighbor’s vaccination or a well-kept garden others enjoy, means part of the benefit is unpaid. Private demand understates social benefit, so the market quantity is too low. A subsidy can close that gap. The graph tells you the direction; the size of the tax or subsidy is an empirical question.
Key terms
- Externality. A cost or benefit of a transaction that falls on someone who is not the buyer or seller.
- Social cost. Private cost plus external cost.
Common mistakes
- Shifting demand for a production pollution problem that belongs on the cost side.
- Calling every government action an efficiency fix when some are mainly about equity.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A binding price ceiling is set below equilibrium in the market for rental apartments. What is the most likely result?
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
Education has a positive externality. Is the unregulated market quantity too high or too low?
Answer. Too low, because buyers consider private benefits, not the extra social benefits.
MSB lies above demand, so the private equilibrium misses some beneficial units.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Microeconomics course page
Official six-unit framework for markets, costs, and market failure.
College Board · All rights reserved · accessed 2026-10-01
Open sourceCC BY — attribution required
Principles of Microeconomics: Scarcity and Social Provisioning
Open microeconomics background for scarcity, markets, and elasticity.
Open Textbook Library listing · CC BY · accessed 2026-10-01
Open source