How do costs and benefits that spill onto third parties cause a market to over- or under-produce, and how can policy fix it?
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.
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What this topic is asking
Topic 6.2 examines externalities: costs or benefits that fall on third parties not involved in a transaction. The College Board wants you to distinguish negative from positive externalities, show how private and social cost or benefit diverge, identify the resulting overproduction or underproduction and deadweight loss, and explain corrective taxes, subsidies, regulation, and property rights. It is the most heavily examined market failure.
Negative externalities
The market sets output where demand meets the private cost curve (supply), ignoring the external cost. Because the true social cost is higher, the market overproduces relative to the efficient quantity (where demand meets ). The over-produced units, whose social cost exceeds their social benefit, create a deadweight loss, the triangle between and demand from the efficient quantity out to the market quantity.
Positive externalities
Here the market sets output where the private benefit curve (demand) meets supply, ignoring the external benefit. Because the true social benefit is higher, the market underproduces relative to the efficient quantity (where meets supply). The units that should be produced but are not, whose social benefit exceeds their social cost, create a deadweight loss.
The general rule and the policy logic
The unifying idea is internalising the externality: a corrective tax or subsidy makes the private decision-maker face the full social cost or benefit, so the market quantity moves to where . The size of the efficient tax or subsidy equals the marginal external cost or benefit at the efficient quantity. Negative externalities call for a tax (to cut overproduction); positive externalities call for a subsidy (to raise underproduction). Getting the direction right, tax for "too much," subsidy for "too little," is the core exam skill.
Try this
Q1. A good generates a positive externality. State whether the market over- or under-produces and the corrective policy. [2 points]
- Cue. It underproduces (marginal social benefit exceeds marginal private benefit); a per-unit subsidy equal to the marginal external benefit corrects it.
Q2. State the relationship between marginal social cost and marginal private cost for a negative production externality. [1 point]
- Cue. Marginal social cost exceeds marginal private cost by the marginal external cost.
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 2018 (style)1 marksMultiple choice. In the presence of a negative externality in production, the free market will (A) underproduce, and the social cost is below the private cost. (B) overproduce, and the social cost is above the private cost. (C) produce the efficient quantity. (D) underproduce, and the social benefit exceeds the private benefit. (E) overproduce, and the social benefit is above the private benefit.Show worked answer →
The answer is (B). A negative production externality (such as pollution) means the marginal social cost is above the marginal private cost; the market, ignoring the external cost, overproduces beyond the efficient quantity.
(A) reverses the cost relationship. (C) ignores the externality. (D) and (E) describe positive (benefit-side) externalities, not a negative cost-side one.
AP 2021 (style)5 marksFree response. A factory emits pollution. (a) Draw a correctly labelled graph showing marginal private cost, marginal social cost, and demand. (b) Identify the market quantity and the socially efficient quantity. (c) Show the deadweight loss. (d) Identify a corrective policy and the size of an efficient per-unit tax. (e) Explain how the tax restores efficiency.Show worked answer →
A five-point externality FRQ.
(a) (1 point): demand (MSB), marginal private cost (MPC = supply), and marginal social cost (MSC) above MPC by the external cost.
(b) (1 point): market quantity where demand meets MPC; efficient quantity where demand meets MSC (smaller).
(c) (1 point): deadweight loss is the triangle between MSC and demand from the efficient quantity out to the market quantity (the overproduced units).
(d) (1 point): a corrective (Pigouvian) per-unit tax equal to the marginal external cost; this shifts MPC up to MSC.
(e) (1 point): the tax makes producers internalise the external cost, raising their cost to the social cost, so the market quantity falls to the efficient quantity and the deadweight loss is eliminated.
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 6.3 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.
- 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 2.8 The Effects of Government Intervention in Markets: analyze binding price ceilings and floors, per-unit taxes and subsidies, and quantity controls, showing the resulting shortages, surpluses, tax incidence, and deadweight loss.
A focused answer to AP Microeconomics Topic 2.8, covering binding price ceilings and floors, per-unit excise taxes and subsidies, tax incidence and elasticity, quantity controls (quotas), and the deadweight loss intervention creates, 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)