Externalities
Market Failures & Spillovers

Explore how third-party costs and benefits cause markets to produce inefficient outcomes, and learn how governments intervene to restore efficiency.

AP Microeconomics

What Is an Externality?

An externality is a cost or benefit that affects a party who did not choose to incur that cost or benefit. Externalities arise when private decisions create spillover effects on third parties that are not reflected in market prices.

Two Types: Negative & Positive

Negative Externality

A cost imposed on third parties. Example: a factory pollutes a river, harming downstream fisheries. The social cost exceeds the private cost.

Positive Externality

A benefit enjoyed by third parties. Example: getting vaccinated protects others through herd immunity. The social benefit exceeds the private benefit.

Why Externalities Matter

In a perfectly competitive market with no externalities, the equilibrium quantity maximizes total surplus. But externalities create a wedge between private and social costs or benefits, leading to deadweight loss.

1

Negative externalities cause overproduction: the market quantity $ Q_M $ exceeds the socially optimal quantity $ Q^* $.

2

Positive externalities cause underproduction: the market quantity $ Q_M $ falls short of the socially optimal quantity $ Q^* $.

3

In both cases, a deadweight loss exists that can potentially be corrected by government policy.

The Core Rule: $ MSC = MSB $

Social efficiency requires that the Marginal Social Cost equals the Marginal Social Benefit at the optimal quantity. The key relationships are:

Negative Externality Graph

Positive Externality Graph

Production vs. Consumption Externalities

	Production	Consumption
Source	Arises from the production process	Arises from the consumption of a good
Negative Example	Factory pollution	Secondhand cigarette smoke
Positive Example	Beekeeper near an orchard	Education, vaccinations
Graph Shift	$ MSC \neq MPC $ (supply side)	$ MSB \neq MPB $ (demand side)

Government Solutions

Corrective Tax

A per-unit tax set equal to the external cost. Shifts MPC up to MSC. Also called a Pigouvian tax.

Subsidy

A per-unit payment set equal to the external benefit. Shifts MPB up to MSB, increasing production.

Regulation

Command-and-control policies: emission limits, mandates, or bans that directly restrict harmful activity.

Tradable Permits

Government sets a cap on total emissions and firms trade permits. Market-based approach to reach $ Q^* $.

The Coase Theorem

The Coase Theorem states that if property rights are well-defined and transaction costs are low, private parties can negotiate to reach the socially efficient outcome regardless of which party holds the rights.

1

Property rights must be clearly assigned to either the polluter or the affected party.

2

Transaction costs must be negligible: few parties involved, easy to identify and negotiate.

3

Outcome is efficient but the distribution of gains depends on who holds the initial property rights.

How to Analyze Externalities

1

Identify the externality. Is a third party harmed (negative) or helped (positive)?

2

Determine production or consumption. Does it shift the supply side ($ MSC \neq MPC $) or the demand side ($ MSB \neq MPB $)?

3

Draw both curves. Show the private curve and the social curve. The gap is the external cost or benefit.

4

Compare $ Q_M $ and $ Q^* $. Negative: $ Q_M > Q^* $ (overproduction). Positive: $ Q_M < Q^* $ (underproduction).

5

Identify the correction. Tax = external cost for negative; subsidy = external benefit for positive. Both move the market to $ Q^* $.

Example 1: Pollution Tax

A steel factory produces pollution that imposes an external cost of $ \$10 $ per ton on nearby residents. The private market equilibrium is at $ Q_M = 500 $ tons at $ P = \$50 $.

Example 2: Education Subsidy

Education generates positive consumption externalities: an educated population benefits everyone through innovation, lower crime, and civic engagement. Suppose the external benefit is $ \$3{,}000 $ per student.

Example 3: Cap-and-Trade System

Common Mistakes

✕ Shifting the Wrong Curve

A negative production externality shifts the supply curve (MSC above MPC), not the demand curve. Check whether the externality is on the cost side or the benefit side.

✕ Direction of Output Error

Negative externalities cause overproduction ($ Q_M > Q^* $), not underproduction. Positive externalities cause underproduction. Don't mix them up.

✕ Ignoring the Coase Conditions

Students cite the Coase Theorem without checking that transaction costs are low and property rights are defined. It fails with many parties or unclear rights.

Summary Comparison

	Negative Externality	Positive Externality
Third-Party Effect	Cost (harm)	Benefit (help)
Social Curve	$ MSC > MPC $	$ MSB > MPB $
Market Outcome	Overproduction ($ Q_M > Q^* $)	Underproduction ($ Q_M < Q^* $)
Market Price	Too low ($ P_M < P^* $)	Too low ($ P_M < P^* $)
Government Fix	Tax / regulation / permits	Subsidy / public provision
Private Fix	Coase bargaining (if feasible)	Coase bargaining (if feasible)

Question 1 of 5

A factory discharges waste into a river, harming downstream fisheries. This is best described as a:

The pollution arises from the production process and imposes a cost on third parties (fisheries). This makes it a negative production externality: $ MSC > MPC $.

Question 2 of 5

When a negative externality exists in a competitive market, the market equilibrium quantity is:

With a negative externality, private costs understate true social costs. The market ignores the external cost, so firms produce more than the socially efficient level: $ Q_M > Q^* $.

Question 3 of 5

To correct a positive externality, the government should:

A positive externality means the market underproduces the good. A per-unit subsidy equal to the external benefit shifts the MPB curve up to MSB, increasing quantity to $ Q^* $. A tax would reduce quantity further, making the problem worse.

Question 4 of 5

According to the Coase Theorem, private bargaining can resolve externalities efficiently if:

The Coase Theorem requires two conditions: well-defined property rights (assigned to either party) and low transaction costs. It doesn't matter who holds the rights or whether the externality is production vs. consumption. Large numbers of parties usually mean high transaction costs, violating the theorem's conditions.

Question 5 of 5

In a cap-and-trade system for carbon emissions, the permit price is determined by:

In a cap-and-trade system, the government sets a fixed cap (vertical supply of permits). Firms that need to emit trade for permits. The market price emerges where the fixed supply meets the downward-sloping demand for permits, just like any market.

ExternalitiesMarket Failures & Spillovers

What Is an Externality?

Two Types: Negative & Positive

Why Externalities Matter

The Core Rule: \( MSC = MSB \)

Negative Externality Graph

Positive Externality Graph

Production vs. Consumption Externalities

Government Solutions

The Coase Theorem

How to Analyze Externalities

Example 1: Pollution Tax

Example 2: Education Subsidy

Example 3: Cap-and-Trade System

Common Mistakes

Summary Comparison

Question 1 of 5

Question 2 of 5

Question 3 of 5

Question 4 of 5

Question 5 of 5

Externalities
Market Failures & Spillovers