Chapter 10: Externalities
When markets ignore bystanders — and what economists propose to fix them
Section 1
What Are Externalities?
Markets work beautifully when every cost and benefit lands on the buyer or seller making the decision. But sometimes a transaction affects bystanders who have no say in it. Mankiw defines an externality as the uncompensated impact of one person's actions on the well-being of a bystander.
Because markets only reflect private costs and private benefits, they ignore external effects. The result: the market equilibrium is no longer socially optimal — it produces either too much or too little of the good.
Negative Externality
The action harms bystanders. Private costs understate true social costs, so the market overproduces.
Examples: factory smoke, carbon emissions, noise pollution, secondhand smoke
Positive Externality
The action benefits bystanders. Private benefits understate true social value, so the market underproduces.
Examples: education, vaccines, R&D, restoring a historic building
An externality means that the private market equilibrium does not maximize total social welfare. Some gains from trade go uncaptured (positive externality) or some trades impose hidden costs (negative externality). Either way, the invisible hand fails to account for society's full interests.
Section 2
Negative Externalities and Overproduction
Consider a steel mill that dumps pollution into a river while producing steel. The mill pays its labor, capital, and materials — those are its private costs. But it also imposes costs on downstream fishermen and residents. Those external costs are real, but the mill ignores them because it doesn't pay them.
The social cost is what matters for society's well-being:
Because the social cost curve lies above the private supply curve by the amount of the external cost, the market produces more than the social optimum. The difference — units produced between Qopt and Qmkt — generates a deadweight loss: each of those units costs society more than it is worth.
The DWL triangle represents the units of output where the social cost exceeds the social value. Those units get produced in the market but should not be — they reduce total welfare. This is why negative externalities are a form of market failure.
Section 3
Positive Externalities and Underproduction
Now consider education. When you learn to read, write, and think critically, you become a better employee, neighbor, and citizen — your classmates benefit from your better contributions, employers benefit from a more productive workforce, and democracy benefits from a more informed electorate. Those external benefits are real, but you don't get paid for them.
The social value includes both what you privately gain and what bystanders gain:
The social value curve lies above the private demand curve by the amount of the external benefit. The market stops producing at Qmkt, but society would benefit from expanding to Qopt. The units between Qmkt and Qopt that don't get produced represent DWL — gains from trade that never happen.
Notice the symmetry: both negative and positive externalities create DWL, but in opposite directions. Negative externalities push quantity too high; positive externalities push it too low. In both cases, the market "gets the quantity wrong" from society's perspective.
Section 4
Government Solutions: Pigovian Taxes & Cap-and-Trade
When markets fail due to externalities, governments have two broad toolkits: command-and-control (regulation) and market-based solutions (taxes, subsidies, tradable permits). Economists generally prefer market-based approaches because they achieve efficiency at lower total cost.
Command & Control
Government sets standards or mandates: emission limits, required vaccinations, minimum education laws. Simple but often inflexible and costly.
Pigovian Tax (negative)
Tax set equal to the external cost. Raises private cost to match social cost, pushing market quantity to the optimum. Named after economist A.C. Pigou.
Pigovian Subsidy (positive)
Subsidy equal to the external benefit. Lowers private cost (or raises private demand), pushing market quantity up to the social optimum.
Cap and Trade
Government caps total pollution, issues permits, lets firms trade. Achieves the target quantity at lowest cost; permit price acts like a Pigovian tax.
Pigovian Tax: How It Works
A Pigovian tax on a negative externality works by making polluters pay for the harm they cause. When the tax equals the external cost, the supply curve shifts up by exactly that amount — matching the social cost curve. The new market equilibrium equals the social optimum.
The beauty of this approach: it doesn't require the government to know each firm's costs. The price signal does the work, and firms with the lowest abatement costs reduce pollution most.
Drag the slider to change the Pigovian tax and see how it shifts the market toward the social optimum.
Market Data
Pigovian optimum!
Cap-and-Trade
Cap-and-trade achieves the same result as a Pigovian tax but by setting the quantity rather than the price. The government issues a fixed number of pollution permits (the "cap"), then lets firms buy and sell them. Firms that can reduce pollution cheaply will sell permits to firms for which reduction is expensive.
The permit price that emerges in this market is equivalent to a Pigovian tax — it internalizes the externality. The key difference: a tax fixes the price of pollution and lets the quantity adjust; cap-and-trade fixes the quantity and lets the price adjust.
Both approaches achieve the efficient outcome when the government knows the right target. Under uncertainty: if it's more costly to get the quantity slightly wrong (steep damage curves), prefer a tax. If more costly to get the price wrong, prefer permits. In practice, the U.S. uses both — carbon taxes and regional cap-and-trade programs.
Section 5
Private Solutions: The Coase Theorem
Government isn't the only solution. Sometimes private parties can negotiate their way to efficiency without any government intervention. Economist Ronald Coase showed that this is possible under specific conditions.
If property rights are well-defined and transaction costs are low, private parties will bargain to an efficient outcome regardless of who holds the initial property rights.
Example: A railroad emits sparks that burn a neighboring farmer's crops. Who is "responsible"? Coase showed it doesn't matter for efficiency. Suppose the damage is $100/year and the cost of a spark-catcher is $50. If the railroad has the right to emit sparks, the farmer will pay the railroad up to $100 to install the catcher (and will, since $50 < $100). If the farmer has the right to clean air, the railroad installs the catcher anyway to avoid liability. Either way: catcher installed, efficient outcome.
Why Coase Often Fails in Practice
The Coase theorem is elegant in theory but fragile in practice. Three big obstacles:
- Transaction costs: When many parties are affected (thousands of people downwind of a factory), organizing them to bargain is prohibitively expensive.
- Information asymmetry: Parties may misrepresent their true costs or damages to gain bargaining advantage, breaking down the negotiation.
- Holdout problems: Even if most parties agree, a single holdout can block a deal (common with many affected parties).
This is why externality problems involving many people — air and water pollution, climate change — typically require government intervention. Private bargaining works best for small-number, local disputes (two neighboring property owners, a small number of firms sharing a resource).
Quick Review
Fast Checks
Click each question to reveal the answer. Use these to test yourself before the exam.
Summary
Key Takeaways
- Externalities are uncompensated impacts on bystanders. When external effects exist, the market equilibrium is not socially optimal.
- Negative externalities (pollution) cause overproduction: social cost > private cost, so the social cost curve lies above supply.
- Positive externalities (education, vaccines) cause underproduction: social value > private value, so the social value curve lies above demand.
- Pigovian tax = external cost per unit. It shifts the supply curve up to match the social cost curve, restoring the social optimum.
- Cap-and-trade fixes the quantity of pollution and lets permit prices adjust. Achieves the same efficiency as a Pigovian tax.
- Coase theorem: with clear property rights and zero transaction costs, private bargaining achieves efficiency — but these conditions rarely hold for large-scale problems.
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