Concept/Economics/No. 0355
Externality
An externality is a cost or benefit that affects others but is not fully reflected in a decision maker’s incentives or prices. In economics, Arthur Pigou studied this gap between private and social returns; pollution is a negative externality, while shared knowledge can be positive.
- Evidence
- Well established
- Read
- 6 min
- Links
- 11 connections
01You've seen this when…
- in life
Your upstairs neighbor practices drums after dinner. He gets better; your children take longer to fall asleep.
- at work
A warehouse moves deliveries to 4 a.m. to avoid traffic. Its schedule gets faster; nearby residents lose sleep.
- out in the world
A farmer restores a wetland on her land. It holds back stormwater, reducing flood damage downstream, but the town doesn’t pay her for that protection.
02The idea
The warehouse’s savings appear in its accounts. The neighbors’ lost sleep doesn’t. Both are consequences of the same decision, but only one helps determine whether the warehouse goes ahead.
An externality is a cost or benefit that falls on others and isn’t fully reflected in the decision maker’s incentives or the transaction price. The effect sits outside the calculation. It may land just across a fence.
A negative externality imposes a cost: exhaust, noise, contaminated water. A positive externality provides a benefit: a vaccination can reduce infection risk for others, and knowledge developed by one organization can help people elsewhere.
The key distinction is between what makes sense for the person choosing and what makes sense once everyone affected is counted. A delivery schedule can be profitable for a warehouse while costing its neighbors more than it saves. A wetland restoration can be worthwhile overall while offering its owner too little financial return to proceed.
The label describes a gap in incentives. Further analysis must establish the gap’s size and which remedy, if any, would improve the outcome.
03Why it matters
- Individual gains can add up to shared losses. Each driver may save time by taking a particular road, while adding a little delay for everyone already using it. Across thousands of drivers, that congestion externality becomes substantial. Nobody needs to be reckless for the combined result to be bad.
- Beneficial activities can receive too little support. If the farmer pays for wetland restoration but downstream households receive much of the flood protection, her private return understates its total benefit. Useful work can go undone because the beneficiary and the payer are different people.
- Changing incentives can change the result. A corrective tax can attach a cost to pollution. A subsidy or payment can reward a benefit. Rules, liability, and agreements can also make consequences harder to ignore. These approaches change the calculation rather than merely asking people to care more.
- Efficiency and fairness remain separate questions. Making pollution costly may reduce it without compensating those already harmed. Paying for wetland protection still leaves a question about who should fund it. A policy can improve the total outcome while distributing its gains and burdens unfairly.
This is why a low sticker price isn’t always evidence of a low cost. Some of the bill may be arriving somewhere else.
04A worked example
Electric utilities burn coal to generate power. Customers receive electricity and utilities receive revenue. But sulfur dioxide emissions also contribute to acid deposition that damages ecosystems, and to fine-particle pollution that harms health. Some consequences fall far from the plant.
What it looks like A generator uses an inexpensive fuel to offer competitively priced electricity. Its accounts record what it spends on coal and equipment and what it pays workers. Customers can compare the price with another supplier’s.
What’s actually going on Without an effective constraint or charge, those accounts leave out costs borne by people and ecosystems downwind. The market can reward the cheaper generator even when part of its apparent advantage comes from shifting costs onto others.
What would have helped Making emissions matter to the generator’s operating decisions. The U.S. Acid Rain Program, created under the 1990 Clean Air Act Amendments and beginning its first phase in 1995, did this through a sulfur dioxide cap-and-trade system. Covered plants needed an allowance for each ton emitted. Each plant could decide how much to reduce emissions and whether to buy allowances or sell those it no longer needed.
An allowance used today could no longer be sold or saved. Pollution therefore carried an economic cost even when a plant received allowances without buying them. Cleaner fuel and pollution controls became more attractive.
The program shows how a rule can bring an external cost into a business decision. It doesn’t show that all remaining harm disappeared, that the cap perfectly valued every damage, or that affected communities received compensation.
05Where people trip up
- An externality calls for weighing an activity’s benefits and full costs. A decision to prohibit an activity because of an externality needs to account for the benefits even a noisy delivery can provide. The question is whether another delivery’s benefits justify its full costs, and whether quieter equipment or a different schedule could preserve the benefit. That’s marginal analysis: deciding how much activity its benefits and full costs justify.
- They assume bargaining will happen automatically. A warehouse and one neighbor might reach an agreement. A power plant and thousands of downwind households face difficulties identifying harm and organizing themselves to reach and enforce a deal. These transaction costs are central to whether bargaining is practical.
- Positive externalities and public goods describe different features. People confuse these overlapping concepts. A vaccine dose is a private product; reduced infection risk around its recipient is a spillover. A public good is difficult to exclude people from, and one person’s use leaves just as much available for others.
- They stop at naming the problem. The size of an externality may be uncertain, and it can range from tiny to enormous. Before proposing a remedy, identify who is affected and how much they are affected, then assess whether the remedy costs more than the damage it avoids. Cost-benefit analysis helps, provided important harms aren’t silently omitted because they’re hard to price.
- Pollution payments change incentives and raise revenue. Addressing an existing illness or restoring a damaged lake requires separate action. Preventing harm differs from compensating people for it or cleaning up the damage.
06When it isn’t an externality
A new competitor opens nearby and your store loses customers. You are worse off, but this is usually a competitive effect, not an externality. In a reasonably functioning competitive market, price changes and competition are part of how resources move toward different uses.
Effects transmitted only through market prices are often called pecuniary effects. A rival lowering prices differs from a rival sending smoke through your windows: one works through the market’s existing mechanism; the other leaves a cost outside it.
This distinction isn’t a guarantee that every price-mediated effect is harmless. Missing markets, financial constraints, and other distortions can complicate the picture. But counting every competitive loss as an external cost would confuse ordinary market-wide adjustments with the specific incentive gap this concept identifies.
07Roots
Alfred Marshall wanted to explain why firms became more productive when an industry gathered in one place. A workshop could benefit from a local pool of skilled workers and nearby specialized suppliers without creating those advantages itself. In Principles of Economics (1890), he described such benefits as external economies. His category was broader than today’s externality: some of those advantages worked through ordinary market prices.
Arthur Pigou, Marshall’s successor at Cambridge, sharpened the problem in The Economics of Welfare (1920). Consider railway engines throwing sparks that damage nearby woods. The railway’s own costs and the costs of its operation to everyone affected can diverge. Pigou examined these gaps between private and social returns and how taxes or subsidies could address them.
Ronald Coase later challenged the habit of jumping straight from an external harm to a government charge. In his 1960 paper, The Problem of Social Cost, he examined disputes involving activities such as cattle damaging crops. Who held the legal rights, and how difficult was agreement? His analysis made bargaining and its practical obstacles central to the story. The modern idea draws on both traditions: Pigou’s missing costs and benefits, and Coase’s attention to rights and bargaining.
08How solid is this?
The distinction between private and social effects is foundational in economics, and pollution spillovers are extensively measured. Their size, who bears them, and which remedy works best are empirical questions that require evidence beyond the label.
09Connections
- Often confused withPublic Good
- Countered byCoase Theorem, Pigouvian Tax
- Part of Cost-Benefit Analysis, Unintended Consequences
- IncludesCongestion Externality, Tragedy of the Commons
- See alsoTransaction Costs, General Equilibrium Effects, Marginal Analysis, Zero-Sum vs. Non-Zero-Sum
+ 1 more in the list
10Origin and sources
Alfred Marshall described external economies in Principles of Economics (1890). Arthur Pigou developed the analysis of divergences between private and social returns in The Economics of Welfare (1920).
- [1]Marshall, A. (1890). Principles of Economics. Macmillan and Co.
- [2]Pigou, A. C. (1920). The Economics of Welfare. Macmillan and Co.
- [3]Coase, R. H. (1960). The Problem of Social Cost. The Journal of Law and Economics, 3, 1–44.
- [4]U.S. Environmental Protection Agency. Acid Rain Program.
Suggest an edit· Updated 2026-10-02