Which situation is the clearest example of a negative externality?
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Economics · Foundations
Externalities
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An Externality A cost or benefit from a market transaction that falls on a third party who is not a buyer or seller in the exchange. Full entry → is a cost or benefit from a transaction that lands on a third party who is not part of the deal. Pollution is a Negative externality An external cost imposed on third parties, such as pollution, which leads the market to produce more of the activity than is socially efficient. Full entry →: the seller's Private cost The cost of an activity borne by the producer or decision-maker, such as labor and materials, excluding costs imposed on others. Full entry → leaves out the harm to others, so the market produces too much. Vaccination and education are positive externalities: the private benefit understates the Social benefit The full benefit of an activity to society: the private benefit to the decision-maker plus any external benefit that spills over to third parties. Full entry →, so the market produces too little. Either way the market misprices the activity, and the fix is to make the decision-maker feel the missing cost or benefit.
Why this matters
Externalities are the standard economic case for why an otherwise-competitive market can still land on the wrong quantity, and they sit behind some of the biggest policy debates: pollution and carbon pricing, congestion, vaccines, and public support for research and education. Learning the model tells you why free markets over-produce dirty goods and under-produce beneficial ones, and it gives you the vocabulary — private versus Social cost The full cost of an activity to society: the private cost plus any external cost imposed on third parties. Full entry →, Pigouvian taxes, cap-and-trade, the Coase theorem The proposition that when property rights are clearly defined and transaction costs are low, private parties can bargain to an efficient outcome regardless of which party holds the right; named for Ronald Coase. Full entry → — to compare the tools governments actually use. It also sharpens judgment: you can separate the settled economics of how externalities distort a market from the contested politics of which remedy to choose.
The college version
What an externality is
Most of the supply-and-demand model assumes that everyone affected by a trade is a party to it: the buyer pays, the seller produces, and the two of them weigh all the costs and benefits involved. An externality is what happens when that assumption breaks. It is a cost or a benefit created by a transaction that falls on a third party who is outside the exchange and never agreed to it. A factory that dumps smoke on a neighborhood, a driver who adds to congestion, a neighbor whose loud party keeps you awake — each imposes a cost on someone who is not buying or selling anything. Externalities can also run the other way. A homeowner who restores a historic house raises the value of the whole street; a person who gets vaccinated lowers the chance that others catch the disease. Economists call the harmful kind a negative externality and the beneficial kind a Positive externality An external benefit enjoyed by third parties, such as the protection others gain from a vaccination, which leads the market to produce less of the activity than is socially efficient. Full entry →. The defining feature in both cases is a gap between what the decision-maker personally experiences and what society as a whole experiences, and it is that gap that pushes the market away from the efficient outcome.
Negative externalities and over-production
To see the distortion, split the cost of an activity into two parts. Private cost is what the producer actually pays — labor, materials, energy. Social cost is the private cost plus any external cost imposed on third parties, such as pollution damage. When a firm decides how much to make, it looks only at its private cost, so its supply curve sits below the true social-cost curve. The market clears where private cost meets demand, but the socially efficient quantity is where social cost meets demand — a smaller amount. Because the external cost is invisible to the market, the good is under-priced and over-produced. This is a Market failure A situation in which a market, left on its own, does not reach the efficient quantity; an externality is one cause because the price omits an external cost or benefit. Full entry →: a competitive market, left alone, delivers more of the activity than is efficient, not because anyone is behaving irrationally but because the price does not carry the full cost. The standard picture is pollution. Steel, electricity, and plastics are cheaper to buy than they would be if their smoke and runoff were priced in, so we consume more of them, and more pollution, than we would if the damage appeared on the receipt.
Positive externalities and under-production
Positive externalities are the mirror image. Split the value of an activity into private benefit — what the decision-maker personally gains — and social benefit, which adds the spillover benefits enjoyed by others. When someone weighs an activity by its private benefit alone, and the social benefit is larger, they do too little of it from society's point of view. A firm that invents a new technology captures only part of the value it creates, because rivals and customers learn from it; so the private sector, left alone, invests less in research than the social return would justify. Education works the same way: a more educated workforce benefits employers, neighbors, and civic life beyond the wage the student earns. Vaccination is the textbook health case — getting vaccinated protects not just you but the people around you, yet an individual weighing only personal protection may under-value the shot. In each case the market quantity falls short of the socially efficient quantity, so positive externalities lead to under-production, which is why governments subsidize research, schooling, and immunization.
Internalizing externalities: taxes, subsidies, and regulation
The general remedy is to internalize the externality — to make the decision-maker face the full social cost or benefit so the private incentive lines up with the social one. The economist Arthur Pigou set out the classic version in The Economics of Welfare (1920), building on Alfred Marshall: tax activities that create negative externalities to discourage them, and subsidize activities that create positive ones to encourage them. A tax equal to the external damage is now called a Pigouvian (or Pigovian) tax, and the matching payment a Pigouvian subsidy. A carbon tax and a research-and-development subsidy are modern examples. A different approach is command-and-control regulation, which sets allowable pollution limits directly or mandates specific control technologies. Command-and-control cleaned up a great deal of air and water pollution, but economists note two weaknesses: it gives a firm no reason to cut pollution below the legal ceiling, and a uniform rule ignores the fact that some firms can reduce pollution far more cheaply than others. These are policy tools with real trade-offs, and which one to use — and how high to set it — is a normative question this lesson does not try to settle.
Permits and bargaining: cap-and-trade and the Coase theorem
Two more tools try to harness markets rather than override them. A marketable-permit or cap-and-trade program issues a fixed number of pollution permits — the cap — and lets firms buy and sell them. Firms that can cut pollution cheaply do so and sell their spare permits; firms that would find cuts expensive buy permits instead. The cap guarantees the total reduction while the trading steers the cuts to wherever they are cheapest, and regulators can tighten the cap over time. The other tool relies on property rights and private bargaining. Ronald Coase, in 'The Problem of Social Cost' (1960), argued that if property rights are clearly defined and the cost of bargaining is low, the affected parties can negotiate their way to an efficient outcome without a tax or a rule at all — and, strikingly, the efficient quantity is the same regardless of which side the law gives the right to; the assignment changes who pays, not what is efficient. This result is the Coase theorem. Its own conditions mark its limits: when many parties are involved or bargaining is costly — as with global air pollution — negotiation breaks down, and that is exactly where taxes, permits, or regulation come back in.
Eli explains
The same idea, in plain words
Explain it like I’m 10
When you buy something, the price is supposed to cover everything the deal costs. But sometimes a deal quietly dumps a cost on someone who had no say. A factory sells cheap plastic, and a family downwind breathes the smoke for free — that hidden cost is a negative externality, and because it is missing from the price, we buy too much. Other times a deal hands a free gift to bystanders: when you get a flu shot you also protect everyone you would have sneezed on. That is a positive externality, and because you do not get paid for the gift, we get too little of it. The repair is to put the missing cost or gift back into the price — a tax on the smoke, a discount on the shot.
Picture it like this
Think of a shared apartment. If one roommate cooks fish and the smell fills the whole place, they enjoy the meal while everyone else pays in stink — a negative externality. If another roommate cleans the bathroom, everybody benefits but only one person did the work — a positive externality. The house runs better once you set up a rule: whoever makes the smell chips in, and whoever does the chores gets a break on rent.
Where the picture stops working
The apartment has a handful of roommates who all know each other, so they can just talk it out — which is basically the Coase bargaining case. Real externalities like carbon emissions involve billions of strangers who cannot sit down together, so bargaining fails and society needs taxes, permits, or regulation instead.
Worked example
OpenStax works a refrigerator market. Ignore pollution and the market clears at a price of $650 and a quantity of 45,000 refrigerators. Now suppose producing each refrigerator imposes $100 of pollution damage on third parties. That external cost is added to the private cost, shifting the supply (social-cost) curve up by $100. The new equilibrium is a price of $700 and a quantity of 40,000. Notice two things. First, the socially efficient quantity, 40,000, is lower than the unregulated market's 45,000, which confirms that a negative externality makes the market over-produce. Second, the price rises by only $50, not the full $100, because buyers and sellers split the burden as the higher cost moves them along the demand curve. A Pigouvian tax of $100 per unit would push the market to exactly this efficient outcome.
Key takeaway
An externality is a cost or benefit that spills onto a third party, so the market price is wrong: negative externalities make markets over-produce and positive ones make them under-produce. Remedies — Pigouvian taxes and subsidies, regulation, cap-and-trade, and Coasean bargaining — all work by putting the missing cost or benefit back into the decision.
Quick check
3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.
Because a good with a negative externality has a social cost greater than its private cost, an unregulated market will tend to:
Choose an answer, then check it.
A refrigerator market clears at $650 and 45,000 units when pollution is ignored. Once a $100-per-unit external pollution cost is included, the new equilibrium is $700 and 40,000 units. What does this show?
Choose an answer, then check it.
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related
You’ll learn to
- Define an externality and distinguish negative from positive externalities.
- Explain why a negative externality leads the market to over-produce and a positive externality leads it to under-produce.
- Distinguish private cost from social cost and private benefit from social benefit.
- Describe how Pigouvian taxes and subsidies, regulation, and cap-and-trade internalize an externality.
- Explain the Coase theorem and the conditions under which private bargaining can resolve an externality.
Common mistakes
Thinking an externality means any cost or any side effect.
An externality specifically falls on a third party outside the transaction. A cost the buyer or seller bears themselves — even an unpleasant one — is a private cost, not an externality.
Assuming a negative externality means the market makes too little and a positive one too much.
It is the reverse. A negative externality makes the good look too cheap, so the market over-produces; a positive externality makes it look less valuable than it is, so the market under-produces.
Believing a Pigouvian tax is just the government raising revenue or punishing a business.
The point of the tax is to make the producer face the external cost, moving output toward the efficient quantity. The revenue is a by-product; a subsidy does the same corrective job for positive externalities.
Reading the Coase theorem as 'externalities always solve themselves without government.'
Coase's result holds only when property rights are clear and transaction costs are low. With many parties or costly bargaining — like global air pollution — private negotiation fails and taxes, permits, or regulation are needed.
Treating externalities and public goods as the same market failure.
They are related but distinct. Public goods are non-excludable and non-rival, a separate topic; externalities are about spillover costs and benefits attached to an otherwise ordinary market activity.
Easily confused
Negative externality vs. Positive externality
A negative externality imposes an external cost, so social cost exceeds private cost and the market over-produces; a positive externality creates an external benefit, so social benefit exceeds private benefit and the market under-produces.
Pigouvian tax or subsidy vs. Cap-and-trade (marketable permits)
A Pigouvian tax sets a price on the externality and lets the quantity adjust; cap-and-trade fixes the quantity of pollution and lets the market set the permit price.
Coasean bargaining vs. Government intervention (tax, permit, or rule)
Coasean bargaining resolves the externality privately when property rights are clear and few parties are involved; government tools are used when many parties or high transaction costs make bargaining impractical.
Key vocabulary
- Externality
- A cost or benefit from a market transaction that falls on a third party who is not a buyer or seller in the exchange.
- Negative externality
- An external cost imposed on third parties, such as pollution, which leads the market to produce more of the activity than is socially efficient.
- Positive externality
- An external benefit enjoyed by third parties, such as the protection others gain from a vaccination, which leads the market to produce less of the activity than is socially efficient.
- Private cost
- The cost of an activity borne by the producer or decision-maker, such as labor and materials, excluding costs imposed on others.
- Social cost
- The full cost of an activity to society: the private cost plus any external cost imposed on third parties.
- Social benefit
- The full benefit of an activity to society: the private benefit to the decision-maker plus any external benefit that spills over to third parties.
- Market failure
- A situation in which a market, left on its own, does not reach the efficient quantity; an externality is one cause because the price omits an external cost or benefit.
- Pigouvian tax (and subsidy)
- A tax on an activity equal to the external cost it imposes (or a subsidy equal to the external benefit it creates), designed to make private incentives match social ones; named for Arthur Pigou.
- Cap-and-trade (marketable permits)
- A policy that fixes the total quantity of pollution through a set number of permits and lets firms buy and sell them, so pollution is cut where it is cheapest to do so.
- Coase theorem
- The proposition that when property rights are clearly defined and transaction costs are low, private parties can bargain to an efficient outcome regardless of which party holds the right; named for Ronald Coase.
Sources & references
- Principles of Economics 3e, Section 12.1: The Economics of Pollution — OpenStax (Rice University)
- Principles of Economics 3e, Section 12.2: Command-and-Control Regulation — OpenStax (Rice University)
- Principles of Economics 3e, Section 12.3: Market-Oriented Environmental Tools — OpenStax (Rice University)
- Principles of Economics 3e, Chapter 13: Positive Externalities and Public Goods (13.1 and chapter summary) — OpenStax (Rice University)
- Arthur Cecil Pigou (Biography), The Concise Encyclopedia of Economics — Library of Economics and Liberty (Econlib), Liberty Fund
- Externalities, The Concise Encyclopedia of Economics — Library of Economics and Liberty (Econlib), Liberty Fund
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Researched 2026-08-19
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