An externality is a cost or benefit from a transaction that lands on someone who wasn't part of it. Externalities are the standard microeconomics answer to the question "if markets are efficient, why do they sometimes get things so wrong?" This explainer shows why an externality makes a market misprice a good, and walks through the three standard fixes a course expects you to know.
What an Externality Is
When you buy a coffee, the price reflects two things: what the coffee is worth to you, and what it cost the shop to make. The market works because price captures both sides. An externality is what happens when a third party — someone neither buying nor selling — feels a cost or a benefit that the price ignores.
There are two kinds:
- A negative externality is a spillover cost. A factory that emits pollution imposes health and cleanup costs on neighbors who never bought its product.
- A positive externality is a spillover benefit. A homeowner who vaccinates against a contagious disease, or restores a historic building, benefits others who didn't pay for it.
The defining feature is that the cost or benefit is external to the price — the buyer and seller don't account for it because it isn't theirs to pay or collect.
Why an Externality Makes the Market Misprice
Here's the core mechanism. A free market settles where the buyer's private benefit meets the seller's private cost. The trouble is that those private figures leave out the externality.
With a negative externality, the seller's private cost is lower than the social cost — the true cost once the spillover is included. Because the firm prices off its private cost, the good looks cheaper than it really is to society, so the market produces too much of it. The pollution-heavy factory runs at an output where the last units cost society — counting the pollution — more than buyers value them. Those units shouldn't be made, but the price signal says "go." That gap between the market quantity and the efficient quantity is a deadweight loss.
With a positive externality, the logic flips. The buyer's private benefit is lower than the social benefit, because some of the good's value goes to third parties. Buyers price off their private benefit, so they buy too little — vaccination and education are classic cases of socially valuable goods that a pure market under-supplies. Either way, the externality breaks the price signal, and the market quantity misses the efficient one. (For the triangle that gap creates, see the explainer on deadweight loss.)
Fix 1: Corrective Taxes and Subsidies
The first standard fix is to make the price tell the truth — to internalize the externality.
For a negative externality, the government can levy a corrective tax (often called a Pigouvian tax) equal to the spillover cost per unit. If a factory's pollution costs society $3 per unit, a $3-per-unit tax forces the firm to pay that cost. The firm's private cost now equals the social cost, it cuts output to the efficient quantity, and the mispricing is gone. A carbon tax is the textbook example.
For a positive externality, the mirror tool is a corrective subsidy: pay producers or buyers an amount equal to the spillover benefit. Subsidizing vaccinations or tuition lowers the price buyers face so the quantity rises to the efficient level. The principle is symmetric — tax what there's too much of, subsidize what there's too little of, in the exact amount of the externality.
Fix 2: The Coase Theorem
The second fix says government intervention may not be needed at all. The Coase theorem holds that if property rights are clearly defined and the cost of bargaining is low, private parties can negotiate their way to the efficient outcome on their own — regardless of who holds the right.
Suppose a factory's pollution costs a neighboring farm $1,000, but the factory would only gain $600 from polluting. If the farm has the right to clean air, the factory won't pay $1,000 to buy permission for a $600 gain — so it stops. If the factory has the right to pollute, the farm will happily pay the factory somewhere between $600 and $1,000 to stop, which the factory accepts. Either assignment of the right leads to the same efficient result — the pollution stops, because stopping is worth more than continuing. Coase's insight is that the clarity of the right matters more than its initial owner. The catch: it only works when the parties are few and bargaining is cheap. With thousands of affected people — global emissions, say — negotiation costs are prohibitive, and this fix fails.
Fix 3: Regulation and Tradable Permits
The third approach is direct regulation — and a market-based version of it.
Plain command-and-control regulation sets a rule: a cap on emissions, a mandatory standard, a required scrubber. It's simple and predictable, but blunt — it doesn't let firms that can cut pollution cheaply do more of the cutting.
The smarter version is a cap-and-trade system of tradable permits. The government caps total pollution and issues permits for that amount, which firms can buy and sell. A firm that can cut emissions cheaply will do so and sell its spare permits; a firm for which cutting is expensive will buy permits instead. The cap guarantees the total falls, and trading ensures the cuts happen where they're cheapest — achieving the target at the lowest overall cost. This combines the certainty of regulation with the efficiency of a price.
Conclusion
An externality is a cost or benefit that spills onto a third party and escapes the price. Negative externalities make a market over-produce; positive externalities make it under-produce — in both cases the price signal is wrong and the efficient quantity is missed. The standard fixes all aim to put the missing cost or benefit back into the decision: corrective taxes and subsidies adjust the price, the Coase theorem lets private bargaining do it when parties are few, and tradable permits cap the total while letting the market find the cheapest cuts.
