Market Failures II: Externalities
The hidden third party
Last chapter, every boat that motored onto the lake thinned the stock for every other boat — and paid nothing for the harm. That was not a quirk of fisheries. It was one case of a much larger thing, and this chapter gives it its name.
A student buys gasoline, a cheap burger, a flight, or a fast-fashion hoodie. The buyer says yes. The seller says yes. Money changes hands. In the clean market story, that voluntary exchange looks like a win.
But sometimes a third person is quietly pulled into the deal. Someone breathes the pollution, sits in the congestion, hears the airport noise, absorbs the carbon risk, or lives downstream from the water damage. The buyer and seller both said yes. But did everyone affected by the trade get a say?
That hidden third party is the beginning of externalities. The problem is not that the buyer and seller said yes. The problem is that someone else was affected without being part of the yes.
Externalities
A voluntary trade can still affect someone outside the trade.
An externality is a cost or benefit that falls on someone outside the transaction. In the simplest market story, the buyer receives the benefit, the seller receives the payment, and the price coordinates the exchange. Externalities complicate that story because the price may leave out someone affected by the decision.
Voluntary exchange often looks like a Pareto improvement: the buyer buys because the good is worth more than the price, and the seller sells because the price is worth more than the cost. Both parties say yes. But if a third party is harmed, the trade is no longer a clean Pareto improvement. Someone outside the deal may be worse off.
Externalities can be negative or positive, and they can come from production or consumption. A factory polluting while producing is a negative production externality. Secondhand smoke from consumption is a negative consumption externality. A flu shot can create a positive consumption externality by reducing spread. Research can create a positive production externality when knowledge spills over to other firms.
| Type | Who creates it? | Example | Likely result |
|---|---|---|---|
| Negative production | Producer | Factory pollution | Too much output |
| Negative consumption | Consumer | Secondhand smoke | Too much consumption |
| Positive consumption | Consumer | Vaccination | Too little consumption |
| Positive production | Producer | Research spillovers | Too little production |
MisconceptionExternalities are always negative — they're just a word for pollution.▸
Positive externalities are just as real and just as common: vaccines, education, research, a well-kept garden on a shared street. With these, the market produces too little, not too much, because the decision-maker does not capture the full benefit. And the fix flips too — a positive externality calls for encouragement (a subsidy), not punishment (a tax). Half of this topic is about doing more of a good thing, not less of a bad one.
Quick checkA commuter drives downtown and adds congestion for everyone behind them. What kind of externality is this?▸
Private Cost vs. Social Cost
The decision-maker may not face the full cost of the decision.
The key distinction is private versus social. Marginal private cost is the cost faced by the person or firm making the decision. Marginal social cost is the full cost of one more unit once third-party harm is included.
The formula is the spine of the graph:
MSC = MPC + external cost
If a factory pays for labor, materials, and machines but does not pay for the smoke damage it imposes on neighbors, its private cost is lower than the true social cost. The market price then tells only part of the truth.
| Term | Meaning | Graph role |
|---|---|---|
| MPC | Cost to the private decision-maker. | Supply curve when external costs are ignored. |
| MSC | Cost to society, including external cost. | Above MPC for a negative externality. |
| MPB | Benefit to the private decision-maker. | Demand curve when external benefits are ignored. |
| MSB | Benefit to society, including external benefit. | Above MPB for a positive externality. |
MisconceptionThe efficient amount of pollution is zero.▸
Almost never. The efficient quantity is where marginal social benefit equals marginal social cost — and for most harmful activities that point is well above zero. Driving pollution to zero would mean shutting off the electricity, transport, and manufacturing that produce it, sacrificing enormous value to erase the last, cheapest-to-tolerate ton of harm. The goal of externality policy is not no harm; it is the right amount of harm — the amount where one more unit's social cost finally outweighs its social benefit.
Retrieval practiceIf MPC is $8 and the external cost is $3, what is MSC?▸
The Negative Externality Graph
When other people pay part of the cost, the market produces too much.
The required AP graph for this chapter is the negative production externality. The market uses MPC because producers face private costs. Society cares about MSC because smoke, congestion, emissions, or other third-party costs are real costs too. Since external cost is positive, MSC lies above MPC.
The market quantity, Qm, occurs where MPB = MPC. The socially efficient quantity, Qs, occurs where MPB = MSC. Because the market ignores the external cost, it produces too much: Qm > Qs.
MisconceptionA negative externality means the market produces too little of the good.▸
The opposite. Because the price leaves out the external cost, the good looks cheaper to make than it really is, so the market makes too much of it: Qm > Qs. The deadweight loss is the wedge of over-production — the units that were only worth producing while someone else was silently paying part of the cost. Flip the sign and you flip the policy: the cure is to shrink output toward Qs, not expand it.
Quick checkOn a negative externality graph, why is MSC above MPC?▸
Positive Externalities
When other people benefit, the market may produce too little.
Externalities can also be beneficial. A vaccination protects the person who receives it, but it also reduces the chance that others get sick. Education can raise a student's earnings, but it may also create civic and productivity benefits for others. Research can help the firm that performs it, while also creating knowledge that other firms build on.
With a positive externality, the private decision-maker does not receive the full social benefit. The market follows MPB, but society cares about MSB. Since the external benefit is positive, MSB lies above MPB. The result is underproduction or underconsumption.
Retrieval practiceWhy might vaccines be underconsumed in a private market?▸
Coase, Bargaining, and Transaction Costs
Some externalities can be bargained away. Many cannot.
Chapter 16 taught that markets need property rights, courts, contracts, and rule of law. That matters here. The Coase theorem says that if property rights are clear and transaction costs are low, private bargaining may solve some externalities.
Coase is not "externalities solve themselves." It says bargaining can work under demanding conditions. The parties must know who has the right, who is harmed, how large the harm is, how to negotiate, and how to enforce the deal. Real-world transaction costs often block bargaining, especially when many people are affected.
A factory gains $400 from producing smoke. Residents suffer $700 in harm.
- Reducing smoke gives up $400 of factory benefit.
- Reducing smoke prevents $700 of resident harm.
- Net social gain = $700 - $400 = $300.
- If bargaining is possible, a payment between $400 and $700 could make both sides better off.
MisconceptionAn externality means someone broke a rule or cheated.▸
No one cheated. The buyer and seller made a legal, voluntary trade and both came out ahead. The problem is structural, not moral: a third party who bore part of the cost was never at the table to say no. That is exactly why you usually cannot fix an externality by punishing wrongdoing — there often isn't any. The repair is to put the missing cost or benefit into the price, not to find a villain.
This chapter diagnosed the disease and stopped there: the price omits a cost or a benefit, so the market makes too much or too little, and private bargaining repairs it only when the parties are few and the rights are clear. For the vast externalities where bargaining cannot reach — a million tailpipes, a warming atmosphere — the question becomes what policy can do that bargaining cannot. The next chapter is the repair kit: taxes that put the missing cost back into the price, subsidies that pay for the missing benefit, and the hard catch that someone has to know the right number and set it honestly. Chapter 19
Graph the hidden third party
The moves you can now make: classify an externality, draw the negative-externality graph with MSC above MPC, and reason about when bargaining can or cannot fix it.
Classification
A factory produces steel and releases pollution that damages nearby crops. Identify the externality type.
Reveal answer▸
Negative production externality. The producer's activity imposes a cost on third parties.
Graph move
On a negative production externality graph, where are Qm and Qs?
Reveal answer▸
Qm is where MPB/D intersects MPC/S. Qs is where MPB/D intersects MSC. Since MSC is above MPC, Qm is greater than Qs.
Bargaining
A noisy activity creates $250 of benefit for the producer and $600 of harm for neighbors. What is the net social gain from stopping it?
Reveal answer▸
Net social gain = $600 - $250 = $350. If bargaining costs are below $350, there may be a mutually beneficial deal to stop or reduce the activity.
A chemical plant and downstream residents
A chemical plant sells a product in a competitive market. Producing the product releases waste into a river, imposing costs on residents downstream. The plant does not pay for the damage.
- 1 pt Identify the type of externality.
- 2 pts Draw a correctly labeled negative production externality graph with MPB/D, MPC/S, and MSC.
- 2 pts Label Qm and Qs and explain why Qm is greater than Qs.
- 1 pt Identify the deadweight loss area.
- 1 pt Explain why voluntary exchange is not enough in this case.
- 2 pts Explain how clear property rights and low transaction costs could allow bargaining, and why high transaction costs might prevent it.
Reveal scoring guide▸
- 1 ptNegative production externality.
- 1 ptCorrect axes and MPB/D plus MPC/S labels.
- 1 ptMSC drawn above MPC.
- 1 ptQm at MPB = MPC.
- 1 ptQs at MPB = MSC and Qm > Qs.
- 1 ptDWL shown for excess output between Qs and Qm.
- 1 ptExplains third-party harm is outside the buyer-seller transaction.
- 1 ptExplains bargaining can work with clear rights and low transaction costs.
- 1 ptExplains high transaction costs, many affected residents, or hard-to-measure damages may block bargaining.
Reveal common mistakes▸
- Drawing MSC below MPC. For a negative externality, MSC sits above MPC by the external cost.
- Saying the market underproduces. A negative externality means over-production: Qm > Qs.
- Forgetting that positive externalities are under-produced and call for subsidies, not taxes.
- Assuming the efficient quantity of the harmful good is zero rather than where MSB = MSC.
Externalities show that voluntary exchange is not enough when some affected people are outside the exchange: a deal can benefit buyer and seller while pushing costs or benefits onto hidden third parties.
they would make polluters pay. Tax bads, not goods. The aim is not to punish production; it is to stop pretending a cost disappears just because it is pushed onto someone who never agreed to bear it.
- Negative externalities cause overproduction when external costs are ignored.
- Positive externalities cause underproduction when external benefits are ignored.
- Coase bargaining can fix some externalities, but only under demanding conditions.
- How should society measure third-party costs and benefits?
- Which tool fits best: liability, bargaining, taxes, fees, cap-and-trade, or regulation?
- How much of the harmful activity should remain, given that the efficient amount is rarely zero?
Before you judge a cheap product, ask who isn't in the price. The bill for the smoke, the noise, or the carbon lands on someone — often neither the buyer nor the seller.
If your operation creates a cost outsiders pay — noise, emissions, congestion — expect it to be priced eventually. The cheapest time to handle an externality is before a regulator handles it for you.
