COMM1100 Chap.9 Market Failure: Externalities and Private Information
Market Failure: Externalities and Private Information
Markets maximise surplus only when their conditions hold, and the Week 3 lecture lists the main reasons they fail: imperfect competition, externalities and private information. This chapter covers the last two.
An externality is a cost or benefit that lands on bystanders who are not part of the deal; private information arises when one side of a trade knows something the other does not.
You will classify externalities, show on a graph why a polluting market overproduces and a beneficial one underproduces, calculate the efficient quantity and the lost surplus when the external effect is valued per unit, and explain how hidden quality can make a market unravel.
The Week 4 practice set tests each of these, and the case study usually needs at least one of them.
Market failure is also the bridge to the rest of the course. It explains why the lecture says markets need the rule of law and healthy supervision, and why the following chapters on contract, consumer, competition and employment law matter for business decisions.
It also leaves room for a responsible business to do better than the market outcome on its own.
What this chapter covers
- 01
Externalities: costs and benefits that fall on bystanders
- 02
Classifying positive, negative and no externality
- 03
Private versus social marginal cost and benefit
- 04
Overproduction with a negative externality
- 05
Underproduction with a positive externality
- 06
Why a ban rarely maximises surplus
- 07
Private information and the unravelling market
Worked example · free
A factory's emissions and the efficient output
- +1Market: 100 − Q = 10 + Q gives Q = 45 and P = $55.
- +1Social marginal cost adds the damage: P = 30 + Q.
- +1Efficient: 100 − Q = 30 + Q gives Q = 35.
- +1Lost surplus is the triangle ½ × (45 − 35) × 20 = $100.
Key terms
- Externality
- An effect on third parties that the people trading leave out of their calculations, whether a harm or a gain.
- Social marginal cost
- Private marginal cost plus any external cost the extra unit imposes on others.
- Social marginal benefit
- Private marginal benefit plus any external benefit the extra unit gives others.
- Market failure
- A situation in which a market fails to produce the quantity that maximises total economic surplus.
- Private information
- Knowledge held by one party to an exchange but not the other, such as hidden product quality.
Market Failure: Externalities and Private Information FAQ
Are the wages a new factory pays a positive externality?
No. Wages and hiring are part of the transaction between the firm and its workers, so the benefits are internal. An externality would be an effect on outsiders, such as pollution, traffic or better local roads.
Why does a negative externality lead to too much output?
Producers decide output using only their own costs. Because the harm to others is left out, they keep producing units whose full social cost is higher than their value to buyers.
Would banning a harmful product maximise surplus?
Usually not. A ban removes the overproduced units but also all the units whose value to buyers exceeds their full social cost. The efficient quantity is lower than the market quantity, not zero.
How does private information cause a market to fail?
If buyers cannot see quality, they pay a price based on average quality. Sellers of high-quality goods may withdraw, average quality falls, the price falls again, and trades that would have created surplus never happen.
Can businesses themselves reduce market failures?
Yes. The lecture notes that market and government failures leave scope for businesses to increase surplus for their stakeholders, for example by disclosing how a product is made so buyers are no longer guessing, or by cutting harmful emissions even where no rule requires it.
Assessment move
Start every externality question by naming the bystander; if you cannot, there probably is no externality. Practise the graph in both directions, a cost curve shifted up and a benefit curve shifted up, and practise one calculation with a per-unit external value. For private information, rehearse the four-step unravelling story with numbers.
These are the tools most likely to anchor the economic part of your case-study analysis.
Working through Market Failure: Externalities and Private Information in COMM1100? Sia is AskSia’s AI Economics tutor — ask any COMM1100 Market Failure: Externalities and Private Information question and get a clear, step-by-step explanation grounded in how COMM1100 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
