Syllabus points
- Define market failure and explain the four externality cases.
- Explain welfare loss from externalities using MSC/MSB analysis.
- Explain common pool resources and their overuse.
- Evaluate policies to correct externalities.
Market failure and externalities
Market failure occurs when the free market fails to allocate resources efficiently. A common cause is an externality — a cost or benefit affecting a third party not involved in the transaction.
The key idea is a gap between private and social costs or benefits:
- Social cost = private cost + external cost
- Social benefit = private benefit + external benefit
At the free-market output, marginal social cost (MSC) does not equal marginal social benefit (MSB), creating a welfare (deadweight) loss.
The four cases
| Case | Example | Problem |
|---|---|---|
| Negative production externality | Factory pollution | MSC > MPC → over-production |
| Positive production externality | Firm training raises wider skills | MSC < MPC → under-production |
| Negative consumption externality | Smoking, driving | MSB < MPB → over-consumption |
| Positive consumption externality | Vaccination, education | MSB > MPB → under-consumption |
Negative externalities → the market over-produces/over-consumes; positive externalities → it under-produces/under-consumes.
Common pool resources
Common pool (common access) resources — such as fish stocks, forests and clean air — are rival but non-excludable. Because no one owns them and use is free, each user ignores the cost imposed on others, leading to overuse and depletion (the "tragedy of the commons"). This threatens sustainability.
Key definitions
| Term | Exam-ready definition |
|---|---|
| Externality | A cost or benefit falling on a third party outside the transaction. |
| Welfare loss | The loss of social surplus when output differs from the social optimum. |
| Common pool resource | A resource that is rival in use but non-excludable, prone to overuse. |
Policies to correct externalities
- Indirect taxes on negative externalities (internalise the external cost).
- Subsidies for positive externalities (encourage under-provided goods).
- Regulation and legislation (limits, bans, standards).
- Tradable permits (cap-and-trade) for pollution.
- Education/awareness to shift demand.
Each has trade-offs: taxes need the harm measured accurately; regulation needs enforcement; permits need a well-set cap.
Worked example
A power station emits pollution: marginal social cost exceeds marginal private cost, so the market over-produces electricity and creates welfare loss. A per-unit tax equal to the external cost raises the firm's private cost towards the social cost, cutting output towards the social optimum — provided the harm can be measured and the tax enforced.
Common exam mistakes
- Mislabelling MSC/MSB diagrams or the welfare-loss triangle.
- Confusing production and consumption externalities.
- Presenting a single policy as a perfect fix — always evaluate.
Exam technique
Draw the correct diagram (MPC, MSC, MPB, MSB), mark the market and social optimum, and shade the welfare loss. Then evaluate at least two policies by cost, enforceability and information needs.
Quick revision
- Social cost/benefit = private + external.
- Negative → overproduce/overconsume; positive → underprovide.
- Common pool resources are overused because non-excludable.