In simple terms
A friendly intro before the formal notes — no formulas yet.
The Lighthouse and the Overfished Sea
Some goods, like a lighthouse beam or street lighting, benefit everyone but are almost impossible to charge for — so a profit-seeking firm won't supply them. Other resources, like fish in the open sea, belong to no one and are free to take — so they get used up too fast. Both are market failures, but for opposite reasons.
Imagine your street wants a security guard to patrol at night. Everyone would feel safer, but there is no way to switch off the safety for anyone who refuses to chip in. So you wait for your neighbour to pay, and they wait for you — and in the end nobody pays and no guard is hired. That is the free-rider problem, and it is why markets under-provide public goods.
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Ask two questions about any good: is it RIVALROUS (does my use reduce what's left for you)? Is it EXCLUDABLE (can non-payers be kept out)?
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Those two answers sort every good into four boxes: private goods, club goods, common pool resources, and public goods.
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Public goods (e.g. national defence) are non-rivalrous AND non-excludable, so the free-rider problem leads to under-provision.
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Common pool resources (e.g. ocean fish stocks) are rivalrous BUT non-excludable, so self-interest leads to over-use — the tragedy of the commons.
Explore the concept
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Full topic notes
Formal explanation with the rigour you need for the exam.
The two building blocks: rivalry and excludability
Every good in economics can be described by two independent properties. Getting these definitions exactly right is the foundation of the whole topic, because both market failures in this lesson are just consequences of particular combinations of the two.
Rivalry: a good is RIVALROUS if one person's consumption reduces the amount or quality available to others. A slice of pizza is rivalrous; a radio broadcast is non-rivalrous, because my listening does not stop yours.
Excludability: a good is EXCLUDABLE if the provider can prevent non-payers from consuming it. A concert ticket makes the show excludable; the reduced crime from police patrols is largely non-excludable, because you cannot switch off the safety for one household on a patrolled street.
Classifying goods: the four-box matrix
Combining the two properties sorts all goods into four categories. Learn this matrix — an examiner can test any cell of it, and the two problem cases (public goods and common pool resources) are simply the two non-excludable boxes.
Private goods — rivalrous AND excludable (a chocolate bar, a car). The market allocates these efficiently.
Club (toll) goods — non-rivalrous but excludable (satellite TV, a streaming subscription, an uncongested toll road). Markets can supply these because non-payers can be shut out.
Common pool resources — rivalrous but non-excludable (open-sea fish, common grazing land). These are over-used: the tragedy of the commons.
Public goods — non-rivalrous AND non-excludable (national defence, street lighting, flood defences). These are under-provided: the free-rider problem.
Public goods and the free-rider problem
A pure public good has two features that together defeat the market. Because it is NON-EXCLUDABLE, a firm cannot stop people who refuse to pay from consuming it, so it cannot reliably collect revenue. Because it is NON-RIVALROUS, the efficient price for one more user is actually zero — serving an extra person costs nothing — yet a firm needs a positive price to cover its costs.
Put those together and you get the free-rider problem. Each rational consumer reasons: 'the good will be provided whether or not I pay, and I cannot be excluded, so I will let others pay and consume for free.' When everyone reasons this way, willingness to pay is understated, firms cannot cover their costs, and the good is UNDER-provided or not provided at all — even though the total social benefit exceeds the total cost. This is a market failure, because a good society values is missing purely because of how it must be paid for, not because it isn't worth producing. Note the causal chain examiners look for: non-excludability → free-riding → no revenue → under-provision.
Common pool resources and the tragedy of the commons
Common pool resources flip one characteristic. They are still NON-EXCLUDABLE — no one can be kept out — but they are RIVALROUS: every unit consumed is one unit less for everyone else. Open access plus rivalry is a dangerous mix. Each user captures the full private benefit of taking more, but the cost of the resulting depletion is spread across all users. So each individual has a private incentive to take as much as possible, as fast as possible, before someone else does.
When every user follows that logic, the resource is over-exploited and can collapse entirely — the tragedy of the commons. In effect there is a negative externality of consumption: the marginal social cost of using the resource (which includes the depletion imposed on others and on the future) exceeds the marginal private cost each user actually faces. This directly threatens SUSTAINABILITY, because a resource used faster than it can regenerate — a fishery, a forest, an aquifer — cannot support future generations.
Government responses
Because both problems come from the same root — non-excludability that the price mechanism cannot handle — the responses all work by having the state step in where the market cannot. Match the intervention to the failure.
Evaluation matters in Paper 1 too: direct provision requires the government to estimate the right quantity (it lacks the price signals a market would give); quotas need costly monitoring and can be evaded; and property rights can be impossible to define for truly global resources. No single response is a complete fix — the best policy depends on how easily the good's use can be measured, priced and enforced.
Direct provision (for public goods): the government supplies the good itself — national defence, street lighting, flood defences — and finances it through compulsory taxation. Taxation defeats the free-rider problem because contribution is no longer voluntary; everyone pays, so the good gets provided.
Regulation and legislation (mainly for common pool resources): quotas (e.g. total allowable catch), restricted seasons, gear limits, or outright bans on harmful activity cap the rate of use and give the resource time to regenerate — protecting sustainability.
Assigning property rights (for common pool resources): granting ownership — individual transferable fishing quotas, land title over grazing land, or emissions allowances — converts open access into managed access. An owner who bears the future cost of depletion now has an incentive to conserve. This directly targets the non-excludability at the root of the problem.
Supporting tools: Pigouvian taxes (raising the private cost of use toward the social cost) and tradable permits / cap-and-trade (a fixed cap with a market price for using the resource) are widely used for pollution and emissions.
International cooperation: because oceans and the atmosphere cross borders, no single government can assign rights or regulate alone. Agreements such as the Paris Agreement are needed — though they are hard to negotiate and to enforce.
Common mistakes examiners penalise
Defining a public good as 'anything the government provides' — the label is about NON-RIVALRY and NON-EXCLUDABILITY, not about who pays. State schooling and hospital care are government-provided but are rivalrous and excludable, so they are merit/private goods, not public goods.
Confusing public goods with common pool resources — both are non-excludable, but public goods are NON-rivalrous (→ under-provision), whereas common pool resources are RIVALROUS (→ over-use). Getting rivalry the wrong way round reverses the entire analysis.
Naming the free-rider problem without explaining the mechanism — you must show the chain: non-excludability → consumers free-ride → firms cannot collect revenue → under-provision. 'People don't pay' on its own is not the mechanism.
Saying the market UNDER-provides common pool resources — no: it OVER-uses them. Under-provision is the public-good failure; over-consumption is the common-pool failure.
Treating a club good (e.g. cable TV, a toll road) as a public good — it is non-rivalrous but EXCLUDABLE, so the market can and does supply it. Non-excludability is the essential ingredient of a public good.
Key concepts in this lesson
This lesson develops three of the course's nine key concepts. Efficiency is lost in both cases — public goods are under-provided and common pool resources over-used relative to the social optimum. Sustainability is central to common pool resources, where over-use can deplete a fishery, forest or aquifer beyond recovery. And intervention is the response: direct provision, regulation and property rights are all ways the government steps in where the price mechanism cannot. Linking a key concept to real-world material is specifically rewarded in the internal assessment, so keep these three in view.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Two households, A and B, live on a dark street and could share a single streetlight costing £200. The light is worth £150 to each household. They decide independently whether to contribute £100 each; if only one pays, the light is not installed and the payer loses their £100. What outcome does self-interest produce, and why is it a market failure?
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Because the light is non-excludable (once lit, it shines on both households whether or not they paid) and non-rivalrous (both benefit fully at once), this is a public good, and we can show the free-rider logic with a payoff matrix.
A lake's fish sell for £10/kg. With 1 boat the catch is 100kg; with 2 boats, 180kg total (90kg each); with 3 boats, 240kg total (80kg each); with 4 boats, 280kg total (70kg each). Each boat costs £750 to operate, and the lake is open to anyone. How many boats will actually fish the lake, and how many SHOULD there be? Explain the divergence.
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Because the lake is non-excludable, anyone may fish; because fish are rivalrous, each extra boat reduces the catch per boat. That is the common pool set-up.
Paper 1, part (a): Explain why public goods are not provided by a free market, with reference to the characteristics of non-rivalry and non-excludability. [10]
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Model answer: A public good is defined by two characteristics: it is NON-RIVALROUS — one person's consumption does not reduce the amount available to others (e.g. one more ship using a lighthouse beam does not dim it for other ships) — and NON-EXCLUDABLE — it is impossible, or prohibitively costly, to prevent non-payers from consuming it. Examples include national defence, street lighting and flood defences.
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Try to recall each definition before you reveal it.
Quick check
Answer in your head first — then tap to check. No pressure.
Revision flashcards
Flip the card. Test yourself before the exam.
Public Good
A good that is both non-rivalrous and non-excludable, e.g. national defence, street lighting, flood defences. Because non-payers cannot be excluded, a private market under-provides it.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
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Rivalry: a good is RIVALROUS if one person's consumption reduces the amount or quality available to others. A slice of pizza is rivalrous; a radio broadcast is non-rivalrous, because my listening does not stop yours.
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Excludability: a good is EXCLUDABLE if the provider can prevent non-payers from consuming it. A concert ticket makes the show excludable; the reduced crime from police patrols is largely non-excludable, because you cannot switch off the safety for one household on a patrolled street.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Get a Paper 1 (a) answer marked: explain why the free market does not provide public goods
Get a Paper 1 (a) answer marked: explain why the free market does not provide public goods
Extra simulations & links
PhET, GeoGebra and other curated tools — open in a new tab.
Frequently asked
Checkpoint
One marked question is worth ten re-reads — close the loop before you move on.
Reading it isn’t knowing it — prove it.
Before you move on: do Get a Paper 1 (a) answer marked: explain why the free market does not provide public goods on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.