IB Economics: Public Goods & Common Access Resources FAQ

A public good (like a lighthouse or street lighting) is non-excludable and non-rivalrous — one person's use doesn't stop another's, and no one can be charged for it. A common access resource (like ocean fish stocks) is non-excludable but rivalrous — overuse by one person reduces what's left for others.

The free rider problem happens because public goods are non-excludable, so people can benefit without paying, giving firms no incentive to supply them privately. Result: the good is under-provided or not provided at all, which is why governments typically fund defence, street lighting and flood barriers directly.

The tragedy of the commons describes over-exploitation of a common access resource because no one owns it, so private marginal benefit exceeds social marginal benefit — each user ignores the cost their consumption imposes on others. Classic IB examples: overfishing, deforestation, and groundwater depletion.

Common IB examples of public goods: national defence, street lighting, flood defences, basic research, and free-to-air broadcasting. Common access resource examples: ocean fisheries, the atmosphere (as a carbon sink), tropical rainforests, and groundwater aquifers — all used repeatedly in past Paper 1 and Paper 2 data-response questions.

It's tested across all three papers. Paper 1 asks 10/15-mark essays comparing market failure types or evaluating government responses; Paper 2 uses data-response extracts on real resources like fisheries; Paper 3 (HL only) can bring in quantitative skills, such as calculating marginal social benefit from a demand schedule.

For common access resources, draw the standard negative externality diagram but relabel it: private marginal benefit (or cost) versus social marginal benefit (or cost), with the free market quantity Qm exceeding the social optimum Qopt, and a welfare loss triangle in between. Public goods don't get a market diagram at all — there's no supply curve because there's no market.

Structure matters more than content volume here. Define both characteristics precisely (non-excludable, non-rivalrous or rivalrous), explain the specific mechanism of failure, use a real-world example named in the question or your own, then evaluate at least two government interventions with a clear judgement in your final paragraph.

The main policy options are quotas (e.g. fishing quotas or catch limits), tradeable permits, taxes on extraction, and international agreements or regulation. Each has trade-offs: quotas are hard to enforce and monitor, permits need a credible cap, and taxes require accurate valuation of the external cost — all fair game for evaluation marks.

The core concept is identical at SL and HL — both study units are under Theme 1 Market Failure and use the same command terms. The difference is exposure: HL students face Paper 3, which can require applying quantitative skills (like calculating welfare loss from given figures) to a public goods or common access scenario.

It's not conceptually difficult, but it's easy to lose marks through imprecise language — mixing up 'non-excludable' and 'non-rivalrous', or drawing the wrong diagram. In my experience marking mocks, most lost marks come from vague definitions rather than a genuine misunderstanding of the theory.

Past paper practice on Paper 1 and Paper 2 data-response questions is the single best use of revision time here, since the same real-world examples (fisheries, rainforests, defence spending) recur often. Topical worksheets that isolate market failure sub-types, alongside condensed revision notes with the key diagrams, help consolidate the theory quickly.

Most families spend nothing beyond textbooks and school materials for a single micro topic like this — it doesn't need dedicated paid support. Where it's worth investing is in broader subject-wide resources (a full question bank or structured revision notes) used across the whole syllabus, not topic-by-topic purchases.