What market failure means
Market failure occurs when the price mechanism allocates resources inefficiently — too much of some goods, too little of others, or none at all — so the outcome is not best for society. It handles ordinary private goods well; failure is the specific cases where it does not: externalities, merit and demerit goods, public goods, and abuse of monopoly power. Governments then intervene, which is why most real economies are mixed.
Externalities and social cost
Decisions weigh only private costs and benefits. Activities also create external costs or benefits — spillovers to third parties. So social cost = private cost + external cost (and social benefit = private benefit + external benefit). A negative externality (pollution) makes social cost exceed private cost, so a good is over-produced; a positive externality (vaccination) makes social benefit exceed private benefit, so it is under-produced. The market fails.
Merit, demerit and public goods
Merit goods (education, healthcare) have positive externalities and are under-consumed and under-produced. Demerit goods (cigarettes, alcohol) have negative externalities and are over-consumed and over-produced. Public goods (street lighting, defence) are non-rival (one user does not reduce the amount for others) and non-excludable (non-payers cannot be stopped). This free-rider problem means private firms make no profit, so a pure market provides none.
Drawn from real examiner reports.
Public good is not just govt-provided
A common error is to call a public good simply a good the government provides. Its defining features are economic: non-rival (one user does not reduce what is left for others) and non-excludable (non-payers cannot be stopped). The free-rider problem means firms cannot charge and will not supply it. Anchor the answer in non-rival and non-excludable.
June 2023 Paper 2 (Paper 21, Q4b): only a few candidates could explain a public good — many described it simply as a good provided by the government, and few mentioned non-rival or non-excludable.
Policy named, impact not explained
Asked how to reduce consumption of a demerit good, weaker answers just name a measure — a tax, an advert — without explaining how it cuts consumption. A tax raises the price so people buy less; a campaign warns of the harm. Do not treat subsidising merit goods as a way to cut demerit-good use. Carry each point through to its effect.
June 2022 Paper 2 (Paper 21, Q3b): weaker answers identified a way of reducing demerit-good consumption without explaining the impact, and a few wrongly treated subsidising merit goods as an alternative. June 2023 Paper 2 general comment: candidates identified factors but did not develop them.
One-sided or unexplained discuss
The 8-mark discuss on correcting market failure needs both sides and a supported judgement. Do not just state that a tax reduces pollution — explain the chain, then weigh it against limits: inelastic demand (so consumption barely changes), a regressive effect, or an externality that is hard to value. Reward comes from depth on both sides, not asserted outcomes.
November 2022 Paper 2 key messages: to reach Level 3 on (d), candidates must examine the question in depth — many just stated points or commented on unlikely outcomes without explaining. June 2023 Paper 2 (Paper 22): Level 3 (d) answers considered both sides with depth.
External cost vs social cost
Do not treat these as the same. An external cost is only the spillover onto third parties. The social cost is the total: social cost = private cost + external cost (and social benefit = private benefit + external benefit). Examiners reward the equation, not just the words. Mixing them up loses the point that social cost exceeds private cost.
Which externality over-produces?
Link the direction correctly. A negative externality (social cost exceeds private cost, e.g. pollution) leads to over-production. A positive externality (social benefit exceeds private benefit, e.g. education) leads to under-production. Students often attach the wrong one to over-production. Remember: bad spillover → too much made, good spillover → too little made.
Merit vs demerit good
Keep the pair straight. A merit good (education, healthcare) has external benefits and is under-consumed and under-produced. A demerit good (cigarettes, alcohol) has external costs and is over-consumed and over-produced. Some candidates swap the two or forget the direction. Merit = good for you, under-provided; demerit = harmful, over-provided.
June 2023 Paper 2 (Paper 21, Q4b): some candidates confused merit and demerit goods.
Match the command word and develop
Command words matter. A define (2) needs only a precise meaning; explain (4) develops a point; analyse (6) is a sustained one-way chain (no evaluation); discuss (8) is two-sided with a judgement. The biggest mark-winner is developing each point through to its effect.
State the direction of the change
When you name an effect, state its direction: demand up or down, output over- or under-produced, price up or down. On merit and demerit goods, some named the good but did not say whether demand would rise or fall. One directional word can win the mark.
Evaluate a policy: it depends on PED
A strong evaluation move on a tax: its effect depends on the price elasticity of demand. For an addictive, inelastic good a higher price cuts quantity only a little, so it mainly raises revenue rather than correcting the failure. Naming this reaches toward Level 3.
Market failure happens when the price mechanism, left to itself, allocates resources in a way that is not the best for society — producing too much of some goods, too little of others, or none at all of some. The market still allocates most ordinary private goods well; market failure is about the specific cases where it does not.
The main causes you must know are:
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