Externalities
~40 min · WEC11 · 1.3.5
WEC11 · 1.3.5 · 40 min
A free market gets the price right for the people trading in it — but when a third party who never agreed to the trade pays part of the cost or pockets part of the benefit, the market is answering a question nobody actually asked, and it answers it wrong in a specific, drawable, way.
Key terms in this lesson
Before you read on
Two or three questions on exactly what this lesson teaches. Being wrong here is fine — it's the fastest way to find out what to pay attention to next.
Market failure, and the five names it goes by
A free market gets one thing right by construction: the price settles wherever the last unit's to the seller equals its marginal benefit to the buyer — the same unit-by-unit logic as : "marginal" always means what the very next unit adds, never an average across every unit already traded. That's efficient exactly on condition that seller and buyer are the only two people affected by the trade. is the spec's name for what happens when that condition breaks — the market still finds an equilibrium, it just finds the wrong one, producing more or less than the quantity that would make society as a whole best off. Spec 1.3.5.1 names five distinct ways this happens: , the free-rider problem behind non-provision of public goods, imperfect market information, moral hazard, and speculation/market bubbles. This lesson builds the first one in full, because it's the one every other market-failure lesson on this paper leans on for vocabulary (private vs social, marginal analysis, ). The other four get their own lesson next.
Every transaction has up to three parties whose costs and benefits could, in principle, be counted: the buyer, the seller, and anyone else affected without ever agreeing to the trade — a third party. Private cost is what the producer itself pays; private benefit is what the consumer itself gets. External cost and external benefit are whatever spills onto third parties — costs or benefits that never show up in either party's own decision, because neither of them is the one paying or receiving it. Add private and external together and you get the true total: social cost = private cost + external cost, social benefit = private benefit + external benefit. A market that only responds to private cost and private benefit is, by construction, blind to the external piece — not making a mistake, just never being asked the question.
Spec 1.3.5.2.c draws a distinction most students collapse without noticing: an externality can arise from producing a good or from consuming it, and these are genuinely different mechanisms that shift genuinely different curves. A — positive or negative — changes the *cost* side: it's about what making the good does to third parties, so it shifts marginal private cost away from , leaving demand (marginal benefit) untouched. Pollution from a factory, or clothing manufacturing waste water, are production externalities regardless of who eventually buys the product — the damage is done in the making, before any consuming happens. A — positive or negative — changes the *benefit* side: it's about what using the good does to third parties, so it shifts marginal private benefit away from , leaving supply (marginal cost) untouched. Vaccination is the standard positive-consumption case — the person vaccinated gets the private benefit of not getting sick, and everyone around them who's now less likely to catch it from them gets an external benefit the vaccinated person never priced into their own decision. Smoking in a shared space is the standard negative-consumption case — a good overconsumed for exactly this reason, because its private benefit to the consumer outstrips its true social benefit once the externality is counted, is the spec's demerit good. The spec (1.3.5.2.e) names five contexts this shows up in on the real exam: transport, health, education, environment, and financial markets — worth having a real example ready in each before an exam, not just the two this lesson builds diagrams for.
Two of the remaining three contexts, worked the same who/how/third-party way as the diagrams above. Transport — driving into a congested city centre is a negative consumption externality: the driver gets the private benefit of the journey, but every extra car on the road adds to journey time for every other road user already there, a real cost that never enters the driver's own decision (this is the standard justification for congestion charging — effectively a Pigouvian tax on the externality, priced per entry rather than per unit of a physical good). Financial markets — a bank taking on excessive risk is a negative production externality: if the bank fails, the cost isn't confined to its own shareholders, it spills onto other institutions through contagion and onto taxpayers through bailout costs, a third-party cost the bank's own risk decision never priced in (this is the standard economic case for bank capital requirements, which function the same way a Pigouvian tax does — forcing the institution to internalise a cost it would otherwise externalise). Education is the fifth named context — a positive-consumption externality (an educated population benefits people beyond the individual who paid for the education) — but doesn't get a worked example of its own here; have one ready before the exam rather than relying on the vaccination case to stand in for it.
Mechanism
Why an externality moves output away from the social optimum — not just "harms someone"
The market equilibrium and the social optimum are answers to two different equations, and an externality is exactly the gap between them. A firm and a consumer, left alone, settle at the output where marginal private benefit equals marginal private cost (MPB = MPC) — each side is optimising its own payoff, and neither has any reason to look past its own cost and benefit curves, because nothing in the transaction requires it to. The socially optimal output is where marginal social benefit equals marginal social cost (MSB = MSC) — the output that would be chosen by someone who had to weigh every cost and every benefit the good actually generates, including the pieces that land on people who aren't in the room. Whenever external cost or external benefit is zero, these two equations are identical and the market gets it right by accident of there being nothing external to miss. The moment an externality exists, MSB ≠ MPB or MSC ≠ MPC for at least one side, and the two equations stop being the same equation — the market keeps solving MPB=MPC because that's genuinely all it has access to, while the efficient answer has moved to wherever MSB=MSC now sits. This is why the direction of the error is fully predictable rather than a coincidence to memorise: a missing cost (negative production externality) means the market's stopping condition is met too late, at an output beyond the social optimum — overproduction. And because that extra output is still sold along the same downward-sloping demand curve, the price it clears at is also too low relative to the truly efficient price (Pₘ < P_opt) — a separate mark-scheme-credited point (confirmed: "Pme below Pso shows price paid is below social optimum," Oct 2023 MS Q12e KAA indicative content), not just a description of the quantity gap. A missing benefit (positive consumption externality) means the market's stopping condition is met too early, at an output short of the social optimum — underproduction. Whether it's the wrong side (too much or too little) follows mechanically from whether the thing being ignored is a cost or a benefit, not from the specific numbers involved.
Worked, in full
Deriving the welfare-loss triangle — negative externality of production
- 01
Take a factory whose output has no consumption externality (MSB = MPB = 100 − Q, a standard downward-sloping demand curve) but does have a production externality: constant marginal private cost MPC = £20, plus a constant marginal external cost of £30 imposed on a downstream water-treatment company for every unit produced. Marginal social cost is therefore MSC = MPC + external cost = 20 + 30 = £50, also constant.
Earns: K — MSC built explicitly from MPC plus the external cost, not asserted as a separate curve from nowhere.
- 02
The market equilibrium is where MPB = MPC (the factory only weighs its own cost): 100 − Q = 20, so Qₘ = 80, at price £20. The social optimum is where MPB = MSC (the efficient stopping point, weighing the full cost): 100 − Q = 50, so Q_opt = 50. Qₘ (80) > Q_opt (50) — the market overproduces, exactly as the mechanism above predicts for a missing cost. The same two equations also pin down two different prices, not just two different quantities: the market settles at Pₘ = £20 (read off MPC, the only cost the firm charges against), while the socially efficient price is £50 (read off MSC). Pₘ (£20) sits strictly BELOW P_opt (£50) — a second, separately creditable symptom of the same missing cost, not a restatement of the output gap: the market doesn't just make too much, the price it settles on is also too low, because it was never made to reflect the externality being ignored.
Earns: An1 — both quantities AND both prices solved from the actual equations, not read off a generic diagram shape; the output gap (Qₘ>Q_opt) and the price gap (Pₘ<P_opt) are two distinct, separately mark-scheme-credited observations, not the same point made twice.
- 03
Consider one specific unit between Q=51 and Q=80 — say the 65th. It gets made because at that output MPB (100−65=£35) still exceeds MPC (£20), so the firm profits from making it. But its true social cost is MSC=£50, which is £15 more than the £35 of benefit it delivers. That single unit is a net loss to society of £15, even though it was privately profitable to produce.
Earns: An2 — the mechanism shown for one concrete unit before generalising, so the triangle isn't just an abstract shape.
- 04
Every unit between Q_opt (50) and Qₘ (80) has this same property — MSC (a flat £50) exceeds MSB (=MPB, falling from £50 at Q=50 to £20 at Q=80) — so every one of those 30 units is individually a net loss. Summing the loss across all of them is exactly the area of the triangle bounded by MSC above, MSB below, between Q=50 and Q=80: base = Qₘ − Q_opt = 80 − 50 = 30, height = MSC − MPB at Qₘ = 50 − 20 = 30. Welfare loss = ½ × base × height = ½ × 30 × 30 = £450 (verified by direct calculation, not estimated from the diagram).
Earns: Eval — the triangle derived as a sum of per-unit losses across the whole overproduced range, which is what makes it deadweight loss rather than a labelled shape on a diagram.
x-axis: Quantity, Q · y-axis: Price and cost, £
- MPB = MSB = AR = D
- Demand curve, unaffected — no consumption externality in this case, so private and social benefit are the same curve. Plotted using the worked-chain's own numbers: MPB = MSB = 100 − Q.
- MPC = S
- The firm's own supply/marginal private cost curve — what the firm actually pays. Constant at £20 per unit in the worked-chain example, which is why it plots flat rather than sloped.
- MSC
- MPC plus the marginal external cost, drawn strictly above MPC by that constant £30 gap (worked-chain: MSC = 20 + 30 = £50) — never below it, since an external cost can only add to social cost, never subtract from it.
- Qₘ, Pₘ
- Market equilibrium — where MPB = MPC (100 − Q = 20). The output the free market actually reaches, ignoring the externality entirely: Qₘ = 80 at price £20.
- Q_opt
- Social optimum — where MPB (=MSB) = MSC (100 − Q = 50), always to the LEFT of Qₘ for a negative production externality. Solves to Q_opt = 50 at £50, the socially efficient price — the point where the true full cost of the next unit just equals what it's worth to the buyer. Pₘ (£20) sits strictly BELOW this £50 socially efficient price — a second, separately mark-scheme-credited point distinct from the output gap: the market doesn't just make too much, it also charges too little for what it makes.
- Welfare loss triangle
- Bounded by MSC (above) and MPB=MSB (below), between Q_opt (50) and Qₘ (80) — every unit in this range costs society more than it's worth, worth exactly £450 when summed unit-by-unit (½ × 30 × 30), derived above rather than just shaded in.
Common error: Drawing MSC below MPC, or shading a triangle without deriving why it represents lost welfare rather than just the gap between two curves.
Correct: MSC strictly above MPC (an external COST can only add to social cost), Q_opt strictly left of Qₘ, and the triangle explicitly between those two quantities — matching the confirmed examiner-report finding that many candidates "omit adding the welfare loss area, social optimum and market equilibrium which are useful to identify on the diagram" [Oct 2023 ER, Q12e]. The same series' real mark scheme credits the price comparison as its own separate point too, not folded into the quantity one — "Pme below Pso shows price paid is below social optimum" [Oct 2023 MS, Q12(e)] — so a complete answer states both that Qₘ is too high AND that Pₘ is too low, not just one of the two.
In your own words
In one sentence: why is the area between MSC and MPB, from Q_opt to Qₘ, a genuine loss to society rather than just "extra output that happened to cost more"?
Complete it yourself
Complete the chain — welfare loss from a positive consumption externality
- 01
A vaccination programme: the private demand for a dose is MPB = 80 − Q. Getting vaccinated also protects people around the vaccinated person from catching the disease — a constant external benefit of £20 per dose that never enters the vaccinated person's own decision. So MSB = MPB + external benefit = 100 − Q.
- 02
The clinic's marginal cost of administering a dose is constant at MPC = MSC = £10 (no production externality here). The market only responds to MPB, so the market equilibrium is where MPB = MPC: 80 − Q = 10, giving Qₘ = 70 doses at price £10.
x-axis: Quantity, Q · y-axis: Price and cost, £
- MPB = AR = D
- The individual's own private demand curve — what the consumer alone is willing to pay for, ignoring the benefit to anyone else. From the chain-drill: MPB = 80 − Q.
- MSB
- MPB plus the marginal external benefit, drawn strictly above MPB by that constant £20 gap (chain-drill: MSB = MPB + 20 = 100 − Q) — never below it, since an external BENEFIT can only add to social benefit.
- MPC = MSC = S
- Supply/marginal cost curve, unaffected — no production externality in this case, so private and social cost are the same curve. Constant at £10 per unit, which is why it plots flat rather than sloped.
- Qₘ, Pₘ
- Market equilibrium — where MPB = MPC (80 − Q = 10). The output the free market actually reaches, underprovided relative to the social optimum: Qₘ = 70 at price £10.
- Q_opt
- Social optimum — where MSB = MSC (100 − Q = 10), always to the RIGHT of Qₘ for a positive consumption externality. Solves to Q_opt = 90 — the point where the true full benefit of the next dose (private plus the protection it gives everyone else) just equals what it costs to provide.
- Welfare loss (gain foregone) triangle
- Bounded by MSB (above) and MSC=MPC (below), between Qₘ (70) and Q_opt (90) — the benefit society is missing out on by not reaching the efficient quantity, worth exactly £200 when summed unit-by-unit (½ × 20 × 20), derived in the chain-drill above.
Common error: Drawing MSB below MPB (the reverse of what a positive externality means), or leaving Q_opt unlabelled relative to Qₘ so the direction of underprovision isn't visible on the diagram itself.
Correct: MSB strictly above MPB, Q_opt strictly right of Qₘ, and the foregone-gain triangle traced to the exact MSB/MSC intersection that defines Q_opt — the same completeness standard the examiner report applies to the negative-externality diagram [Oct 2023 ER, Q12e].
Named traps
- who-how-development-gap
- The single most consistently confirmed weakness on this topic, verified across four separate series. Candidates correctly identify an external cost or benefit from a data-response extract, then stop — without naming the specific third party affected or the mechanism connecting the externality to them. Confirmed directly: "When asked to identify the production external costs most could identify the relevant example of deforestation in the Mekong region. Where responses were lacking they failed to explain how third parties were affected by this." [Oct 2021 ER, Q12e]. "The key when using the external costs is to develop who and how the external costs impact third parties." [Oct 2023 ER, Q12e]. "To gain the knowledge marks these needed linking to the external costs and then the analysis marks would be for the impact on the third party." [Jan 2024 ER, Q12d]. The confirmed model of what "developed" looks like, from a real examiner report: "the carbon emissions were linked to global warming and this linked to the impact on sea level rises and flooding of those near the coast. This gained two marks." [Jan 2022 ER, Q12d] — externality → named mechanism → named third party → stated effect, not just "third parties are harmed."
- diagram-completeness-gap
- Naming the right curves isn't the same as completing the diagram. Confirmed directly: "Many candidates omit adding the welfare loss area, social optimum and market equilibrium which are useful to identify on the diagram." [Oct 2023 ER, Q12e]. Both diagrams above need all three landmarks — Qₘ, Q_opt, and the shaded triangle between them — not just the curves themselves.
- define-the-cost-not-just-the-symptom
- Not every correct-sounding definition earns full marks. Confirmed directly on a define-external-costs question: "Most correctly defined it as a negative impact on third parties and were awarded both marks. Many said external costs were costs to third parties which achieved one mark. A less common approach was to say that MSC>MPC which was awarded full marks also." [Oct 2022 ER, Q12a]. "Costs to third parties," alone, is worth one mark, not two — it names WHO but not the mechanism. "A negative impact on third parties" or the formal MSC>MPC statement both clear the full definition.
- production-is-not-consumption
- The spec (1.3.5.2.c) treats production and consumption externalities as genuinely separate cases with separate diagrams, but they're routinely conflated in practice — usually by shifting the demand curve for what is actually a production-side (cost) externality, or vice versa. The test is always: does the externality arise from making the good, or from using it? Factory pollution, deforestation for raw materials, and manufacturing waste water are all production externalities and shift MPC away from MSC, leaving demand untouched — regardless of who eventually buys the product. Vaccination, education, and passive smoking are consumption externalities and shift MPB away from MSB, leaving supply untouched — regardless of how the good was made. Shifting the wrong curve draws a diagram that looks plausible but answers a different question than the one asked. One case this test can mislead on if applied too literally: a good used as an INPUT into someone else's further production. A farmer applying fertiliser to grow crops, or a factory using a chemical in its own manufacturing process, is technically "using" that good — but because that use is itself part of making something else, the externality stays production-side (the cost curve), not consumption-side. Confirmed directly: a real exam question asking students to discuss the external costs of the production AND the use of fertiliser is credited under a single diagram — "Diagram showing MSC above MPC" [Oct 2023 MS, Q12e] — covering both the manufacturing stage and the farmland-application stage, and the examiner report records that "most correctly drew the external costs of production diagram" [Oct 2023 ER, Q12e]. Only a FINAL consumer's own use — a smoker, someone vaccinated, a driver stuck in traffic — triggers the consumption-side (benefit curve) diagram; an intermediate producer applying an input to make something else does not.
- unconditional-conclusion
- "Government intervention will always correct a negative externality" or "a Pigouvian tax always restores efficiency" are unconditional claims. The confirmed Evaluation level descriptors draw the line explicitly: Level 2 evaluation is capped where "the conclusion is not conditional," while Level 3 requires "well-reasoned, conditional perspective consistent with the analysis" [Jan 2025 MS]. State the condition under which the intervention actually works — see the conditional-judgement drill below — in the same sentence as the conclusion, not as an afterthought.
- evaluating-the-claim-not-just-the-policy
- Every conditional-judgement drill below, and the L3-top band of the level-exemplar above, evaluates whether a POLICY response — a tax, a subsidy, private bargaining — would actually work. That's the right skill when a policy is named in the question. But just as many real Section C essays ask a student to "discuss" or "examine" the external costs (or benefits) of an activity with no policy mentioned anywhere in the stem, and that question type rewards a different evaluative skill entirely: weighing the externality CLAIM itself, not judging a fix for it. Three techniques do this, all confirmed against the same real mark scheme. (1) Magnitude — don't just assert that a cost or benefit is significant; weigh it against a comparator. Confirmed: "1.4% of total carbon emissions- small relative to other sectors/a significant impact" [Oct 2023 MS, Q12e] — the identical figure supports both a "small" and a "significant" reading depending what it's set against, which is exactly why stating the comparator, not just the number, is what earns the mark. (2) Netting off — a single activity can generate external costs AND external benefits at the same time, and a complete answer weighs one against the other rather than treating them as two separate, unconnected claims. Confirmed: "Fertiliser use has helped increase crop production four-fold- increasing revenues for crop growers and manufacturers" and "Fertiliser production results in external benefits in terms of increased food supply and employment opportunities" [Oct 2023 MS, Q12e] — both credited as Evaluation marks on a question about fertiliser's external COSTS, precisely because netting the benefits off against the costs is what makes the answer evaluative rather than one-sided. (3) Measurement/valuation difficulty — that external costs and benefits are often hard to measure or put a monetary value on is itself a standalone evaluative point about the externality claim, distinct from the conditional-judgement drill's own "can government measure it precisely enough to set the correct tax rate" below (that's a condition on a POLICY; this is a limit on the CLAIM itself, and it applies even when no policy is mentioned at all).
The conditional move
Complete: "A tax on a good with a negative production externality will move output to the socially optimal level only if ___."
Complete: "A government subsidy is the appropriate response to a positive consumption externality only if ___."
Complete: "Affected parties bargaining privately, instead of a Pigouvian tax, can reach the socially optimal output only if ___."
Complete: "Stating that fertiliser production causes carbon emissions is a genuinely evaluative point, rather than just descriptive analysis, only if ___."
Complete: "An answer that lists fertiliser's external costs is a complete evaluation of the externality, rather than one-sided analysis, only if ___."
Beyond the spec
The spec asks you to draw the externality diagrams and identify the welfare loss but names no economist and offers no theory of WHY a tax (rather than some other fix) is the standard textbook response, or whether government correction is even the only lens available. Both gaps are exactly where a genuinely evaluative Section D answer earns marks that a merely correct diagram doesn't.
Arthur Cecil Pigou, in The Economics of Welfare (1920), is the origin of the standard fix taught above: a tax (or subsidy) set exactly equal to the marginal external cost (or benefit) at the socially optimal quantity, which forces the private decision-maker to internalise the externality — face the true social cost or benefit as if it were their own — without the government needing to ban or ration anything directly. This is why such a tax is called a Pigouvian tax. Ronald Coase's 1960 paper "The Problem of Social Cost" (Journal of Law and Economics) is the classic challenge to Pigou's framing: Coase argued that if property rights are clearly defined and the cost of bargaining between the affected parties is low enough, the parties themselves can negotiate their way to the efficient outcome without any government tax or subsidy at all — whoever values the resource more ends up using it, and compensation flows privately between the two sides. The catch, and the reason this doesn't replace the Pigouvian answer on most of the exam's real contexts, is Coase's own condition: it only works when the number of affected parties is small enough and property rights clear enough for bargaining to actually happen at low cost — a single factory and a single downstream farm can plausibly negotiate; a factory and every resident of a city breathing its air cannot. Naming Coase as the reason a Pigouvian tax is not automatically "the" answer, rather than simply preferring one policy to another, is what turns "evaluate government intervention" into a genuinely conditional judgement instead of a coin flip between two named policies.
Retrieval — with feedback on every choice
A chemical plant's marginal private cost of production is a constant £15 per unit. Every unit produced imposes a constant external cost of £5 on nearby residents. Demand for the plant's output is P = 45 − Q.
What is the value of the welfare loss caused by this externality? (VERIDIAN-original, testing the same welfare-loss-triangle calculation derived in the worked chain above.)
A national park suffers overcrowding and litter as visitor numbers rise, degrading the experience for other visitors and local wildlife. Is this best classified as a production or a consumption externality?
A candidate answers a 2-mark "define external costs" question with: "External costs are costs to third parties." How many marks does this earn, and why?
A furniture manufacturer's private marginal cost of production is a constant £30 per unit. Producing furniture also releases chemical waste that costs a downstream water-treatment company an extra £10 for every unit made. Market demand for the furniture is P = 60 − Q.
Which of the following correctly identifies the free-market output and the socially optimal output, and correctly explains why they differ? (VERIDIAN-original, written in the style of a real Section C "examine" question — not a reproduction of any specific past paper.)
Same question, every level
Evaluate the view that a Pigouvian tax is always the best policy to correct a negative externality arising from production. (VERIDIAN-original question, written in the style of the real Section C extended sub-question — the Q12(e)-type "discuss/evaluate... illustrate with a diagram" essay worth 8 KAA marks + 6 Evaluation marks (14 total, no Level 4 on either strand), confirmed verbatim against the Jan 2025 MS [Q12e]. This is a different essay type from the 20-mark Section D "Evaluate" essay, which carries a 12-mark KAA strand with its own Level 4 band. Not a reproduction of any single past-paper question.)
14 marks available
A tax makes the firm pay more, so it produces less. This is good because it helps reduce pollution and protects the environment.
KAA Level 1 (1-3/8): descriptive, no formula, no diagram, no named mechanism connecting the tax to a specific curve or quantity. Eval Level 1 (1-2/6): the "this is good" claim is asserted, not supported.
Same question, every level
Discuss the external costs associated with the production and use of chemical fertiliser by farmers. Illustrate your answer with an appropriate diagram. (VERIDIAN-original question, matching the real Section C Q12(e)-type "discuss/illustrate" essay worth 8 KAA marks + 6 Evaluation marks (14 total, no Level 4 on either strand), confirmed against the Oct 2023 MS and ER [Q12e] — not a reproduction of the real October 2023 question's own extract-based wording. Deliberately has NO policy in the stem, unlike the Pigouvian-tax exemplar above, because this is the more common real shape of a Q12(e)-family question: [Oct 2021 ER, Q12e], [Jan 2024 ER, Q12d] and [Jan 2022 ER, Q12d], all cited in the trap-taxonomy above, are every one of them a "discuss/identify the external costs of X" question with no policy named at all.)
14 marks available
Fertiliser causes pollution when it's made and when farmers use it on their fields. This is bad for the environment.
KAA Level 1 (1-3/8): descriptive, no named third party, no mechanism, no diagram. Eval Level 1 (1-2/6): "this is bad" is asserted, not supported.
- MSC = MPC + external cost. MSB = MPB + external benefit. Market: MPB=MPC. Optimum: MSB=MSC.
- Negative production externality → MSC>MPC → market OVERproduces, and the price it settles at (Pₘ) sits below the efficient price (P_opt). Triangle: MSC above MPB, between Q_opt and Qₘ.
- Positive consumption externality → MSB>MPB → market UNDERproduces. Triangle: MSB above MPC, between Qₘ and Q_opt.
- Production = shifts the cost curve (making it). Consumption = shifts the benefit curve (using it). Don't swap them.
- Always name WHO is affected and HOW — 'costs to third parties' alone is one mark, not two.
- No diagram, or missing Qₘ/Q_opt/welfare-loss area = capped below the top level.
Not affiliated with or endorsed by Pearson Edexcel. Every quotation and figure attributed to a mark scheme or examiner report in this lesson was independently verified against the primary Pearson document by the WEC11 facts-bank research pass this session, not carried over from prior course material — including one citation deliberately re-pointed to a series that was actually read (October 2022) after the prior build's attribution (June 2024) could not be verified.
A chemical plant's marginal private cost of production is a constant £15 per unit. Every unit produced imposes a constant external cost of £5 on nearby residents. Demand for the plant's output is P = 45 − Q.
What is the value of the welfare loss caused by this externality? (VERIDIAN-original, testing the same welfare-loss-triangle calculation derived in the worked chain above.)
- A£5
This is just the external cost per unit — it isn't the total welfare loss, which depends on how many units are overproduced as well as the size of the gap per unit.
- £12.50
Correct. MSC = 15+5 = £20. Market: 45−Q=15 → Qₘ=30. Social optimum: 45−Q=20 → Q_opt=25. Welfare loss = ½ × (30−25) × (20−15) = ½ × 5 × 5 = £12.50.
- C£25
This comes from using the full base-times-height rectangle (5×5) rather than the triangle — the welfare loss is a triangle, not a rectangle, so it needs the ½ factor.
- D£0, because the market still reaches equilibrium
The market reaching an equilibrium (MPB=MPC) doesn't mean that equilibrium is efficient — it's efficient only where MPB=MSC. Here the two are different quantities (30 vs 25), so a real, calculable welfare loss exists between them.
Traps tested: Reports per unit cost not total loss · Forgets half triangle formula · Confuses market clearing with efficiency
A national park suffers overcrowding and litter as visitor numbers rise, degrading the experience for other visitors and local wildlife. Is this best classified as a production or a consumption externality?
- AProduction — parks are provided by the state
Who provides a good doesn't determine the classification. The damage here comes from people USING (visiting) the park, not from any production process — that makes it a consumption externality regardless of who runs the park.
- Consumption — the externality arises from visitors using the park, not from any production process
Correct. The overcrowding and litter are caused by the act of consuming (visiting) the park — this shifts MPB away from MSB on the demand side, leaving whatever it costs to maintain the park (supply-side) unaffected.
- CIt cannot be classified without knowing the park's entry price
The classification depends on which activity generates the externality (making vs using the good), not on price. A free park with the same overcrowding problem would be classified identically.
- DNeither — this is a free-rider problem, not an externality
The free-rider problem is a distinct source of market failure specifically about non-excludable, non-rival goods where people consume without paying. Overcrowding degrading other visitors' experience is a negative consumption externality — a real cost imposed by one consumer's use on others.
Traps tested: Production consumption confusion · Overclaims uncertainty · Wrong concept entirely
A candidate answers a 2-mark "define external costs" question with: "External costs are costs to third parties." How many marks does this earn, and why?
- A2 marks — it correctly names third parties
It names WHO bears the cost but not the mechanism (a negative impact/spillover, or the formal MSC>MPC relationship) — the confirmed mark scheme awards only partial credit for this exact phrasing.
- 1 mark — it identifies third parties but not the negative-impact mechanism
Correct, and directly confirmed: an examiner report on this exact definition records "external costs were costs to third parties which achieved one mark," against "a negative impact on third parties" or MSC>MPC earning full marks [Oct 2022 ER, Q12a].
- C0 marks — "third parties" is not part of the correct definition
Third parties ARE central to the correct definition — the issue isn't that this phrase is irrelevant, it's that naming who is affected without saying how (a negative impact, or MSC>MPC) only clears the partial-credit bar, not the full one.
- DIt depends on the extract used in the question
This specific under-development pattern — third parties named, mechanism missing — is confirmed as a general one-mark outcome in the examiner report, not something that varies by which extract the question used.
Traps tested: Assumes naming third parties is sufficient · Overcorrects to zero · Overclaims uncertainty
A furniture manufacturer's private marginal cost of production is a constant £30 per unit. Producing furniture also releases chemical waste that costs a downstream water-treatment company an extra £10 for every unit made. Market demand for the furniture is P = 60 − Q.
Which of the following correctly identifies the free-market output and the socially optimal output, and correctly explains why they differ? (VERIDIAN-original, written in the style of a real Section C "examine" question — not a reproduction of any specific past paper.)
- Qₘ = 30, Q_opt = 20. The market produces where MPB=MPC (60−Q=30), but the socially optimal output is where MPB=MSC, and MSC = 30+10 = £40 (60−Q=40). The market overproduces by 10 units because the factory's own decision never accounts for the £10 external cost it imposes on the water company, so it keeps producing units that cost society more than they're worth.
Correct — both quantities correctly solved, MSC correctly built as MPC plus the external cost (not the market price), and the direction (overproduction) tied to the actual mechanism rather than just stated.
- BQₘ = 30, Q_opt = 30. The externality doesn't change the output decision at all — it only affects who ends up bearing the cost of the pollution, not how much furniture gets made.
This treats the externality as a pure distribution question, but it isn't — the whole reason externalities cause market FAILURE (not just an unfair outcome) is that they change the efficient quantity itself. If Q_opt really equalled Qₘ here, there would be no welfare loss and nothing for a market-failure question to be asking about.
- CQₘ = 20, Q_opt = 30. The market under-produces furniture because it ignores the external cost, so output needs to rise to compensate the water company for the damage already being done.
This reverses both the quantities and the direction. A negative production externality means the market OVERproduces relative to the optimum (missing a cost pushes output up, not down) — and raising output further would make the water company's damage worse, not compensate for it.
- DQₘ = 30, Q_opt = 40. Subtracting the external cost from the firm's marginal cost gives MSC = 30−10 = £20, so the socially efficient output (60−Q=20) is actually larger than the market output.
This subtracts the external cost from MPC instead of adding it — an external COST can only raise the true cost to society above what the firm privately pays, never lower it. The sign error also produces the wrong direction: a missing cost should reduce, not raise, the efficient output relative to the market's.
Traps tested: Treats externality as distributional only · Direction reversed · Sign error subtracted not added
Practice this for real
This site teaches the mechanism; the exam is sat on Pearson's own real questions. Go find and attempt these yourself — nothing here substitutes for actually sitting a timed paper.
Pearson's official past-papers portalSelect International Advanced Level → Economics → any series, then look for WEC11.
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A public good can't be switched off for someone who hasn't paid, and that single fact about excludability — not any claim about people being cheap — is what forces the free-rider problem into existence and keeps a private market from ever supplying one.
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