A workshop at the end of your street runs loud equipment at 6 a.m. Its production is cheap partly because the noise cost lands on you, not on its books. Meanwhile, your flu shot protects your coworkers, who paid nothing for that protection. In both cases the market price omits real costs or benefits to bystanders — and when prices lie, even competitive markets produce the wrong quantities. That is market failure, and it's the core of the Market Failure and Role of Government content category on the CLEP exam (8–14% of questions).
An externality is a cost or benefit that falls on a third party — someone outside the transaction. Four curves organize everything:
| Curve | Meaning |
|---|---|
| MPC | Marginal private cost — the supply curve (producers' own costs) |
| MSC | Marginal social cost = MPC + external cost |
| MPB | Marginal private benefit — the demand curve (buyers' own benefit) |
| MSB | Marginal social benefit = MPB + external benefit |
Markets settle where MPB = MPC. Society wants MSB = MSC. Whenever the private and social curves split apart, the market quantity is wrong and deadweight loss appears.
Pollution, noise, congestion — external costs, usually from production.
[GRAPH: Negative externality. X-axis "Quantity", Y-axis "Price/Cost". Demand D = MPB = MSB. Supply S = MPC. A second curve MSC parallel and ABOVE MPC, with the vertical gap labeled "marginal external cost". Market equilibrium at Qmkt where D crosses MPC. Social optimum at Qopt < Qmkt where D crosses MSC. DWL triangle between MSC and MSB from Qopt to Qmkt, with its vertex at Qopt. A per-unit tax equal to the external cost shifts MPC up onto MSC.]
Vaccination, education, home improvements that lift the whole block — external benefits, usually from consumption.
[GRAPH: Positive externality. Demand D = MPB. A second curve MSB parallel and ABOVE MPB, gap labeled "marginal external benefit". Supply S = MPC = MSC. Market equilibrium at Qmkt where MPB crosses S. Social optimum at Qopt > Qmkt where MSB crosses S. DWL triangle between MSB and MSC from Qmkt to Qopt, vertex at Qopt.]
Orientation tricks: - Negative externality → the extra curve is on the cost side (MSC above MPC) → optimum lies left of the market quantity (produce less). - Positive externality → the extra curve is on the benefit side (MSB above MPB) → optimum lies right of the market quantity (produce more). - The DWL triangle always sits between the two quantities, with its vertex at the social optimum (where MSB = MSC). - Size the policy to the externality. A tax larger than the external cost overshoots — output falls below Qopt and a new deadweight loss appears on the other side.
Demand (MPB = MSB): P = 30 − Q. Supply (MPC): P = 6 + Q. Each unit imposes $4 of pollution cost. - Market: 30 − Q = 6 + Q → Qmkt = 12. - MSC = (6 + Q) + 4 = 10 + Q. Optimum: 30 − Q = 10 + Q → Qopt = 10. - DWL = ½ × (12 − 10) × $4 = $4 — base is the overproduced units, height is the external cost.
| Excludable | Non-excludable | |
|---|---|---|
| Rival | Private goods (pizza, jeans) | Common resources (fish stocks, groundwater) |
| Non-rival | Club/toll goods (streaming services, uncongested toll roads) | Public goods (national defense, tornado sirens, streetlights) |
Public goods (non-rival + non-excludable) invite free riders: you receive the good whether or not you pay, so voluntary payment collapses, private firms can't profitably supply it, and the market underprovides — often to zero. This is the textbook case for government provision funded by taxes. Careful: "provided by government" is the remedy, not the definition — public schools are actually rival and excludable; national defense is the real thing.
Common resources (rival + non-excludable) suffer the tragedy of the commons: each user captures the full benefit of additional use but shares the depletion cost with everyone, so the resource is overused. An HOA's unmetered community well, open-ocean fisheries, and free street parking all fit. Remedies: quotas, permits, metering, assigning property rights.
Symmetry worth memorizing: public goods behave like positive-externality machines (underprovided); common resources behave like negative-externality machines (overused).
Q1 — D. The defining feature of an externality is a spillover to a third party — someone outside the transaction bears a cost or receives a benefit. A describes an ordinary profitability problem internal to the firm. B is the definition of positive economic-vs-accounting comparison, unrelated to spillovers. C reverses cause and effect — a tax can be the remedy for an externality, not the externality itself. E describes a market-functioning problem, not a spillover. Fix rule: externality = effect on a bystander; if only buyer, seller, or government appear in the option, it's a distractor.
Q2 — C. The shop equates MPB with its own MPC, ignoring the fume costs, so it produces past the point where MSC = MSB — overproduction. A puts the wedge on the benefit side, which describes a positive externality. B ignores the externality entirely; supply-equals-demand is efficient only when private and social curves coincide. D is internally contradictory — the whole problem is that the external cost does not appear in the shop's own costs. E gets the price distortion backwards: the market price is too low because it omits the external cost. Fix rule: negative externality → market overproduces and underprices; the missing cost belongs to society, not the firm.
Q3 — A. Efficiency from society's viewpoint requires counting all benefits and all costs: MSB = MSC. B is where the unregulated market lands — efficient only if no externality exists. C would just mean there is no external cost, a description, not an optimum. D likewise describes the absence of external benefits. E is wrong because maximizing consumer surplus alone ignores production costs and producer surplus. Fix rule: market sits at MPB = MPC; society wants MSB = MSC — write both, then compare.
Q4 — B. A Pigouvian tax equal to the $30 marginal external cost lifts MPC onto MSC, so the market internalizes the noise cost and chooses the optimal quantity. A pays producers to make more of the harmful good — the direction error. C is a price control that creates a shortage; it doesn't align private costs with social costs. D raises no per-unit disincentive — a lump sum doesn't change marginal decisions. E overshoots: the optimal quantity of blowers is positive as long as some uses are worth more than their full social cost. Fix rule: tax negative externalities per unit, sized exactly at the marginal external cost.
Q5 — C. The protection of coworkers and family is an external benefit, so MSB lies above MPB and the market, which stops at MPB = MPC, falls short of the optimum. A invents a bias with no basis in the externality model. B is exactly what fails here — buyers weigh only their own benefits. D locates the wedge on the cost side, which is the negative-externality diagram. E reverses the direction of the failure. Fix rule: external benefits → MSB > MPB → the market under-produces; the wedge sits on the benefit side.
Q6 — E. MSC = MPC + $4 = 10 + Q. Set MSB = MSC: 30 − Q = 10 + Q → Q = 10. A is the gap between market and optimal quantities (12 − 10), not a quantity itself. B is the external cost mistaken for a quantity. C is the supply intercept. D comes from subtracting the $4 externality from the market quantity of 12 — a tempting shortcut with no basis. Fix rule: build MSC by adding the external cost to supply, then solve MSB = MSC — never adjust the market quantity directly.
Q7 — B. Streetlight benefits are non-excludable: every resident gets the light whether they contributed or not, so each has an incentive to free-ride on neighbors — and voluntary funding collapses below the efficient level. A is false: light is non-rival — one household's use takes nothing from others. C mislabels a benefit spillover as a cost. D reverses the facts; if exclusion were possible, ordinary pricing would work. E is backwards and irrelevant — the problem is understated willingness to pay, not overstated valuations. Fix rule: non-excludable benefit → free riders → voluntary provision falls short; look for the "benefit without paying" option.
Q8 — C. A tornado siren is non-rival (my warning doesn't diminish yours) and non-excludable (everyone in range hears it) — a public good. A is excludable (password-protected) and non-rival: a club good. B is excludable (tolls) and, while congested, rival — effectively a private good. D is rival and non-excludable — a common resource, the other quadrant. E is rival and excludable — a private good. Fix rule: test both properties separately — public good requires non-rival AND non-excludable; one out of two isn't enough.
Q9 — B. Between the market quantity and the optimum lie units for which MSB > MSC — value society never collects because buyers, counting only private benefits, don't purchase them. A describes the deadweight loss of a negative externality. C confuses efficiency with revenue; DWL is lost surplus, not lost taxes. D has nothing to do with it — the market here can be perfectly competitive with normal profit. E misstates the price relationship and, in any case, DWL comes from the missing quantity, not the price level. Fix rule: positive externality DWL = surplus on the units that go un-produced between Qmkt and Qopt.
Q10 — D. The water is rival (each gallon used is gone) and non-excludable (no meters, no gates), so each household captures the full benefit of watering while spreading the depletion cost across all forty — classic tragedy of the commons. A misclassifies: a public good is non-rival, but this water visibly depletes. B mislabels a depletion problem as a benefit spillover. C fails because no single agent controls the well. E is wrong because no price control exists — there's no price at all, which is precisely the problem. Fix rule: rival + non-excludable = common resource = overuse; non-rival + non-excludable = public good = underprovision.
Q11 — E. The optimal Pigouvian tax equals the $30 marginal external cost; a $50 tax pushes MPC above MSC, cutting output below the social optimum and creating a fresh deadweight loss from under-production. A treats pollution reduction as always worth more — but past the optimum, forgone production is worth more than the avoided harm. B confuses tax incidence (who pays) with tax size (how much correction is needed). C is false: only a tax equal to the external cost lands the market on the optimum. D denies the market failure that motivated the tax. Fix rule: the corrective tax matches the marginal external cost exactly — bigger is not better past MSB = MSC.
Q12 — A. A subsidy equal to the $15 marginal external benefit closes the wedge between MPB and MSB, moving quantity exactly to the optimum. B taxes the very activity society wants more of — the direction error. C overshoots: pushing consumption past the point where MSB = MSC creates new deadweight loss. D lowers the legal price, which reduces the quantity supplied and creates a shortage rather than more vaccinations. E ignores the $15 spillover, leaving the under-provision in place. Fix rule: subsidize positive externalities per unit, sized exactly at the marginal external benefit — tax what you want less of, subsidize what you want more of.
Q1 — D. The defining feature of an externality is a spillover to a third party — someone outside the transaction bears a cost or receives a benefit. A describes an ordinary profitability problem internal to the firm. B is the definition of positive economic-vs-accounting comparison, unrelated to spillovers. C reverses cause and effect — a tax can be the remedy for an externality, not the externality itself. E describes a market-functioning problem, not a spillover. Fix rule: externality = effect on a bystander; if only buyer, seller, or government appear in the option, it's a distractor.
Q2 — C. The shop equates MPB with its own MPC, ignoring the fume costs, so it produces past the point where MSC = MSB — overproduction. A puts the wedge on the benefit side, which describes a positive externality. B ignores the externality entirely; supply-equals-demand is efficient only when private and social curves coincide. D is internally contradictory — the whole problem is that the external cost does not appear in the shop's own costs. E gets the price distortion backwards: the market price is too low because it omits the external cost. Fix rule: negative externality → market overproduces and underprices; the missing cost belongs to society, not the firm.
Q3 — A. Efficiency from society's viewpoint requires counting all benefits and all costs: MSB = MSC. B is where the unregulated market lands — efficient only if no externality exists. C would just mean there is no external cost, a description, not an optimum. D likewise describes the absence of external benefits. E is wrong because maximizing consumer surplus alone ignores production costs and producer surplus. Fix rule: market sits at MPB = MPC; society wants MSB = MSC — write both, then compare.
Q4 — B. A Pigouvian tax equal to the $30 marginal external cost lifts MPC onto MSC, so the market internalizes the noise cost and chooses the optimal quantity. A pays producers to make more of the harmful good — the direction error. C is a price control that creates a shortage; it doesn't align private costs with social costs. D raises no per-unit disincentive — a lump sum doesn't change marginal decisions. E overshoots: the optimal quantity of blowers is positive as long as some uses are worth more than their full social cost. Fix rule: tax negative externalities per unit, sized exactly at the marginal external cost.
Q5 — C. The protection of coworkers and family is an external benefit, so MSB lies above MPB and the market, which stops at MPB = MPC, falls short of the optimum. A invents a bias with no basis in the externality model. B is exactly what fails here — buyers weigh only their own benefits. D locates the wedge on the cost side, which is the negative-externality diagram. E reverses the direction of the failure. Fix rule: external benefits → MSB > MPB → the market under-produces; the wedge sits on the benefit side.
Q6 — E. MSC = MPC + $4 = 10 + Q. Set MSB = MSC: 30 − Q = 10 + Q → Q = 10. A is the gap between market and optimal quantities (12 − 10), not a quantity itself. B is the external cost mistaken for a quantity. C is the supply intercept. D comes from subtracting the $4 externality from the market quantity of 12 — a tempting shortcut with no basis. Fix rule: build MSC by adding the external cost to supply, then solve MSB = MSC — never adjust the market quantity directly.
Q7 — B. Streetlight benefits are non-excludable: every resident gets the light whether they contributed or not, so each has an incentive to free-ride on neighbors — and voluntary funding collapses below the efficient level. A is false: light is non-rival — one household's use takes nothing from others. C mislabels a benefit spillover as a cost. D reverses the facts; if exclusion were possible, ordinary pricing would work. E is backwards and irrelevant — the problem is understated willingness to pay, not overstated valuations. Fix rule: non-excludable benefit → free riders → voluntary provision falls short; look for the "benefit without paying" option.
Q8 — C. A tornado siren is non-rival (my warning doesn't diminish yours) and non-excludable (everyone in range hears it) — a public good. A is excludable (password-protected) and non-rival: a club good. B is excludable (tolls) and, while congested, rival — effectively a private good. D is rival and non-excludable — a common resource, the other quadrant. E is rival and excludable — a private good. Fix rule: test both properties separately — public good requires non-rival AND non-excludable; one out of two isn't enough.
Q9 — B. Between the market quantity and the optimum lie units for which MSB > MSC — value society never collects because buyers, counting only private benefits, don't purchase them. A describes the deadweight loss of a negative externality. C confuses efficiency with revenue; DWL is lost surplus, not lost taxes. D has nothing to do with it — the market here can be perfectly competitive with normal profit. E misstates the price relationship and, in any case, DWL comes from the missing quantity, not the price level. Fix rule: positive externality DWL = surplus on the units that go un-produced between Qmkt and Qopt.
Q10 — D. The water is rival (each gallon used is gone) and non-excludable (no meters, no gates), so each household captures the full benefit of watering while spreading the depletion cost across all forty — classic tragedy of the commons. A misclassifies: a public good is non-rival, but this water visibly depletes. B mislabels a depletion problem as a benefit spillover. C fails because no single agent controls the well. E is wrong because no price control exists — there's no price at all, which is precisely the problem. Fix rule: rival + non-excludable = common resource = overuse; non-rival + non-excludable = public good = underprovision.
Q11 — E. The optimal Pigouvian tax equals the $30 marginal external cost; a $50 tax pushes MPC above MSC, cutting output below the social optimum and creating a fresh deadweight loss from under-production. A treats pollution reduction as always worth more — but past the optimum, forgone production is worth more than the avoided harm. B confuses tax incidence (who pays) with tax size (how much correction is needed). C is false: only a tax equal to the external cost lands the market on the optimum. D denies the market failure that motivated the tax. Fix rule: the corrective tax matches the marginal external cost exactly — bigger is not better past MSB = MSC.
Q12 — A. A subsidy equal to the $15 marginal external benefit closes the wedge between MPB and MSB, moving quantity exactly to the optimum. B taxes the very activity society wants more of — the direction error. C overshoots: pushing consumption past the point where MSB = MSC creates new deadweight loss. D lowers the legal price, which reduces the quantity supplied and creates a shortage rather than more vaccinations. E ignores the $15 spillover, leaving the under-provision in place. Fix rule: subsidize positive externalities per unit, sized exactly at the marginal external benefit — tax what you want less of, subsidize what you want more of.