Cambridge International AS and A Level Economics: Efficiency, market failure and externalities (9708)

This guide follows the Cambridge International AS & A Level Economics (9708) syllabus for examination in 2026, 2027 and 2028. It covers section 7.3 (Efficiency and market failure) and section 7.4 (Private costs and benefits, externalities and social costs and benefits). The syllabus treats topics 1 to 6 as AS Level and 7 to 11 as A Level, so this unit is A Level only. AS content is assumed knowledge for Papers 3 and 4. Every figure below is invented for illustration: the firms, towns and markets are fictional.

See the printable checklist and the course hub. Pair this page with the revision notes and the practice questions. The free diagnostics show where to start. Related AS pages: reasons for intervention and methods of intervention.

What you must be able to do

Section	Outcome
7.3.1, 7.3.2	Productive and allocative efficiency: definitions and conditions
7.3.3, 7.3.4	Pareto optimality; dynamic efficiency
7.3.5, 7.3.6	Market failure and its reasons
7.4.1, 7.4.2	SC = PC + EC and SB = PB + EB, with marginal versions, calculated
7.4.3 to 7.4.5	Positive and negative externalities of consumption and production; deadweight welfare loss
7.4.6, 7.4.7	Asymmetric information and moral hazard; costs and benefits in decisions (net present value not required)

7.3.1 and 7.3.2 Productive and allocative efficiency

Productive efficiency means producing at the lowest possible cost per unit. For a firm, the condition is output at minimum average cost (AC), where marginal cost (MC) = AC. For the economy, it means being on the production possibility curve.

Allocative efficiency means resources go to the goods and services that people value most. The condition is price = marginal cost (P = MC). The last unit is then worth exactly what it costs to make. When externalities exist, the condition becomes marginal social benefit = marginal social cost (MSB = MSC).

Worked example: a firm that misses both conditions

A fictional firm has total cost TC = 900 + Q² (output Q in thousands, costs in currency units, CU). So AC = 900/Q + Q and MC = 2Q.

1. AC is lowest where MC = AC: 2Q = 900/Q + Q, so Q² = 900 and Q = 30. AC = 30 + 30 = 60, and MC = 60. This is the productively efficient output.
2. The firm is a monopoly facing demand P = 150 − 2Q, so MR = 150 − 4Q. Profit maximisation, MR = MC, gives 150 − 4Q = 2Q, so Q = 25 and P = 100.
3. At Q = 25, AC = 900/25 + 25 = 61, just above the minimum of 60. Productive efficiency is narrowly missed.
4. P = 100 but MC = 50, so the 25th unit is worth far more to buyers than it costs. Allocative efficiency is missed.
5. P = MC needs 150 − 2Q = 2Q, so Q = 37.5 and P = 75.
6. Output is 12.5 below that level. The welfare loss is the triangle between demand and MC over the gap: ½ × (100 − 50) × 12.5 = 312.5 thousand CU.

The two efficiencies are separate: a firm can meet one and miss the other.

7.3.3 Pareto optimality

An allocation is Pareto optimal if no one can be made better off without making someone else worse off. A Pareto improvement makes at least one person better off and nobody worse off.

Allocation	Utility of person A	Utility of person B
W	6	6
X	8	6
Y	8	3
Z	5	9

W to X is a Pareto improvement: A gains and B is unchanged, so W was not Pareto optimal. X to Y and X to Z are not improvements, because B or A loses. X and Z are both Pareto optimal. Pareto optimality says nothing about fairness: one person holding everything can be Pareto optimal.

With perfect competition and no externalities, both conditions hold in every market and the outcome is Pareto optimal. A policy that helps most people but harms some is not a Pareto improvement, even if total welfare rises.

7.3.4 Dynamic efficiency

Dynamic efficiency is improvement in efficiency over time, through investment, innovation and new technology that lower costs or raise quality. Static efficiency is judged at one moment, dynamic efficiency across years.

Worked example: paying for lower costs later

A fictional firm sells 1,000 units a year and invests supernormal profit in new methods. Its unit cost over six years is 60, 56, 52, 50, 48 and 46 CU. A rival that does not invest has a flat unit cost of 55 CU.

1. The investing firm costs more in years 1 and 2.
2. Six-year cost: (60 + 56 + 52 + 50 + 48 + 46) × 1,000 = 312,000 CU. The competitor: 55 × 6 × 1,000 = 330,000 CU.
3. The saving is 18,000 CU, and the gap widens after year 3.

Evaluation. A monopoly may earn the profit to fund research, so dynamic gain can sit alongside static loss. But a protected firm may feel no pressure to innovate. The outcome depends on how profit is used and whether rivals can enter.

7.3.5 and 7.3.6 Market failure

Market failure occurs when the free market leaves resources allocated inefficiently, so that society’s welfare could be raised. It is complete when no good is supplied, and partial when the quantity is wrong.

Reason	What goes wrong
Externalities	Third parties bear costs or gain benefits that prices ignore
Public goods	Non-excludable and non-rival, so firms cannot charge and supply none (complete failure)
Merit and demerit goods	Imperfect information leaves consumption too low or too high
Asymmetric information	One party knows more, so choices are distorted
Market power	Firms restrict output and set P above MC
Immobile factors	Labour and capital move slowly to where they are needed
Unequal incomes	Markets respond to ability to pay, not need

7.4.1 and 7.4.2 Social costs and benefits

• Social cost (SC) = private cost (PC) + external cost (EC). Marginal: MSC = MPC + MEC.
• Social benefit (SB) = private benefit (PB) + external benefit (EB). Marginal: MSB = MPB + MEB.

Private values fall on the parties to the transaction, external values on others. A market follows private values and settles where MPB = MPC. Efficiency requires MSB = MSC.

Worked example: a negative externality of production

A fictional chemical works. Output Q is in thousand tonnes and values are CU per tonne.

• MPB = 90 − 3Q (demand)
• MPC = 18 + 3Q (supply)
• MEC = 2Q (grows with output)

Q	MPB	MPC	MEC	MSC = MPC + MEC
9	63	45	18	63
12	54	54	24	78

1. Market: 90 − 3Q = 18 + 3Q gives Q = 12 and P = 54.
2. Social optimum: MSB = MSC gives 90 − 3Q = 18 + 5Q, so 72 = 8Q, Q = 9 and P = 63.
3. The market overproduces by 3 thousand tonnes.
4. Total external cost is the area under MEC, which is Q². It is 144 at Q = 12 and 81 at Q = 9, so the extra 3 thousand tonnes impose 63 thousand CU of external cost.
5. Welfare loss: the gap between MSC and MPB at Q = 12 is 78 − 54 = 24. The triangle is ½ × 24 × 3 = 36 thousand CU.

7.4.3 and 7.4.4 Positive and negative externalities

A negative externality is a cost borne by someone outside the transaction, and a positive externality is a benefit enjoyed by someone outside it. Each can arise in production or consumption.

	Negative	Positive
Production	Factory pollution: MSC above MPC	A firm trains workers who later move to rivals: MSC below MPC
Consumption	Noise from loud music: MSB below MPB	Vaccination: MSB above MPB

To sketch: an external cost on the supply side lifts MSC above MPC, and an external benefit puts it below. On the demand side, an external cost puts MSB below MPB and an external benefit puts it above.

Worked example: a positive externality of consumption

A fictional vaccination programme, Q in thousand doses and CU per dose.

• MPB = 100 − 5Q
• MEB = 28 − Q
• MPC = MSC = 20 + 3Q

1. MSB = MPB + MEB = 128 − 6Q.
2. Market: 100 − 5Q = 20 + 3Q gives Q = 10, P = 50.
3. Social optimum: 128 − 6Q = 20 + 3Q gives 108 = 9Q, so Q = 12. MSB = MSC = 56.
4. The market under-provides by 2 thousand doses.
5. Welfare loss: at Q = 10, MSB = 68 and MSC = 50, a gap of 18. The triangle is ½ × 18 × 2 = 18 thousand CU.
6. MEB at Q = 12 is 16, the size of the subsidy that would reach the optimum.

7.4.5 Deadweight welfare loss

A deadweight welfare loss is the net loss to society from producing the wrong quantity. With a negative externality, units beyond the social optimum cost society more than they are worth. With a positive externality, units short of the optimum would be worth more than they cost, and are not made.

On a diagram it is the triangle between MSB and MSC, between the market quantity and the social optimum. Its area is half the gap at the market quantity times the quantity difference. Name the curves and both quantities.

7.4.6 Asymmetric information and moral hazard

Asymmetric information exists when one party in a transaction knows more than the other. Moral hazard exists when a party changes behaviour because someone else bears the cost of the risk, so the risk is greater after the contract than before.

Worked example: information

A fictional second-hand car market. Half the cars are good (worth 6,000 CU to a buyer) and half are poor (3,000 CU). Only sellers know which is which.

1. Buyers cannot tell, so offer the average: 0.5 × 6,000 + 0.5 × 3,000 = 4,500 CU.
2. Owners of good cars need at least 5,000 CU, so they withdraw.
3. Only poor cars remain, so buyers pay 3,000. Trades with a gain of 1,000 CU each (6,000 − 5,000) never happen.

Worked example: moral hazard

A fictional insurer covers 1,000 households against a loss of 5,000 CU. Before cover, the chance of a claim is 4%, so the expected loss is 0.04 × 5,000 = 200 CU. It charges 200 CU. With cover, households take less care, the chance rises to 6%, and the expected claim is 300 CU.

1. Premiums: 200 × 1,000 = 200,000 CU. Claims: 300 × 1,000 = 300,000 CU.
2. The insurer loses 100,000 CU.
3. It must charge 300 CU or add excesses, which leaves some people uninsured.

7.4.7 Costs and benefits in analysing decisions

Cost-benefit analysis values all private and external costs and benefits of a decision and compares total social benefit with total social cost. Net social benefit = SB − SC, and the project goes ahead if it is positive. Net present value is not required, but benefits that arrive later are worth less now.

Worked example: a bypass

A fictional town considers a bypass (all figures in CU millions).

Item	Value
Toll income (private benefit)	20
Time saved by drivers (external benefit)	35
Fewer accidents (external benefit)	10
Construction (private cost)	50
Noise and pollution (external cost)	12

1. SB = 20 + 35 + 10 = 65 and SC = 50 + 12 = 62, so net social benefit = +3.
2. A private operator sees 20 − 50 = −30 and would not build.
3. If the time saving is 20% lower, it is 28, SB is 58 and net social benefit is −4. The result flips once the time saving falls below 32.

Evaluation. The result is sensitive to its most uncertain values. Intangibles such as noise are hard to price, and who counts as affected changes the answer.

Common errors

• Defining allocative efficiency as “no waste” (that is productive), or giving MC = AC as its condition.
• Calling any welfare rise a Pareto improvement when someone loses.
• Adding MEC to MPB, or MEB to MPC. External costs add to costs and external benefits add to benefits.
• Shading a whole area under a curve as welfare loss, not the triangle between MSB and MSC.

Official syllabus

Cambridge International, Cambridge International AS & A Level Economics (9708) syllabus for examination in 2026, 2027 and 2028, Subject content, topic 7, sections 7.3 and 7.4.