HookThe 5p that beat 7.6 billion plastic bags
In 2014, England's big supermarkets handed out around 7.6 billion single-use plastic bags — roughly 140 for every person in the country. In October 2015 the government imposed a charge of just 5p per bag. Usage did not dip; it collapsed. Within a few years the main retailers' single-use bag sales had fallen by well over 95%, and by the early 2020s the average person was buying two or three a year instead of 140. The charge was later raised to 10p and extended to every shop in 2021, but the work was already done — by five pence.
Why did the free market get plastic bags so spectacularly wrong? Because the price was lying. A 'free' bag was never free: its true cost included litter, clogged waterways, harm to wildlife and decades in landfill — costs paid by third parties who never stood at the till. When prices leave costs out, markets over-produce the harmful and under-produce the beneficial, however smoothly they appear to run. That is market failure, and 1.5 covers the full arc: why markets misallocate, what governments can do about it — and why intervention itself sometimes fails.
ModelExternalities — costs and benefits that escape the price
Market failure means the free market allocates resources inefficiently — the wrong quantities produced and consumed relative to what would maximise society's welfare. The version this section centres on is the externality: a cost or benefit falling on third parties, people outside the transaction who were never consulted and never compensated.
With a negative externality, the social cost of a good exceeds the private cost: the factory pays for labour and materials but not for the river it pollutes; the driver pays for fuel but not for the congestion and fumes imposed on everyone else. Because producers and consumers respond only to private costs, the market over-produces — the price is too low and the quantity too high relative to the social optimum. With a positive externality, social benefit exceeds private benefit: your vaccination protects people who never paid for it; your education raises the productivity of every team you ever join. Left alone, the market under-provides these, because individuals weigh only their own benefit. The vocabulary is the scaffolding of every answer: private cost plus external cost equals social cost; and the market fails whenever the two diverge and nothing forces the decision-maker to notice.
CaseTwo live UK externalities
Congestion is the cleanest negative-consumption case in Britain. Each driver entering central London weighs private costs — fuel, time — but not the delay, noise and fumes imposed on everyone behind them. So in February 2003 London priced it: the congestion charge launched at £5 a day (it stands at £15 today), and traffic entering the zone fell by around 15% early on, with measured congestion falling considerably more. The externality did not vanish; it finally appeared on the driver's own bill.
Smoking runs the same logic through consumption: the smoker buys the packet, but treating smoking-related illness costs the NHS an estimated £2–3 billion a year, and passive smoking put the harm directly onto bystanders — which is why intervention came as regulation, not just tax: the 2007 ban on smoking in enclosed public places simply removed the choice. And on the positive side, vaccination is the textbook under-provision case solved by a 100% subsidy: each jab protects the vulnerable people the patient will never meet, so the NHS makes the private price zero and provision universal. One habit to build now: for any externality in an extract, name the third party explicitly — the driver behind, the bar worker, the unvaccinated neighbour. That single sentence is usually the application mark.
MechanismThe toolkit — taxes, subsidies, rules, information, price floors
Governments have five main levers, and Edexcel expects you to match lever to failure. Indirect taxes internalise external costs by making the polluter pay: landfill tax started at £7 per tonne in 1996 and had climbed to just over £100 by 2024 — a price signal that helped drive waste from holes in the ground into recycling. Subsidies do the reverse for positive externalities, paying producers or consumers to do more of what benefits others. Regulation commands rather than prices: bans, limits, minimum ages, the 2007 smoking ban. Information provision — labelling, campaigns — attacks failures rooted in ignorance rather than incentives. And minimum prices put a legal floor under goods whose cheapness is the problem: from 1 May 2018 Scotland required every unit of alcohol to sell for at least 50p, a floor raised to 65p in September 2024.
Run the minimum unit price arithmetic. A 3-litre bottle of 7.5% white cider contains 3,000ml × 0.075 = 225ml of pure alcohol — 22.5 units. Before 2018 it sold in Scotland for as little as £3.50–£4. At 50p a unit, its legal floor became 22.5 × £0.50 = £11.25; from September 2024, 22.5 × £0.65 = £14.63. Meanwhile a pub pint of 4% lager (about 2.3 units) was already priced far above any floor and barely moved. That asymmetry IS the policy design: hit the cheapest, strongest drinks — where the heaviest drinkers' units are concentrated — while leaving moderate consumption almost untouched. Early studies suggest Scottish alcohol purchases fell by roughly 3%, concentrated in the heaviest-buying households. Quote the calculation and the targeting logic together and you have knowledge, application and analysis in one paragraph.
CaseGovernment failure — when the cure costs more than the disease
Government failure is intervention that leaves the allocation of resources worse — or costs more than the failure it set out to fix. The causes are structural, not accidental: governments face imperfect information (taxing an externality properly requires valuing it, which nobody can do precisely); interventions trigger unintended consequences; administration itself burns resources; and political time horizons are short.
Britain's defining case is diesel. From 2001, vehicle and company-car taxes were graded by CO2 — and diesels, emitting less CO2 per mile, got favourable treatment. Drivers responded exactly as incentives predicted: diesel's share of new cars roughly tripled from about 14% in 2000 to around half by 2012. Then the evidence hardened that diesels emit far more NOx and particulates — and cities began breaching legal air-quality limits. Policy reversed into surcharges and ULEZ zones; a tax aimed at one externality had subsidised another. The Green Homes Grant told the quicker, cheaper version: a £1.5 billion insulation voucher scheme launched in September 2020 and scrapped within about six months, with fewer than 50,000 of a hoped-for 600,000 homes upgraded. But keep the balance the exam demands: government failure is a risk, not a rule. The 5p bag charge cost almost nothing to run and beat 7.6 billion bags. Weighing which way a given intervention falls — that is the evaluation, and there is no shortcut slogan.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Definitions in this section live or die on the phrase 'third party' — an external cost is a cost imposed on those outside the transaction, and examiners look for those words. Write the accounting identity explicitly too: social cost = private cost + external cost, then locate the failure as the gap between them. For intervention questions, build the chain like a supply-and-demand answer: 'tax raises costs → supply shifts left → price rises, quantity falls toward the social optimum' — and mirror it for subsidies.
Evaluation is where 1.5's big marks sit, and the strong cards are always the same four: elasticity (does the tax actually change quantity, or just raise revenue?); valuation (the right tax rate requires measuring an unmeasurable externality); fairness (duties and price floors bite hardest on low incomes); and government failure (could this intervention cost more than the harm — as diesel did?). Anchor every point to a real, dated policy — the 5p charge, MUP at 50p and 65p, the 2003 congestion charge — because 'a tax on a good with externalities' earns knowledge marks, while 'landfill tax rising from £7 to over £100 a tonne' earns application marks on top.