Difficulty: Introductory | Prerequisites: Resource Allocation and Market Efficiency notes (Doc 1)
This set of notes builds on the surplus and efficiency framework from the previous document. Here you will see what happens when governments intervene in markets through price controls, taxes, and subsidies, and why those interventions create deadweight loss. The final section introduces the equity-efficiency tradeoff, which frames almost every policy debate in economics. You should be comfortable with consumer surplus, producer surplus, and the idea that equilibrium maximises total surplus before working through this material.
Price ceilings (set below equilibrium) cause shortages and deadweight loss; price floors (set above equilibrium) cause surpluses and deadweight loss. Taxes drive a wedge between buyer and seller prices, shrinking the market and creating DWL, with the tax burden falling on whichever side has less elastic demand or supply. Subsidies push output beyond the efficient level. Economists frame all of these as departures from the equity-efficiency tradeoff: redistribution can improve fairness, but it usually comes at the cost of some total surplus.
Price ceiling
A legal maximum price that sellers may charge. It is binding (has an effect) only when set below the equilibrium price.
Think of it as a cap that holds the price down, like rent control.
Price floor
A legal minimum price that must be paid. It is binding only when set above the equilibrium price.
Think of it as a platform that props the price up, like the minimum wage.
Non-binding
A price control set on the "wrong" side of equilibrium (a ceiling above equilibrium or a floor below it). It has no effect because the market clears before the control kicks in.
Production quota
A government-imposed limit on the maximum quantity that may be produced.
Tax incidence
The division of a tax burden between buyers and sellers. It depends on the relative elasticities of demand and supply, not on which side the tax is legally imposed.
In simple terms, whoever finds it harder to walk away from the market ends up paying more of the tax.
Pigovian tax
A tax set equal to the external cost of an activity, designed to bring the market outcome to the socially efficient quantity. Named after economist Arthur Pigou.
Cap-and-trade
A system that sets a total limit (cap) on emissions or production and issues tradeable permits. Firms that can reduce output cheaply sell permits to firms that cannot, so the cap is met at the lowest overall cost.
Subsidy
A payment from the government to producers or consumers, intended to encourage production or consumption of a good. It shifts the supply or demand curve and can lead to overproduction.
Equity
The fairness of the distribution of economic benefits and costs across members of society.
The big tradeoff
The tension between efficiency (maximising total surplus) and equity (distributing it fairly). Redistribution can improve fairness, but it often reduces the total size of the economic pie by weakening incentives.
Utilitarianism
The ethical view that society should maximise total welfare (utility). Because of diminishing marginal utility of income, utilitarianism can favour some redistribution from rich to poor.
A price ceiling is a maximum legal price. It only matters when it is set below equilibrium, because that is when it prevents the market from clearing.
Shortages – at the capped price, quantity demanded exceeds quantity supplied. Some buyers who want the good at that price cannot get it.
Deadweight loss – the units that would have been traded between the ceiling quantity and the equilibrium quantity are lost. The surplus those trades would have created vanishes.
Side effects – shortages breed black markets, higher search costs (time spent queueing or hunting), and quality deterioration (sellers cut costs because they can fill their stock at the capped price anyway).
Rent controls are the most commonly examined price ceiling. They hold the price of rental housing below market equilibrium.
Tenants who secure a unit at the controlled rent benefit.
Landlords receive lower income, reducing their incentive to maintain or build new properties.
Prospective tenants who cannot find a unit are worse off, facing long waiting lists and degraded housing quality.
Black markets emerge: under-the-table payments, key money, or subletting at higher prices.
If the ceiling is set above the equilibrium price, it is non-binding and the market behaves as though the control does not exist.
A price floor is a minimum legal price. It is binding only when set above equilibrium.
Surpluses – at the floor price, quantity supplied exceeds quantity demanded. Sellers want to sell more than buyers want to buy.
Deadweight loss – trades that would have occurred between the floor quantity demanded and the equilibrium quantity do not happen.
Examples – minimum wage laws (a price floor on labour) and agricultural price supports.
The minimum wage sets a floor on the price of labour. When it sits above the market-clearing wage:
Workers who keep their jobs earn more per hour.
Some workers lose their jobs or have their hours cut, because firms demand less labour at the higher wage. The gap between the quantity of labour supplied and the quantity demanded is unemployment.
Increased search costs arise as more workers compete for fewer available positions.
Some studies suggest the minimum wage may reduce job turnover, partially offsetting the employment loss, so the net effect on efficiency is debated.
If the floor is set below the equilibrium wage, it is non-binding and has no effect.
A production quota caps the quantity that firms may produce. The effects mirror a price ceiling in reverse:
Reduced supply pushes the market price above the competitive level.
Deadweight loss arises from the gap between the quota quantity and the efficient quantity.
Enforcement is difficult: firms have an incentive to cheat and overproduce.
A tax on a good creates a wedge between the price buyers pay (price + tax) and the price sellers receive (price - tax). This wedge:
Reduces the quantity traded below the equilibrium level.
Creates deadweight loss on the units no longer traded.
Generates government revenue equal to the tax per unit multiplied by the quantity traded.
The key insight on tax incidence is that the legal assignment of the tax (whether the buyer or seller writes the cheque) does not determine who bears the burden. What matters is relative elasticity:
Inelastic demand, elastic supply – buyers bear most of the burden. They cannot easily reduce their purchases, so the price they pay rises by nearly the full tax.
Elastic demand, inelastic supply – sellers bear most of the burden. Buyers walk away easily, so sellers absorb the tax through a lower received price.
Perfectly inelastic demand – buyers pay the entire tax (price rises by the full tax amount).
Perfectly elastic demand – sellers pay the entire tax (price buyers pay does not change at all).
Pigovian taxes are specifically designed to correct externalities. The tax is set equal to the marginal external cost, shifting the private cost curve up to the social cost curve and restoring the efficient quantity.
Cap-and-trade achieves a similar outcome through quantity rather than price: the government sets a total cap on emissions and issues permits. Firms trade permits among themselves, and the market price of a permit acts as an implicit tax on pollution.
Subsidies are the mirror image of taxes. A government payment to producers or consumers shifts the supply or demand curve outward, leading to:
A quantity traded above the efficient level (overproduction).
Government expenditure that must be funded from elsewhere (taxes on other markets, borrowing).
Examples include agricultural price supports and vaccine subsidies.
Subsidies can be justified when they correct a positive externality (encouraging vaccination, for instance), but they create their own deadweight loss if they push output beyond the socially optimal quantity.
Efficiency asks: is total surplus as large as it can be? Equity asks: is the surplus distributed fairly?
These two goals often pull in opposite directions. Redistributive policies (progressive taxation, transfer payments) shift resources from higher earners to lower earners, which can improve equity. But the taxes and transfers distort incentives: people may work, save, or invest less if they keep a smaller share of the returns. That lost output shrinks the total economic pie.
Utilitarianism says society should maximise total utility. Because of diminishing marginal utility of income (an extra pound means more to a low earner than a high earner), strict utilitarianism can justify significant redistribution. Taken to its logical extreme, it implies equalising incomes entirely.
The practical problem is that full equalisation destroys the incentive to be productive, so total output falls. The optimal policy is somewhere in between: enough redistribution to capture the welfare gains from diminishing marginal utility, without so much that the incentive effects dominate.
This tension between efficiency and equity is called the big tradeoff. Nearly every policy question in microeconomics (and in politics) involves some version of it. A price ceiling on rent improves affordability for some tenants (equity) but creates shortages and deadweight loss (efficiency). A Pigovian tax corrects an externality (efficiency) but may fall disproportionately on lower-income households (equity). Recognising this tradeoff is the starting point for evaluating any market intervention.
Students often think a price ceiling always causes a shortage. It does not. Only a binding ceiling (set below equilibrium) causes a shortage. A ceiling above equilibrium has no effect.
Students often believe tax incidence depends on who legally pays the tax. It does not. The economic burden falls on whichever side of the market is less elastic, regardless of the legal assignment.
Students often assume subsidies are always good because they lower prices. Subsidies can push production past the socially optimal level, creating deadweight loss, and they must be funded by taxes elsewhere.
Students often conflate equity with equality. Equity is about fairness, which may or may not mean equal outcomes. Different ethical frameworks (utilitarianism, Rawlsian fairness) define equity differently.
⚠️ You will likely be asked to draw the deadweight loss triangle created by a price ceiling, price floor, or tax. Practise until you can do it without hesitation.
⚠️ Tax incidence questions are common. Be ready to explain, with a diagram, why the burden falls on the more inelastic side, and to work through the extreme cases (perfectly inelastic demand, perfectly elastic demand).
⚠️ Expect a short-answer or essay question asking you to evaluate a specific policy (rent control, minimum wage, a carbon tax) using the efficiency-equity framework. The strong answer acknowledges both sides of the tradeoff.
⚠️ "Binding vs non-binding" is a frequent multiple-choice trap. Always check whether the control is above or below equilibrium before analysing its effects.
True or false: a price ceiling set above equilibrium creates a shortage.
False. A ceiling above equilibrium is non-binding and has no effect.
Fill in the blank: the side of the market with more ________ demand or supply bears less of the tax burden.
Elastic.
True or false: a Pigovian tax is designed to maximise government revenue.
False. It is designed to correct an externality by setting the tax equal to the marginal external cost.
Fill in the blank: the big tradeoff is the tension between ________ and ________.
Efficiency and equity.
True or false: a subsidy always improves market efficiency.
False. A subsidy can cause overproduction and deadweight loss.
Q: A city imposes a rent ceiling of £800/month in a market where the equilibrium rent is £1,000/month. Describe two consequences.
A: (1) A shortage develops because the quantity of housing demanded at £800 exceeds the quantity supplied. (2) Deadweight loss arises because some mutually beneficial tenancies (where tenants would have paid and landlords would have supplied between the controlled and equilibrium quantities) no longer occur.
Q: A per-unit tax of £2 is imposed on a good with perfectly inelastic demand. Who bears the tax, and why?
A: Buyers bear the entire tax. Perfectly inelastic demand means buyers purchase the same quantity regardless of price, so they absorb the full £2 increase. The price sellers receive does not change.
Q: Explain why a subsidy on a good with no externalities leads to deadweight loss.
A: The subsidy pushes production beyond the efficient quantity (where MSB = MSC). For the extra units produced, the cost of production (MSC) exceeds the benefit to consumers (MSB). The surplus lost on those excess units is the deadweight loss.
Q: A government must choose between a policy that maximises total surplus and one that distributes surplus more equally. What is this tension called, and what does utilitarianism say about it?
A: This is the big tradeoff between efficiency and equity. Utilitarianism argues that redistribution can increase total utility (because of diminishing marginal utility of income), but only up to the point where the incentive effects of redistribution start reducing total output.
Price controls and taxes are direct applications of the surplus and deadweight loss framework from the previous set of notes. This material also connects forward to externalities and public goods (the next set of notes), where Pigovian taxes and cap-and-trade reappear as corrective tools. The equity-efficiency tradeoff underpins virtually every policy topic you will encounter later in the course, from healthcare economics to environmental regulation.
Price ceiling, price floor, rent control, rent ceiling, minimum wage, binding vs non-binding, shortage, surplus, production quota, tax incidence, tax burden, elasticity and taxes, Pigovian tax, Pigou, cap-and-trade, emissions trading, subsidy, agricultural subsidy, deadweight loss from taxes, equity vs efficiency, the big tradeoff, utilitarianism, redistribution, diminishing marginal utility, fairness in markets, welfare economics, microeconomics, ECON 101