Price Controls and Market Intervention
AdvancedGovernments intervene through price ceilings, floors, quotas and per-unit taxes or subsidies. An effective ceiling creates a shortage, a floor creates a surplus, a quota lifts the price; a per-unit tax is not borne by buyers alone — tax incidence is shared according to supply and demand elasticities.
- A ceiling binds below equilibrium: the price is held down, creating a shortage and non-price competition (queuing, rationing, falling quality)
- A floor binds above equilibrium: supply exceeds demand — a surplus; too high a minimum wage causes unemployment
- A quota caps quantity: the supply curve turns vertical at the quota, quantity falls and the price rises
文體小知識：經濟現象說明文——以供求圖的文字分析講解價格上限、價格下限、配額與從量稅津貼對市場的影響。
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1Market prices guide resources well, but governments often intervene on fairness grounds or to protect the vulnerable. A price ceiling is the maximum legal price a seller may charge. It binds only when set below the equilibrium price: the price cannot return to equilibrium, quantity demanded exceeds quantity supplied, and a shortage appears. If the ceiling is at or above equilibrium, the market price is already lower and the control has no effect.
2With an effective ceiling the shortage persists, and allocation shifts to non-price competition: buyers queue or wait for rationing, sellers pick buyers, quality declines, and side payments may emerge. In the early pandemic, masks were scarce and queues formed outside pharmacies; landlords of rent-controlled flats may attach extra conditions or cut maintenance. The worse the shortage, the fiercer the non-price competition — and the more resources wasted on waiting.
3A price floor is the minimum legal price, binding only above equilibrium, and it creates a surplus: quantity supplied exceeds quantity demanded. The minimum wage in the labour market is the classic case: if the minimum exceeds the equilibrium wage, employers demand less labour while more people seek work — a labour surplus, meaning some job-seekers cannot find employment. Some countries guarantee farmers a floor price for crops, and the government often ends up buying the surplus, a fiscal burden.
4Among quantity interventions, a quota caps the maximum amount that may be sold or produced: quantity falls and the price rises, with the supply curve turning vertical at the quota. A per-unit tax takes a fixed amount for each unit, shifting the supply curve upward; the price rises, but by less than the tax. A per-unit subsidy works in reverse, lowering the price and raising quantity. Neither the tax nor the subsidy is borne by one side alone: the burden or benefit is shared between buyers and sellers according to elasticities — the less elastic the demand, the more buyers bear.
1Summary 重點摘要
Governments intervene through price ceilings, floors, quotas and per-unit taxes or subsidies. An effective ceiling creates a shortage, a floor creates a surplus, a quota lifts the price; a per-unit tax is not borne by buyers alone — tax incidence is shared according to supply and demand elasticities.
2Vocabulary cards Vocabulary
The maximum legal price set by the government; binding below equilibrium, it creates a shortage.
The minimum legal price set by the government; binding above equilibrium, it creates a surplus.
A situation where quantity demanded exceeds quantity supplied at a price — the result of an effective ceiling.
A situation where quantity supplied exceeds quantity demanded at a price — the result of an effective floor.
When prices are controlled, buyers and sellers compete through queuing, rationing and selection instead of price.
A government cap on the amount that may be sold or produced; it cuts quantity and raises the price.
The supply curve under a quota: it turns vertical at the quota, so no price can raise output beyond it.
A fixed tax charged per unit; it shifts supply up, raises the price and cuts the quantity.
A fixed government payment per unit; it shifts supply down, lowers the price and raises the quantity.
How the tax is actually shared between consumers and producers, depending on elasticities; inelastic demand means buyers bear more.
3Step by step Steps
- Step 1: Price ceilings and when they bindA price ceiling is the maximum legal price. To judge whether it binds, compare it with the equilibrium price: only a ceiling below equilibrium is effective — the price cannot rise back to equilibrium, quantity demanded exceeds quantity supplied, and a shortage appears. For example, if a district's equilibrium rent is $15,000 a month and the ceiling is set at $12,000, demand rises to 1,300 units while supply falls to 800 — a shortage of 500 units. A ceiling at $18,000, above equilibrium, changes nothing.
- Step 2: Non-price competition under a shortageAn effective ceiling disables the price mechanism, so the shortage cannot be cured by higher prices and allocation shifts to non-price means: queuing, rationing, sellers picking buyers, falling quality, even side deals. For example, when masks ran short early in the pandemic, residents queued from before dawn for pharmacies to open — waiting time itself is a cost. Landlords of rent-controlled flats may attach extra conditions or cut back on maintenance. In answers, name these phenomena and their costs.
- Step 3: Price floors and surplusesA price floor is the minimum legal price; it binds only above equilibrium and creates a surplus — quantity supplied exceeds quantity demanded. For example, if the equilibrium wage is $40 an hour and the minimum wage is set at $50, employers hire less while more people seek work — a labour surplus, with some workers unemployed. Likewise, countries that guarantee farmers a floor purchase price to stabilise incomes often end up buying the surplus crops, a burden on public finances.
- Step 4: Quotas and the kinked supply curveA quota caps the maximum amount that may be sold or produced — a quantity intervention. The supply curve under a quota is kinked: up to the quota it follows the original shape; beyond it the curve turns vertical, because no price can lift output past the cap. For example, if the government quotas imported fresh milk at 100,000 bottles a day, supply cannot rise no matter how much consumers are willing to pay; the price climbs, and importers holding quotas earn the extra margin.
- Step 5: How a per-unit tax affects price and quantityA per-unit tax charges a fixed amount for each unit. After the tax, sellers offer less at every price: the whole supply curve shifts upward by the tax, the equilibrium price rises and the equilibrium quantity falls. For example, in a market with an equilibrium price of $20, a $6 tax per unit lifts the new equilibrium price to $24 and cuts the volume. Note that the price rise of $4 is smaller than the $6 tax: a per-unit tax is never fully passed on in the price — buyers do not bear the entire tax, a frequent exam point.
- Step 6: Sharing the tax burden — the role of elasticityTax incidence is how the tax is actually shared between consumers and producers, and it depends on elasticities: the less elastic the demand, the harder it is for buyers to cut back and the more they bear; the less elastic the supply, the more sellers bear. Continuing the example: with a $6 tax and a $4 price rise, consumers bear $4 and producers $2. For example, cigarette demand is inelastic, so smokers bear most of the tobacco duty; taxing goods with elastic demand forces sellers to absorb a larger share as customers walk away.
- Step 7: The effect of a per-unit subsidyA per-unit subsidy is a fixed government payment for each unit produced, and it works in the opposite direction of a tax: the whole supply curve shifts downward by the subsidy, the equilibrium price falls and the equilibrium quantity rises. The benefit is likewise shared by elasticity: the less elastic the demand, the more the price falls and the more consumers gain. For example, a $10-per-box subsidy to mask makers lets them expand output and cut prices, so the public buys masks more cheaply — a common way for governments to encourage production.
5Worked example Example
Data question (two parts): a district's private-housing rental market has an equilibrium rent of $15,000 a month with 1,000 units let. 1. The government imposes rent control with a ceiling of $12,000; at that rent, quantity demanded is 1,300 units and quantity supplied is 800. Is the control effective? Find the shortage and give one example of non-price competition. 2. After the control is scrapped, the government levies a per-unit tax of $1,000 on landlords for each unit let; the rent rises to $15,600 and the volume falls to 960 units. Find how much of the tax tenants and landlords each bear.
- (1) Effectiveness: the ceiling of $12,000 lies below the equilibrium rent of $15,000, so the control is an effective price ceiling
- Compute the shortage: at $12,000, quantity demanded 1,300 − quantity supplied 800 = a shortage of 500 units; with rent pinned below equilibrium, the shortage persists
- Non-price competition: landlords select tenants or attach extra conditions, units receive less maintenance, tenants must wait, and side payments may appear
- (2) Tenants' share: the rent rises from $15,000 to $15,600, so tenants pay $600 more per unit — they bear $600
- Landlords' share: after tax they receive 15,600 − 1,000 = $14,600, which is $400 less than before — they bear $400; and $600 + $400 = $1,000, so the tax is shared between the two sides
- Answer: 1. The $12,000 ceiling is below the $15,000 equilibrium, so the control is effective; at $12,000, quantity demanded (1,300) exceeds quantity supplied (800) by 500 units — the shortage. The shortage breeds non-price competition: landlords select tenants or attach extra conditions, and maintenance declines. 2. Of the $1,000 per-unit tax, the $600 rise in rent is borne by tenants, while landlords now net $14,600 — $400 less than before — bearing $400 themselves. The tax is shared between the two sides according to elasticities; it is not passed on in full to tenants.
6Common mistakes Mistakes
7Quick check Quick check
Q1When is a price ceiling effective, and what does it cause?
Q2Under a price ceiling, quantity demanded is 200 units and quantity supplied 120. What is the shortage?
Q3What happens when the minimum wage is set above the equilibrium wage?
Q4A $5 per-unit tax raises the price by $3. How much of the tax do consumers and producers each bear?
Q5How does less elastic demand affect the distribution of tax incidence?
8Recap Recap
- A ceiling binds below equilibrium: the price is held down, creating a shortage and non-price competition (queuing, rationing, falling quality)
- A floor binds above equilibrium: supply exceeds demand — a surplus; too high a minimum wage causes unemployment
- A quota caps quantity: the supply curve turns vertical at the quota, quantity falls and the price rises
- A per-unit tax shifts supply up: the price rises by less than the tax and quantity falls; a subsidy works in reverse
- Tax incidence is shared by elasticity: the less elastic the demand, the more consumers bear