Difficulty: Introductory to Intermediate | Prerequisites: Module 1 (Scarcity, Allocation, and Markets)
Module 1 showed how markets allocate resources through supply and demand. This module asks what happens when governments step in. Price controls (ceilings and floors) override the market's equilibrium price, creating predictable shortages or surpluses. Elasticity measures how sensitive buyers are to price changes, which in turn determines who bears the burden of a tax. If you are not comfortable with supply, demand, equilibrium, shortage, and surplus from Module 1, revisit those notes first.
Governments intervene in markets through price controls and taxes. Price ceilings (maximum prices) cause shortages when set below equilibrium; price floors (minimum prices) cause surpluses when set above it. The elasticity of demand tells you how much quantity demanded responds to a price change, and it determines how the burden of an excise tax is split between buyers and sellers.
Elasticity of demand
The responsiveness of quantity demanded to changes in price. Measured as the percentage change in quantity demanded divided by the percentage change in price.
Think of it as "how much do buyers flinch when the price moves?"
Excise tax
A per-unit tax levied on a specific good. It is added on top of the price for each unit sold.
In simple terms, for every unit sold, the government collects a fixed amount of money.
Incidence of taxation
Who pays what share of a tax. The legal obligation to remit the tax (statutory incidence) is separate from who actually bears the economic cost (economic incidence).
Think of it as "who really ends up paying, regardless of who writes the cheque to the government?"
Price ceiling
A government-imposed maximum price for a good or service. If set below the equilibrium price, it creates a shortage.
In simple terms, the government says "you cannot charge more than this amount."
Price controls
Government rules or laws that inhibit the formation of market-determined prices. Price ceilings and price floors are both types of price control.
Price floor
A government-imposed minimum price for a good or service. If set above the equilibrium price, it creates a surplus.
In simple terms, the government says "you cannot charge less than this amount." The minimum wage is the most common example.
Elasticity tells you the degree to which buyers respond to a price change.
Inelastic demand (|E| < 1): quantity demanded changes less than proportionally to price. Buyers are relatively unresponsive. Examples include necessities like insulin or petrol.
Elastic demand (|E| > 1): quantity demanded changes more than proportionally to price. Buyers are very responsive. Examples include luxury goods or items with close substitutes.
Unit elastic demand (|E| = 1): the percentage change in quantity demanded exactly equals the percentage change in price.
Elasticity varies along a linear demand curve. It is not the same as slope.
Price ceilings set a legal maximum price. When the ceiling is below the equilibrium price, it binds and creates a shortage (Q_D > Q_S at the ceiling price). When set above equilibrium, the ceiling is non-binding and has no effect.
Price floors set a legal minimum price. When the floor is above the equilibrium price, it binds and creates a surplus (Q_S > Q_D at the floor price). When set below equilibrium, the floor is non-binding and has no effect.
In both cases, the binding condition is key: a price control only distorts the market if it prevents the equilibrium price from being reached.
An excise tax drives a wedge between what buyers pay and what sellers receive.
The side of the market that is more inelastic bears a larger share of the tax burden, regardless of whether the tax is legally imposed on buyers or sellers.
If demand is more inelastic than supply, buyers bear most of the tax. If supply is more inelastic than demand, sellers bear most of it.
Elasticity of demand:
E = \frac{\%\Delta Q}{\%\Delta P} = \frac{\Delta Q / Q}{\Delta P / P} = \frac{\Delta Q}{\Delta P} \cdot \frac{P}{Q}Read it as: the percentage change in quantity demanded divided by the percentage change in price.
The fraction (Delta Q / Delta P) is the inverse of the demand curve's slope. Multiplying by (P / Q) adjusts for the point on the curve where you are measuring.
Classification thresholds:
|E| < 1: Inelastic
|E| > 1: Elastic
|E| = 1: Unit elastic
Variable to know:
E = elasticity
Rent control is a classic price ceiling. In cities like New York and San Francisco, capping rent below the market rate produces housing shortages: long waiting lists, deteriorating building quality, and a thriving under-the-table market.
The minimum wage is a price floor on labour. When it is set above the equilibrium wage, it can create a surplus of labour (unemployment), though the magnitude depends on the elasticity of demand for labour in that market.
Government taxes on cigarettes and alcohol rely on the fact that demand for these goods is relatively inelastic, meaning the tax burden falls largely on consumers and generates substantial revenue.
Students often think that a price ceiling always causes a shortage. It does not. A ceiling only causes a shortage when it is set below the equilibrium price (a binding ceiling). A ceiling above equilibrium has no market effect.
The same logic applies to price floors: a floor below equilibrium is non-binding and has no effect.
Students frequently confuse elasticity with slope. Elasticity changes along a straight-line demand curve even though the slope is constant, because the P/Q ratio changes at different points on the curve.
Many students assume that whoever the tax is "imposed on" (buyers or sellers) is the one who bears the burden. This is wrong. Economic incidence depends on relative elasticities, not on statutory incidence.
Expect questions that give you a price control and ask whether it is binding. The test is simple: is the ceiling below equilibrium (binding) or above (non-binding)? Is the floor above equilibrium (binding) or below (non-binding)?
Elasticity calculations are a staple of exam problem sets. Be ready to compute E given numerical changes in P and Q, and to classify the result as elastic, inelastic, or unit elastic.
Tax incidence questions often present two scenarios with different elasticities and ask who bears more of the tax. The rule: the more inelastic side bears more.
True or False: A price ceiling set above the equilibrium price will cause a shortage.
Fill in the blank: If |E| < 1, demand is said to be ______.
True or False: The statutory incidence of a tax determines who bears the economic burden.
Fill in the blank: A per-unit tax is called an ______ tax.
True or False: A price floor set above equilibrium creates a surplus.
Answers: 1. False (it must be set below equilibrium to cause a shortage). 2. Inelastic. 3. False (economic incidence depends on relative elasticities). 4. Excise. 5. True.
Q: The government imposes a price ceiling of 5 dollars on a good whose equilibrium price is 8 dollars. What happens in this market?
A: The ceiling is binding because 5 dollars is below the equilibrium price of 8 dollars. At 5 dollars, quantity demanded exceeds quantity supplied, creating a shortage.
Q: Demand for a good is highly inelastic and supply is highly elastic. The government imposes an excise tax. Who bears most of the tax burden?
A: Buyers (the demand side) bear most of the tax burden because their side of the market is more inelastic.
Q: At a given point on a demand curve, the price is 10 dollars and the quantity demanded is 200 units. The slope of the demand curve (Delta Q / Delta P) is negative 20. Calculate the price elasticity of demand at that point.
A: E = (Delta Q / Delta P) x (P / Q) = (negative 20) x (10 / 200) = negative 1. Demand is unit elastic at that point.
Q: The government sets a minimum wage above the equilibrium wage. What type of price control is this, and what does it create?
A: This is a binding price floor. It creates a surplus of labour, which manifests as unemployment.
This module builds directly on Module 1's supply-and-demand framework. Every price-control question starts with finding the equilibrium price and then checking whether the control is binding.
Elasticity reappears throughout the course. In later modules on market structures, the elasticity of a firm's demand curve determines its pricing power.
Tax incidence connects to welfare economics and deadweight loss, topics covered in more advanced modules.
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