AP Syllabus focus: ‘The effects of taxes and subsidies depend on the price elasticity of demand and supply.’
Elasticity determines how strongly buyers and sellers respond to price changes caused by taxes or subsidies. Those responsiveness differences shape how policy changes prices, quantities, surplus, and the size of market inefficiency.
Core idea: elasticity drives “who changes” and “who pays”
Per-unit taxes and subsidies create a wedge between the price consumers face and the price producers receive. Price elasticity of demand and price elasticity of supply determine:
Tax/subsidy incidence (how the price burden/benefit is split)
The change in equilibrium quantity
The resulting deadweight loss (efficiency cost)
Key elasticity measures
Price elasticity of demand (PED): How responsive quantity demanded is to a change in price, ceteris paribus.
PED is typically reported in absolute value when comparing responsiveness; more elastic means consumers reduce purchases more when price rises.
Price\ Elasticity\ of\ Demand\ (E_d)=\frac{%\Delta Q_d}{%\Delta P}
= responsiveness of quantity demanded to price (unitless)
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FAQ
PED is usually negative because price and quantity demanded move in opposite directions.
Using $|PED|$ makes it easier to compare responsiveness across goods without focusing on the sign.
Over longer periods, buyers and sellers typically find more alternatives:
Consumers adjust habits and switch substitutes.
Firms adjust technology and inputs.
This often makes demand and supply more elastic, increasing quantity changes and DWL for a given per-unit tax.
More close substitutes generally make demand more elastic, limiting how much the consumer price can rise.
Necessities tend to have more inelastic demand, making it easier for the consumer price to rise and shifting more burden to consumers.
With perfectly elastic demand, consumers will not pay a higher price; even a small price increase causes quantity demanded to drop to zero.
So the consumer price cannot rise; the tax is absorbed on the producer side (as a lower net-of-tax price), with a large quantity reduction.
On a straight-line demand curve, the slope is constant but elasticity changes with price and quantity.
A per-unit tax can move the market along the curve to a region with different elasticity, altering the magnitude of quantity changes and DWL compared with what you’d predict from elasticity at the original equilibrium.
