Change in Demand vs. Change in Quantity Demanded vs Excise Tax
Change in Demand vs. Change in Quantity Demanded and Excise Tax are two Supply & Demand concepts in AP Economics that students often mix up. Change in demand is a shift of the demand curve, while change in quantity demanded is a movement along the demand curve. An excise tax is a tax levied on the production or sale of a specific good or service. Here is how they compare side by side.
A change in demand occurs when factors like income, preferences, or prices of related goods change, shifting the entire demand curve. A change in quantity demanded occurs when the price of the good itself changes, causing movement along the existing demand curve.
Excise taxes are typically applied to goods with negative externalities, like cigarettes or alcohol, to discourage consumption and raise revenue. They shift the supply curve upward by the amount of the tax.
Excise Tax vs Shift and Movement: What a Tax Actually Moves
| Change in Demand vs. Change in Quantity Demanded | Excise Tax | |
|---|---|---|
| What the term settles | Whether the curve moved or you moved along it | How much is added to the cost of each unit sold |
| Curve it acts on | Demand, either shifted or traced | Supply, lifted by the tax on every unit |
| What buyers do once the tax lands | Slide along an unchanged demand curve | Face a higher price and buy fewer units |
| Sentence that earns the point | Quantity demanded fell, demand did not change | The tax cut equilibrium quantity, not demand |
| The exception | Only a determinant other than own price shifts demand | A tax collected from buyers does shift demand down by the tax |
| What the diagram must label | A shift arrow, or two points on one curve | Price buyers pay, price sellers keep, and the gap between them |
The tax lands on supply, and buyers answer by sliding down a curve that never moved
Start with Qd equal to 120 minus 4P and Qs equal to 6P minus 20, which clear at a price of 14 with 64 units traded. Impose a tax of 5 per unit collected from sellers. Sellers now need 5 more from every buyer to supply any given quantity, so supply becomes 6P minus 50. Setting 120 minus 4P equal to 6P minus 50 gives a price of 17 to buyers, and quantity of 52. Sellers keep 12. Check the split: buyers pay 3 more than before, sellers keep 2 less, and 3 plus 2 recovers the tax of 5. Revenue is 5 times 52, or 260. Now look at the demand equation. It is still 120 minus 4P, and plugging in the new price of 17 returns 52, the exact quantity traded. Nothing about buyers changed. Their curve was never touched, and the whole demand-side story is a movement from one point on it to another. Writing that the tax reduced demand describes a shift that did not happen and forfeits the point.
The one version of this question where demand really does shift
Collect the same tax from buyers instead and the demand curve moves. Buyers hand over 5 per unit on top of whatever sellers post, so at any posted price P their willingness to buy is what it used to be at P plus 5, and demand becomes 100 minus 4P. Solve against the untouched supply curve, 6P minus 20, and the posted price is 12 while quantity is 52. Buyers still part with 12 plus 5, which is 17. Every real number matches the seller-side version exactly: 52 units, 17 out of the buyer's pocket, 12 into the seller's. That equivalence is the point of the exercise, and it is why the legal side of a tax is treated as bookkeeping rather than economics. Shifting demand here is legitimate precisely because it changes no outcome. Free-response prompts nearly always state that the tax is levied on producers, so the safe default is to shift supply and leave demand alone. Work the burden split at /calculate/tax-incidence and the revenue at /calculate/excise-tax-revenue.
Frequently asked questions
Does an excise tax decrease demand or quantity demanded?
Quantity demanded, in the standard case where the tax is collected from sellers. Supply shifts up by the tax, the market price rises, and buyers move along an unchanged demand curve to a smaller quantity. In the worked example above a tax of 5 lifted the buyer price from 14 to 17 and cut quantity from 64 to 52, while the demand equation stayed exactly as it was.
When does an excise tax shift the demand curve?
Only when the tax is legally collected from buyers. Demand then shifts down vertically by the per-unit tax, because a buyer willing to pay 17 in total will now offer sellers only 12 once a tax of 5 is added at the register. Even then the equilibrium quantity, the buyer price, and the seller price come out identical to the seller-side version.
Why does it not matter which side the tax is collected from?
Both versions open the same gap between what buyers pay and what sellers keep, and only that gap determines quantity. Shifting supply up by 5 or shifting demand down by 5 produces 52 units either way, with buyers at 17 and sellers at 12. Who actually bears the burden depends on relative elasticity, never on which side writes the cheque.
Live Supply and Demand graph. Drag the curves, or open the full version.
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