Change in demand vs. change in quantity demanded
The price of ice cream goes up and people buy fewer cones. Most students say "demand fell". It sounds right, but in economics it is wrong, and it is one of the most common mistakes on tests.
Demand is the whole curve
A demand curve shows how many cones buyers want at each price. It is a list of answers, not one number. On our beach, buyers want 120 cones an hour at $3 and 80 cones at $4 (the rule is: quantity = 240 − 40 × price).
So demand means the whole curve, and quantity demanded means one point on it: the amount at one particular price.
The price changes: move along the curve
Say the price of a cone rises from $3 to $4, and nothing else changes. Buyers now want 80 cones instead of 120. Nothing about the buyers changed: the curve stays where it is, and we just slide up along it, from point A to point B.
Something else changes: the whole curve shifts
Now keep the price at $3, but a heatwave hits the beach. At every price, buyers want 50 more cones than before. At $3 they now want 170 cones, not 120. That is not a slide along the old curve: the whole curve has moved to the right, from D₀ to D₁. Demand increased.
What can shift demand? Anything that changes how much buyers want at a given price:
- Weather and tastes: a heatwave, a new trend.
- Income: people earn more or less.
- Prices of related goods: if popsicles get cheaper, some buyers switch, and demand for cones falls.
- Number of buyers: a busy weekend, a cruise ship in port.
- Expectations: if people think prices will rise next week, they buy more today.
One question sorts it out
| What changed? | On the graph | Say |
|---|---|---|
| The good's own price | Move along the curve | Quantity demanded changed |
| Anything else | The whole curve shifts | Demand changed |
The same rule works for sellers: a change in the price of cones moves the shops along their supply curve (quantity supplied), while a change in costs, like more expensive cream, shifts the whole supply curve.
The trap on tests
A classic question: "A heatwave raises demand for ice cream. The price goes up. Does the higher price then lower demand?" No. The higher price lowers quantity demanded, a move along the new curve. Demand itself, the curve, does not move back. If you write "demand falls because the price rose", you are mixing the two ideas.
Quick check
1. The price of a cone falls from $3 to $2. What changes?
Quantity demanded rises, from 120 to 160 cones (240 − 40 × 2). A move along the same demand curve: only the cone's own price changed.
2. Popsicles get much cheaper. What happens to the demand for cones?
Demand for cones falls: the whole curve shifts left, because some buyers switch to popsicles at every price of cones.
3. A news story says: "Higher prices reduced the demand for cones." What is wrong?
A higher price of cones reduces the quantity demanded, not demand. Demand would change only if something other than the cone's price changed.