The Most Famous Curve in Economics
Demand is the relationship between a good's price and the quantity buyers are willing and able to purchase, holding everything else constant.
The law of demand: when price rises, quantity demanded falls. Two forces drive it:
- Substitution effect: as coffee gets pricier, some drinkers switch to tea.
- Income effect: a higher price shrinks what your budget can cover, so you buy less of everything — including this.
Plotted with price on the vertical axis, demand slopes downward:
Willingness and ability. Wanting a yacht is not demand; demand is desire backed by purchasing power at that price. The curve counts wallets, not wishes.
From individual to market. The market demand curve is the horizontal sum of every buyer's curve: at each price, add up all the quantities individuals would buy. A million small decisions become one smooth line.
Reading the curve. Pick a price, read across: that is the quantity the market will absorb. Pick a quantity, read up: that is the maximum price at which it all sells — the demand price. The curve is a complete menu of price-quantity possibilities, of which the market will select exactly one point once supply enters the story.
Two forces behind the law of demand
| Force | Mechanism | Example |
|---|---|---|
| Substitution effect | Pricier good → switch to alternatives | Coffee up, tea instead |
| Income effect | Higher price shrinks real purchasing power | Buy less of everything |
Common pitfall: Saying "demand fell" when the price rose. A price change moves you along the curve — quantity demanded falls, demand itself is unchanged. Reserve "demand fell" for a leftward shift of the whole curve.
Movement vs Shift: The Distinction That Grades Exams
The most common error in first-year economics is confusing two completely different events on the demand diagram.
Movement along the curve — caused by one thing only: the good's own price. Price falls, you slide down the same curve to a larger quantity. The relationship hasn't changed; you've just read a different row of the same menu.
Shift of the whole curve — caused by anything that changes how much buyers want at every price:
- Income: for normal goods, more income shifts demand right; for inferior goods (instant noodles, bus rides), more income shifts it left.
- Substitute prices: tea gets expensive → coffee demand shifts right.
- Complement prices: printers get cheap → ink demand shifts right.
- Tastes and expectations: a health scare shifts demand for a food left overnight; expecting prices to rise tomorrow shifts demand right today.
- Number of buyers: population growth adds curves to the horizontal sum.
The test that never fails: ask what changed? If the answer is "this good's price" → movement along. If it is anything else → shift.
Key rule: A change in the good's own price moves you along the curve. A change in anything else shifts the whole curve.
Why it matters beyond exams. A sales jump can mean your price cut worked (movement) or that a competitor stumbled (shift). The remedies differ: one is repeatable, the other is luck. Firms that misread which curve event they're living through price themselves into trouble.
The shift catalogue
| Change in… | What happens to demand |
|---|---|
| The good's own price | Nothing shifts — movement along the curve |
| Income (normal good) | Shifts right when income rises |
| Income (inferior good) | Shifts left when income rises |
| Price of a substitute | Substitute pricier → shifts right |
| Price of a complement | Complement cheaper → shifts right |
| Tastes / expectations | Either direction, at every price |
Tip: One question settles every exam item: did the good's own price change, or something else? Own price → along the curve. Anything else → the whole curve moves.