A price change slides you along the demand curve. Everything else picks the curve up and moves it. Getting this distinction right is probably the single most examined skill in the whole of microeconomics — and it is also where most marks are quietly lost.
📚 What you need to know
A non-price determinant is anything except the price of the good itself that changes how much people buy.
A change in one of these shifts the entire curve left or right at every price level.
Shift right = demand increases (D → D₁). Shift left = demand decreases (D → D₂).
The five to learn: income, tastes, prices of related goods, number of consumers, future price expectations.
Substitutes move demand in the same direction as the other good’s price. Complements move it the opposite way.
Price of the good itself → movement along. Anything else → shift.
What a shift looks like
Hold the price completely still. If people now buy more than before, the curve has moved right. If they buy less, it has moved left. The price did not have to change at all for this to happen.
Notice the price line never moves. That is the whole point of a shift: the change happens at every price, not because of a price.
The five non-price determinants
Determinant
What happens
Which way the curve moves
Real income
People can afford more (or less) at every price. For normal goods the link is direct
Income up → right. Income down → left
Tastes and preferences
Fashion, health news, advertising and branding change what people want
More popular → right. Less popular → left
Price of a substitute
A rival good gets dearer, so buyers switch to this one
Substitute’s price up → right for this good
Price of a complement
A good used alongside this one gets dearer, so fewer of both are bought
Complement’s price up → left for this good
Number of consumers
Population size or the age profile of the market changes
More buyers → right. Fewer buyers → left
Future price expectations
If buyers expect prices to rise, they buy now instead of later
Expect a rise → right now. Expect a fall → left now
Six rows for five determinants, because substitutes and complements are two halves of the same idea (“prices of related goods”). Learn them as separate rows anyway — questions almost always ask about one or the other, not both.
Inferior goods: the exception on income
For most things, a rise in income means more demand. These are normal goods. But some goods are bought precisely because money is tight — supermarket own-brand food, long-distance coach travel, second-hand clothing. When incomes rise, people trade up and buy less of these. They are inferior goods, and their demand curve shifts left when income rises.
Inferior does not mean bad quality. It is a statement about how demand responds to income, nothing else. A perfectly good product can be an inferior good.
Substitutes and complements
Ask yourself: would I buy one instead of the other, or one along with the other? That question sorts every related-goods question you will ever get.
Watch the direction carefully. It is the price of the other good that changes, and the demand for your good that shifts. Nobody moved along the coffee curve in that left-hand column — the coffee curve itself moved right.
Movement or shift? The test that always works
Left: one curve, two points. Right: two curves. If your diagram has only one curve on it, you cannot be describing a shift.
🧩 The two-question test
What caused the change? If the answer is “the price of this good”, stop — it is a movement along.
If it was anything else, it is a shift. Now ask: at the old price, would people buy more or less?
More → shift right, an increase in demand. Less → shift left, a decrease in demand.
Then say it properly: “demand increases from D to D₁” for a shift; “quantity demanded extends” for a movement.
Worked examples
WORKED EXAMPLE 1
For the market for domestic flights, state whether each causes a movement along or a shift, and in which direction: (a) airlines cut fares; (b) train ticket prices double; (c) a recession cuts household incomes; (d) an airport tax on passengers is scrapped, lowering ticket prices. [4]
Ask each time: was it the price of flights, or something else?
(a) Price of the good itself → movement along, extension in QD
(b) Price of a substitute rises → shift right, demand increases
(c) Income falls, flights are a normal good → shift left, demand decreases
(d) Ends as a lower ticket price → movement along, extension in QDalong, right, left, along(d) is the trap. The tax changes supply, but what consumers respond to is the lower price, so they move along their curve.
WORKED EXAMPLE 2
Using a diagram, explain the effect on the demand for electric cars of a large fall in the price of home charging units. [4]
Step 1: identify the relationship
Charging units and electric cars are complements — used together.
Step 2: apply the rule
The price of the complement falls, so owning an electric car becomes cheaper overall.
Step 3: state the shift
At every price, more electric cars are wanted: demand shifts right, D to D₁.
Step 4: the diagram
Two parallel downward-sloping curves, the second to the right, with an arrow and both curves labelled.
Complement cheaper → demand for the other rises → shift rightDo not also move the supply curve. The question only changed something on the demand side.
WORKED EXAMPLE 3
Incomes in a country rise sharply. Explain what happens to the demand for supermarket own-brand instant noodles. [3]
Step 1: classify the good
Own-brand instant noodles are typically an inferior good.
Step 2: state the relationship
For inferior goods, income and demand move in opposite directions.
Step 3: conclude with the shift
Richer households trade up to fresh or branded food, so demand shifts left.
Income up → demand for an inferior good fallsAdd a caveat: whether a good is inferior depends on the country and the income level, so this is not automatic.
💡 Exam tip
Draw both curves and label them D and D₁. A shift diagram with one curve scores nothing.
Add an arrow showing the direction of the shift — it is often an explicit mark.
Name the determinant before you describe the shift: “a change in tastes causes…”
Keep the language clean: “demand increases” for a shift, “quantity demanded extends” for a movement.
Check whether the good is normal or inferior before answering any income question.
Only move the curve you were asked about. Shifting demand and supply at once when only one changed is a classic self-inflicted error.
⚠ Common mix-up
Shifting the demand curve when the price of the good changes. That is always a movement along.
Confusing substitutes and complements. Instead of = substitute. Along with = complement.
Thinking “inferior” means poor quality. It only describes the response to income.
Shifting the curve up or down instead of left or right. Describe demand shifts horizontally — right for an increase.
Assuming every good is normal. Always consider whether it might be inferior.
Forgetting expectations. Buyers who think prices will jump next month buy now, which shifts demand right today.
Saying an advertising campaign lowers the price. It changes tastes, which shifts demand right; the price is then determined in the market.
Up next: The Law of Supply and the Supply Curve — the same logic seen from the seller’s side of the counter.
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