Movement Along vs Shift of the Demand Curve

A very common source of confusion — and a frequent exam question — is the difference between a change in quantity demanded and a change in demand itself.

Movement along the demand curve (change in quantity demanded) happens ONLY when the good's OWN price changes, every other determinant held constant. The curve itself does not move; the consumer simply slides to a different point on the SAME curve.

• Downward movement (expansion of demand): price falls → quantity demanded rises → movement down and to the right along the same curve.
• Upward movement (contraction of demand): price rises → quantity demanded falls → movement up and to the left along the same curve.

Shift of the demand curve (change in demand) happens when any determinant OTHER than the good's own price changes — income, tastes, price of related goods, population, expectations, and so on. The ENTIRE curve moves to a new position, and at the SAME price a different quantity is now demanded.

• Rightward shift (increase in demand): e.g. income rises, a favourable change in taste, a substitute's price rises, a subsidy is announced — more is demanded at every price.
• Leftward shift (decrease in demand): e.g. income falls, tastes turn unfavourable, a complement's price rises, an unfavourable government policy — less is demanded at every price.