Foundationsmicroeconomics
Shifts vs. Movements: What Moves Along the Curve and What Moves the Curve
Overview
The single most-tested distinction in introductory microeconomics: a change in the good's own price causes a movement along the demand or supply curve (quantity demanded/supplied changes), while a change in anything else -- income, tastes, input costs, technology -- shifts the entire curve (demand or supply itself changes). This lesson catalogs every major shifter for both curves, teaches the step-by-step method for analyzing market events, and shows how to handle double shifts where both curves move at once.
Learning objectives
- Distinguish a movement along a curve from a shift of the curve.
- List the five demand shifters and six supply shifters.
- Apply the four-step method to analyze any market event.
- Analyze double shifts and identify when results are ambiguous.
The core distinction
Own-price change -> movement along the curve. The demand curve already contains every price-quantity combination, so a price change just slides you to a different point: quantity demanded changes, demand does not. Anything else changing -> the whole curve shifts, because the old price-quantity relationship no longer holds: demand (or supply) itself changes. Mixing these up is the most common error in the subject -- and the easiest to avoid once the rule is automatic.
Price of the good itself changes: MOVE along the curve. Anything else changes: SHIFT the curve.
Demand shifters
Five factors shift demand. (1) Income: for normal goods, higher income shifts demand right; for inferior goods (instant noodles, used clothing), higher income shifts it left. (2) Tastes and preferences: advertising, trends, health news. (3) Prices of related goods: substitutes (Coke/Pepsi) -- a rise in the substitute's price shifts demand right; complements (cars/gasoline) -- a rise in the complement's price shifts demand left. (4) Expectations: expecting higher future prices or income raises current demand. (5) Number of buyers: more buyers shift market demand right. Mnemonic: TRIBE -- Tastes, Related goods, Income, Buyers, Expectations.
Supply shifters
Six factors shift supply. (1) Input prices: higher wages or material costs shift supply left. (2) Technology: improvements shift supply right. (3) Taxes and subsidies: taxes shift left (higher cost), subsidies shift right. (4) Prices of related goods in production: if a farmer can grow wheat or corn, a higher corn price shifts wheat supply left. (5) Expectations: expecting higher future prices can reduce current supply (hold inventory). (6) Number of sellers: entry shifts right, exit shifts left. Mnemonic: TRINE -- Technology, Related goods, Input prices, Number of sellers, Expectations (plus taxes/subsidies).
The four-step method
Analyze any event in order: Step 1 -- decide whether demand, supply, or both are affected (start with the market named in the question). Step 2 -- decide the direction of the shift (right = increase, left = decrease). Step 3 -- draw or imagine the new intersection. Step 4 -- read off the new equilibrium price and quantity, and note the shortage/surplus pressure that drives the adjustment. Example: 'a health study praises oatmeal' -- Step 1: demand for oatmeal; Step 2: tastes improve, shift right; Step 3: new intersection up and right; Step 4: price up, quantity up.
'Steelworkers win a big raise' -> input prices up -> supply of cars shifts left -> shortage pressure at old price -> equilibrium price up, quantity down.
Double shifts and ambiguity
When both curves shift, combine the single-shift results. If both shifts push price the same way, price definitely moves that way; if they push oppositely, the price change is ambiguous without numbers. The same logic applies to quantity. Example: demand rises (P up, Q up) and supply falls (P up, Q down) -> price definitely rises, but quantity could go either way. Stating 'ambiguous' confidently -- with the reason -- is a correct and complete answer, not a cop-out.
Key points
- Own-price change = movement along the curve; other-factor change = shift of the curve.
- Demand shifters (TRIBE): Tastes, Related-goods prices, Income, Buyers, Expectations.
- Supply shifters: input prices, Technology, Taxes/subsidies, Related goods in production, Number of sellers, Expectations.
- Four-step method: which curve, which direction, new intersection, new P and Q.
- Double shifts: same-direction effects are definite; opposing effects are ambiguous without magnitudes.
Common misconceptions
✓ Truth: A price rise moves you up along a fixed demand curve (quantity demanded falls). The curve shifts only when a non-price factor changes. Saying 'demand fell' when you mean 'quantity demanded fell' is the classic wording trap.
✓ Truth: You can often say something definite about one variable. If demand rises and supply falls, price definitely rises (both push it up) while quantity is ambiguous. Analyze price and quantity separately.
✓ Truth: An increase (rightward shift) in supply lowers equilibrium price and raises quantity. The confusion comes from everyday language where 'more demand' raises prices -- for supply it is the reverse. Keep the four shift cases straight.
Practice questions
Q1. The price of smartphones falls from $800 to $600, and quantity demanded rises. This is:
- An increase in demand (rightward shift).
- A movement along the demand curve (increase in quantity demanded).
- A decrease in supply.
- An increase in supply (rightward shift).
The good's own price changed, so we move along the existing demand curve: quantity demanded rises. 'Demand' (the curve) only shifts when a non-price factor -- income, tastes, related prices -- changes.
Q2. A new robot technology halves the cost of assembling bicycles. The bicycle market will see:
- A rightward shift of supply; lower equilibrium price, higher quantity.
- A rightward shift of demand; higher price and quantity.
- A movement up the supply curve.
- No change, because technology is not a price.
Technology is a supply shifter: cheaper production means more bicycles offered at every price -- supply shifts right. The new intersection has lower price and higher quantity. Demand does not shift; buyers simply move down along their curve as price falls.
Q3. Demand for apartments rises while the supply of apartments also rises by a larger amount. What can be said definitively?
- Price definitely rises.
- Quantity definitely rises; the price change is ambiguous.
- Both price and quantity definitely rise.
- Quantity definitely falls.
Demand up pushes P up and Q up; supply up pushes P down and Q up. Quantity effects agree (both up) so quantity definitely rises. Price effects oppose, so the net price change is ambiguous -- it depends on which shift is larger.
Click a question to reveal the answer and explanation.
Frequently asked questions
How do I remember the demand and supply shifters?
Use mnemonics. Demand: TRIBE -- Tastes, Related goods, Income, Buyers, Expectations. Supply: TRINE plus taxes -- Technology, Related goods in production, Input prices, Number of sellers, Expectations, and Taxes/subsidies. Write them from memory before every exam.
Does a change in income cause a movement or a shift?
A shift. Income is not the good's own price, so it is a background factor: the whole demand curve moves. For normal goods it shifts right when income rises; for inferior goods it shifts left. Only the good's own price causes movements along the curve.
What is the difference between a substitute and a complement?
Substitutes satisfy similar wants (tea and coffee): a rise in one good's price shifts demand for the other right. Complements are used together (printers and ink): a rise in one good's price shifts demand for the other left. Cross-elasticity (a later lesson) measures this precisely.
In a double shift, how do I decide what is ambiguous?
List each shift's separate effect on price and on quantity. If both shifts push a variable the same direction, that change is definite. If they push oppositely, the net effect is ambiguous -- it depends on which shift is larger. Report both the definite and the ambiguous result.