Why does the quantity buyers want fall as price rises, and what makes the whole demand curve shift?
Topic 2.1 Demand: state the law of demand, distinguish a change in quantity demanded from a change in demand, and identify the determinants that shift the demand curve.
A focused answer to AP Microeconomics Topic 2.1, covering the law of demand, the difference between a movement along and a shift of the demand curve, the determinants of demand, and the income and substitution effects, with worked exam-style questions.
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What this topic is asking
Topic 2.1 introduces the buyers' side of a market: demand. The College Board wants you to state the law of demand, draw and read the demand curve, distinguish a change in quantity demanded (a movement along the curve) from a change in demand (a shift of the curve), and identify the determinants that shift demand. The movement-versus-shift distinction is the single most tested idea in Unit 2.
The law of demand and the demand curve
Two forces explain the downward slope. The substitution effect: when a good's price rises, it becomes relatively more expensive than substitutes, so buyers switch toward the cheaper alternatives. The income effect: a higher price reduces the real purchasing power of a buyer's income, so they can afford less. Both push quantity demanded down as price rises. Diminishing marginal utility from Topic 1.6 reinforces this: because each extra unit is worth less, consumers will buy more only at a lower price.
Movement along versus a shift
This distinction is where most Unit 2 points are won or lost. If a question says "the price of coffee fell," that is a movement along the demand curve, more coffee is demanded, but the curve does not move. If a question says "incomes rose" or "the price of tea rose," the whole coffee demand curve shifts.
The determinants that shift demand
A useful mnemonic is TRIBE: Tastes, Related goods, Income, Buyers, Expectations.
- Tastes and preferences: a stronger preference for the good shifts demand right; a weaker one shifts it left.
- Related goods: a substitute is a good used instead (tea for coffee); a rise in a substitute's price shifts demand for the good right. A complement is a good used together (cream with coffee); a rise in a complement's price shifts demand for the good left.
- Income: for a normal good, higher income shifts demand right; for an inferior good, higher income shifts demand left (consumers switch away as they get richer).
- Buyers: more buyers in the market shift demand right; fewer shift it left.
- Expectations: if buyers expect a higher future price, current demand rises (shifts right) as they buy now.
A rightward shift is an increase in demand (more demanded at every price); a leftward shift is a decrease.
Try this
Q1. State the law of demand. [1 point]
- Cue. As a good's price rises, the quantity demanded falls, all else equal (an inverse relationship).
Q2. The price of sugar (a complement to coffee) falls. State the effect on the demand for coffee and the direction of any shift. [2 points]
- Cue. Cheaper sugar makes the coffee-and-sugar pair cheaper to enjoy, so demand for coffee increases (shifts right).
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2018 (style)1 marksMultiple choice. A fall in the price of good X, all else equal, causes (A) an increase in demand for X. (B) a decrease in demand for X. (C) an increase in the quantity demanded of X (a movement along the curve). (D) a leftward shift of the demand curve. (E) no change in quantity demanded.Show worked answer →
The answer is (C). A change in a good's own price is a movement along the demand curve, a change in quantity demanded, not a shift. A lower price raises quantity demanded.
(A), (B), and (D) describe shifts of the whole curve, which only non-price determinants cause. (E) contradicts the law of demand.
AP 2021 (style)3 marksFree response (short). (a) State the law of demand. (b) Identify two determinants that would shift the demand curve for coffee to the right. (c) Explain the difference between a change in demand and a change in quantity demanded.Show worked answer →
A three-point short FRQ.
(a) Law of demand (1 point): as the price of a good rises, the quantity demanded falls, all else equal (an inverse relationship).
(b) Determinants (1 point): any two of, for example, a rise in consumer income (if coffee is normal), a rise in the price of a substitute such as tea, an increase in the number of buyers, or a stronger preference for coffee.
(c) Difference (1 point): a change in quantity demanded is a movement along the curve caused by the good's own price; a change in demand is a shift of the whole curve caused by a non-price determinant.
Related dot points
- Topic 2.2 Supply: state the law of supply, distinguish a change in quantity supplied from a change in supply, and identify the determinants that shift the supply curve.
A focused answer to AP Microeconomics Topic 2.2, covering the law of supply, the difference between a movement along and a shift of the supply curve, the determinants of supply, and why the curve slopes upward, with worked exam-style questions.
- Topic 2.6 Market Equilibrium and Consumer and Producer Surplus: find equilibrium price and quantity, identify consumer and producer surplus on a graph, and explain why the competitive equilibrium maximizes total surplus (allocative efficiency).
A focused answer to AP Microeconomics Topic 2.6, covering how supply and demand determine equilibrium price and quantity, the measurement of consumer and producer surplus, total surplus, and why the competitive equilibrium is allocatively efficient, with worked exam-style questions.
- Topic 2.3 Price Elasticity of Demand: calculate price elasticity of demand using the midpoint formula, classify demand as elastic, inelastic, or unit elastic, apply the total revenue test, and identify the determinants of elasticity.
A focused answer to AP Microeconomics Topic 2.3, covering the price elasticity of demand, the midpoint formula, elastic versus inelastic versus unit elastic demand, the total revenue test, perfectly elastic and inelastic cases, and the determinants of elasticity, with worked exam-style questions.
- Topic 2.7 Market Disequilibrium and Changes in Equilibrium: explain how shortages and surpluses arise and self-correct, predict the new equilibrium after a single shift, and handle the indeterminate cases of a double shift.
A focused answer to AP Microeconomics Topic 2.7, covering shortages and surpluses, how price adjusts to clear a market, the four single-shift outcomes, and the indeterminate results of a double shift, with worked exam-style questions.
- Topic 1.6 Marginal Analysis and Consumer Choice: explain diminishing marginal utility, and use the utility-maximizing rule (equal marginal utility per dollar) to find the consumption bundle that maximizes total utility given a budget.
A focused answer to AP Microeconomics Topic 1.6, covering total and marginal utility, the law of diminishing marginal utility, and the utility-maximizing rule that equalises marginal utility per dollar across goods, with worked exam-style questions.
Sources & how we know this
- AP Microeconomics Course and Exam Description — College Board (2023)