Income elasticity of demand (YED) measures how quantity demanded responds to a change in consumer income, calculated as the percentage change in quantity demanded divided by the percentage change in income. A positive YED means the good is normal; a negative YED means it is inferior.
Income elasticity of demand answers a simple question. When consumers get richer (or poorer), how much does the quantity demanded of a good change? You calculate it as the percentage change in quantity demanded divided by the percentage change in consumer income. The price never appears in this formula, which is exactly what makes it different from regular price elasticity.
The number itself matters less than its sign. If income elasticity is positive, people buy more of the good as income rises, so it's a normal good. If it's negative, people buy less as income rises (think ramen noodles or bus rides once you can afford better), so it's an inferior good. Within normal goods, you can go one step further. A YED between 0 and 1 suggests a necessity, since demand grows slower than income. A YED greater than 1 suggests a luxury, since demand grows faster than income. This is the income-determinant version of the elasticity toolkit you build in Topic 2.5.
This term lives in Unit 2: Supply and Demand, Topic 2.5 (Other Elasticities), and it hits all three learning objectives there. You need to define it (AP Micro 2.5.A), explain what the result means for consumer behavior (AP Micro 2.5.B), and calculate it from a table or scenario (AP Micro 2.5.C). The CED is explicit that elasticity can be measured for any determinant of demand, not just price, and income is the headline example. It also closes a loop from earlier in Unit 2. Back in Topic 2.1, you learned that income is a demand shifter and that goods are either normal or inferior. Income elasticity is how economists actually prove which one a good is, by putting a number on that shift.
Keep studying AP Microeconomics Unit 2
Normal Goods and Inferior Goods (Unit 2)
Income elasticity is the measuring stick for this classification. In Topic 2.1 you just memorize that income shifts demand; in Topic 2.5 you compute YED and let the sign decide. Positive means normal, negative means inferior. Same idea, now with a number attached.
Cross-Price Elasticity of Demand (Unit 2)
Its sibling formula in Topic 2.5. Both put quantity demanded of a good in the numerator, but the denominator changes. Cross-price uses the percentage change in another good's price (sign tells you substitutes vs. complements), while income elasticity uses the percentage change in income (sign tells you normal vs. inferior). Learn them as a matched pair.
Price Elasticity of Demand (Unit 2)
The original elasticity from Topics 2.3-2.4 measures responsiveness to the good's own price. Income elasticity reuses the exact same percentage-change logic but swaps the cause. One big difference matters here. With price elasticity you usually ignore the sign; with income elasticity, the sign IS the answer.
Determinants of Demand (Unit 2)
A change in income moves the entire demand curve, not a point along it. Income elasticity quantifies how big that shift is, which is why it connects directly to predicting changes in consumer expenditure and market outcomes when the economy booms or slumps.
Income elasticity shows up in multiple-choice in three flavors. First, straight definition questions, like identifying the term that describes how quantity demanded responds to an income change. Second, calculation questions where you plug numbers into %ΔQd ÷ %ΔIncome. For example, a 15% rise in income causing a 30% drop in quantity demanded gives a YED of -2, an inferior good. Third, interpretation questions where you're given a value like 0.5 and asked to predict what happens to expenditure when incomes fall (it falls, but proportionally less, since 0.5 marks a normal good and a necessity). Trickier stems combine it with other Unit 2 concepts, like asking how producer surplus changes when incomes rise but YED is negative (demand shifts left, so producer surplus shrinks). No released FRQ has leaned on this term verbatim, but FRQs regularly ask you to shift demand after an income change, and YED is the logic behind that shift. Always keep the negative sign here. It carries the answer.
Both are Topic 2.5 'other elasticities' with quantity demanded on top, so they blur together fast. The fix is to look at the denominator. Income elasticity divides by the percentage change in consumer income and sorts goods into normal (positive) or inferior (negative). Cross-price elasticity divides by the percentage change in another good's price and sorts pairs of goods into substitutes (positive) or complements (negative). If the question mentions two different goods, it's cross-price. If it mentions a raise, a recession, or a paycheck, it's income elasticity.
Income elasticity of demand equals the percentage change in quantity demanded divided by the percentage change in consumer income.
A positive income elasticity means the good is normal, and a negative income elasticity means the good is inferior. The sign is the whole point, so never drop it.
Among normal goods, a YED between 0 and 1 suggests a necessity, while a YED greater than 1 suggests a luxury whose demand grows faster than income.
Income elasticity describes a shift of the demand curve, since income is a determinant of demand, not the good's own price.
On the exam, you can be asked to define it (2.5.A), interpret what a given value means for spending (2.5.B), or calculate it from percentages in a scenario (2.5.C).
Quick check with real numbers: if income rises 15% and quantity demanded falls 30%, YED is -2 and the good is inferior.
It's the percentage change in quantity demanded divided by the percentage change in consumer income. In Topic 2.5, economists use it to classify goods as normal (positive YED) or inferior (negative YED).
No. A negative YED just means quantity demanded falls as income rises, which makes the good inferior, not unwanted. People still buy ramen and used clothes; they just buy less of them when their paychecks grow.
Income elasticity divides %ΔQd by the percentage change in consumer income and identifies normal vs. inferior goods. Cross-price elasticity divides %ΔQd of one good by the percentage change in another good's price and identifies substitutes vs. complements. Same numerator, different denominator, different classification.
Both are normal goods (positive YED), so look at the size. A YED between 0 and 1, like 0.5, means a necessity because demand changes proportionally less than income. A YED above 1 means a luxury because demand changes proportionally more than income.
No, and this is a classic exam trap. With price elasticity of demand you often use the absolute value, but with income elasticity the sign carries the answer. A YED of -2 means the good is inferior, and dropping the sign would change your entire conclusion.