A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Income elasticity of demand measures how quantity demanded responds to changes in consumer income, expressed as the percentage change in quantity divided by the percentage change in income. Normal goods have positive income elasticity (demand rises with income), while inferior goods have negative income elasticity (demand falls as income rises). This distinction explains consumption patterns across different income levels.
From your study of price elasticity, you already know that elasticity quantifies responsiveness — how much quantity changes in percentage terms when some other variable changes by one percent. Income elasticity of demand applies the same logic to income rather than price: it equals the percentage change in quantity demanded divided by the percentage change in income. But unlike price elasticity, which is almost always negative, income elasticity can be positive or negative, and its sign reveals something fundamental about how consumers categorize a good.
A normal good has positive income elasticity: as income rises, consumers buy more of it. Most goods fit this description — restaurant meals, clothing, electronics. A subset called luxury goods (or superior goods) have income elasticity greater than 1, meaning the percentage increase in quantity exceeds the percentage income increase. A 10% income rise might produce a 20% increase in demand for foreign travel. By contrast, an inferior good has negative income elasticity: as income rises, consumers actually buy less of it. Classic examples include generic staples such as instant noodles, bus rides, or low-quality cuts of meat — consumers substitute toward more preferred alternatives once they can afford to.
The intuition behind inferior goods is not that the product is inherently bad — it's that it was the best available option when money was tight. Bus rides are genuinely useful; they simply get replaced by car trips as income grows. This is why "inferiority" is income-conditional, not absolute. A good that is inferior for middle-class households may be a normal good for low-income households still rising out of poverty. This context-dependence makes income elasticity a key tool for market segmentation and for forecasting how demand shifts as economies develop.
The income elasticity framework underlies Engel curves — graphs of household expenditure on a good as a function of income. For normal goods, the Engel curve slopes upward. For inferior goods, it eventually bends backward. For luxuries, it rises steeper than proportionally. These curves are the empirical backbone of household budget studies and development economics, connecting directly to your earlier elasticity work by extending the same percentage-ratio logic from prices to income as the driving variable.