income elasticity of demand

When people get richer, they don't just buy more of everything in equal measure — they buy a lot more of some things, slightly more of others, and actually less of a few. Income elasticity of demand measures this: how much the quantity demanded of a good changes when buyers' incomes change. It captures how a product's fortunes ride the rising or falling tide of household budgets, which is why it matters enormously to anyone planning what to sell as a country grows.

It's the percentage change in quantity demanded divided by the percentage change in income. The sign and size tell a story. If the number is positive, the good is normal — people buy more of it as they earn more. Among normal goods, if income elasticity is greater than 1 (a 10 percent income rise lifts demand by, say, 25 percent), it's a luxury, whose demand grows faster than income — restaurant meals, holidays, designer clothes. If it's between 0 and 1, it's a necessity, whose demand grows but more slowly than income — bread, basic clothing. And if the number is negative, the good is inferior: as income rises, people buy less of it, trading up to something better — think bus travel giving way to cars, or instant noodles giving way to fresh food.

Businesses and governments lean on income elasticity to see the future. A firm selling luxuries booms in good times and suffers badly in recessions, while a seller of necessities is steadier through the cycle. As whole economies grow, demand shifts predictably away from inferior and basic goods toward luxuries and services — which is part of why rich countries spend so much more on travel, dining, and entertainment than poor ones. The usual caveat applies: whether a good is a luxury, necessity, or inferior depends on the person and the income level — a bicycle is a luxury for a poor household and may be inferior for a rich one.