law of demand

The law of demand is the most reliable habit of buyers, and you already obey it without thinking: when something costs more, you tend to buy less of it; when it costs less, you buy more. Cinema tickets at half price on a Tuesday fill more seats; a coffee that creeps up to five dollars sees you skip a cup or two a week. The relationship runs the opposite way to the price — up here, down there — which is why economists call it an inverse, or negative, relationship between price and the quantity demanded.

Two quiet forces drive it. First, the substitution effect: when steak gets pricier, chicken suddenly looks like a better deal, so people swap toward the cheaper alternative. Second, the income effect: a higher price makes your money buy less overall, so you feel a touch poorer and trim your buying. Notice the careful wording — the law holds "all else equal" (ceteris paribus). It is only about price changing while tastes, incomes, and other prices stay put. If a price rises but the law seems broken because sales also rose, something else moved too — usually demand itself shifted.

The law of demand is why the demand curve slopes downward, and that downward slope underlies almost everything in market analysis. There are famous-but-rare apparent exceptions: a so-called Giffen good (a staple so essential to the very poor that a price rise, by eating their budget, forces them to buy even more of it) and Veblen goods (luxuries bought partly because they are expensive). These are genuinely unusual edge cases, hard to find in real data — the law itself is one of the sturdiest regularities in all of economics.