Supply and demand

As price rises, buyers want less and sellers offer more. The price where the two quantities match is the one that clears the market. Marshall's point was that neither side sets the price alone: "We might as reasonably dispute whether it is the upper or the under blade of a pair of scissors that cuts a piece of paper, as whether value is governed by utility or cost of production." A shock to either side moves the price. A cap held below the clearing price leaves buyers wanting more than sellers will supply, a shortage rationed by queues, luck or connections. A floor above it leaves a glut.

It holds because price does two jobs at once: it tells people what is scarce, and it rewards them for responding. A high price pulls in new sellers and pushes marginal buyers out, and both responses shrink the gap. Suppress the price and both responses stop while the underlying scarcity remains, so the adjustment moves to quantity. The usual pattern is slow, which is why a price cap can look successful for years: existing supply does not vanish on day one, but new supply is not built and old supply leaks away. The San Francisco rent control study below shows this time lag in numbers.

The curves are not observable, so the model predicts direction well and magnitude poorly; how much quantity moves depends on elasticities that have to be estimated. Markets with a few sellers, with buyers who know less than sellers, or with costs that land on third parties do not clear at the socially right price, and that is where regulation has a case. And demand is willingness plus ability to pay, not need, so a market can clear while people go without. None of this rescues a price cap from the quantity response; it only says that the right fix is often on the supply side, such as building more, rather than on the price.