Supply and Demand

Demand is the willingness and ability of consumers to purchase various quantities of a good at various prices, over a given period. Notice both words matter: wanting a yacht isn't demand if you can't afford one, and being able to afford broccoli isn't demand if you have no interest in eating it.

The law of demand captures an almost universal regularity: as price rises, quantity demanded falls, holding everything else constant. Three forces explain this inverse relationship. The substitution effect — as a good gets pricier, consumers switch toward now-relatively-cheaper alternatives. The income effect — a higher price shrinks your purchasing power, so your same paycheck buys less. And diminishing marginal utility — each additional unit delivers less satisfaction than the last, so consumers are only willing to buy more units at progressively lower prices.

It is critical to distinguish a movement along the curve (caused only by a change in the good's own price) from a shift of the entire curve (caused by anything else — income, the price of related goods, tastes, the number of buyers, or expectations).

Supply is the willingness and ability of producers to sell various quantities of a good at various prices. The law of supply states this relationship is direct: higher prices make production more profitable at the margin, so firms bring more output to market — and, over time, new firms are drawn into a profitable industry.

Just as with demand, changes in the good's own price only move you along a fixed supply curve. A genuine shift comes from the determinants: input prices, technology, the number of sellers, taxes and subsidies, and producer expectations. Notice input prices and taxes work the same direction — both raise the cost of producing each unit, discouraging supply — while technological improvement and subsidies do the opposite.

Two demand curves can look similarly steep on a graph and still behave completely differently as price changes — which is exactly why economists don't rely on slope alone. Price elasticity of demand asks a sharper question: for a given percentage change in price, what percentage change in quantity demanded results?

The most reliable shortcut on the exam is the total revenue test, since it needs no formula at all: calculate TR = P × Q at each price and watch what happens as price changes.

The determinants of PED all boil down to one idea: how easily can a consumer walk away? Necessities (insulin) have few good alternatives, so demand is inelastic; luxuries (a second vacation home) are easy to skip, so demand is elastic. Ready substitutes (name-brand vs. generic aspirin) make demand elastic; no substitutes (addictive goods) make it inelastic. More time to shop around increases elasticity; an emergency purchase decreases it. A purchase that eats a large share of your budget (a car) gets more elastic; a trivial one (a pack of gum) stays inelastic.

The single biggest determinant of supply elasticity is the amount of time producers have to adjust. Given a year, an apple farmer can plant more trees; given an afternoon, they can only pick what's already ripe. This is why economists often distinguish the momentary run (fixed supply — perfectly inelastic) from the short and long run (increasingly elastic as firms can add machinery, then build entirely new factories, then let new competitors enter).

Income elasticity of demand measures how quantity demanded responds to a change in consumer income:

Cross-price elasticity of demand measures how the quantity demanded of good X responds to a change in the price of a related good Y:

At the equilibrium price and quantity, every mutually beneficial trade that could happen, does happen — and no trade that would make anyone worse off is forced through. That's what economists mean by allocative efficiency.

Consumer surplus is the gap between what a buyer was willing to pay and what they actually paid — the area below the demand curve and above price. Producer surplus is the mirror image: the gap between what a seller actually received and the minimum they'd have accepted — the area above supply and below price.

A shortage (excess demand) exists when Qd > Qs at the current price — this happens whenever price sits below equilibrium. A surplus (excess supply) exists when Qs > Qd — whenever price sits above equilibrium. Left alone, market forces close both gaps: sellers facing unsold inventory cut prices; buyers facing empty shelves bid prices up. Either way, the market self-corrects back to Qe, Pe.

Use the tool below to see how each of the four possible shifts moves the equilibrium.

A price ceiling is a legal maximum. It only binds — has any effect at all — when set below equilibrium, where it creates a persistent shortage (think: rent control). A price floor is a legal minimum; it only binds when set above equilibrium, creating a persistent surplus (think: minimum wage or agricultural price supports).

An excise tax drives a wedge between the price buyers pay and the price sellers receive, shrinking quantity below the efficient level. A subsidy does the reverse — it pushes quantity above the efficient level. Either way, whenever quantity is pushed off Qe, deadweight loss appears: it's the triangle of surplus that used to exist and now simply doesn't, because those units are no longer traded.

Compare the domestic no-trade (autarky) equilibrium price to the world price. If the world price is higher than the domestic price, domestic producers have an incentive to sell abroad — the country exports, producer surplus rises, and consumer surplus falls as the domestic price is bid up to match the world price. If the world price is lower, domestic consumers gain access to cheaper goods abroad — the country imports, consumer surplus rises, and producer surplus falls.