Demand, Supply, and Market Equilibrium

Supply and demand is the core model of market economics. It explains how the choices of buyers and sellers interact to determine the prices and quantities we observe every day, and how markets coordinate decisions without anyone being in charge. This chapter develops that model and puts it to work.

Markets

A market is any arrangement that brings buyers and sellers together. Markets can be physical, like a farmers' market, or virtual, like an online marketplace. In a competitive market there are many buyers and many sellers, so no single participant can dictate the price. Each takes the price as given and decides only how much to buy or sell. The model of demand and supply describes how that price comes to be.

Demand

Demand is the relationship between the price of a good and the quantity buyers are willing and able to purchase, holding everything else constant.

The law of demand states that, all else equal, as the price of a good rises, the quantity demanded falls — and as the price falls, the quantity demanded rises. Price and quantity demanded move in opposite directions. Two forces drive this: as a good becomes more expensive, buyers substitute toward cheaper alternatives (the substitution effect), and their fixed income buys less than before (the income effect).

Plotting price against quantity demanded gives the downward-sloping demand curve.

What Shifts Demand

A change in price moves us along a fixed demand curve. A change in any other factor shifts the entire curve. The main shifters are:

• Income — For a normal good, higher income raises demand. For an inferior good, higher income lowers demand.
• Prices of related goods — A rise in the price of a substitute raises demand for the good; a rise in the price of a complement lowers it.
• Tastes and preferences — Changing fashions, health information, or advertising shift demand.
• Expectations — If buyers expect higher prices later, they buy more now.
• Number of buyers — More buyers in the market increases demand.

Supply

Supply is the relationship between the price of a good and the quantity sellers are willing and able to produce.

The law of supply states that, all else equal, as the price rises, the quantity supplied rises. Higher prices make production more profitable and cover the rising costs of expanding output, so firms offer more. This gives the upward-sloping supply curve.

What Shifts Supply

As with demand, a price change moves us along the curve, while other factors shift it:

• Input prices — Cheaper labor, materials, or energy increase supply.
• Technology — Productivity improvements lower costs and increase supply.
• Expectations — Expected future prices can shift current supply.
• Number of sellers — More firms in the market increases supply.
• Taxes and subsidies — Taxes reduce supply; subsidies increase it.

Market Equilibrium

Equilibrium occurs at the price where the quantity demanded equals the quantity supplied — where the demand and supply curves cross. At this equilibrium price, the plans of buyers and sellers are consistent: everyone who wants to buy at that price can, and everyone who wants to sell at that price can.

What forces the market toward equilibrium?

• A surplus exists when the price is above equilibrium: quantity supplied exceeds quantity demanded. Unsold goods pile up, and sellers cut prices to clear them, pushing the price down.
• A shortage exists when the price is below equilibrium: quantity demanded exceeds quantity supplied. Buyers compete for scarce goods and bid the price up.

Only at the equilibrium price are there no leftover goods and no unmet buyers, so there is no pressure to change. This self-correcting tendency is the price mechanism at work.

Changes in Equilibrium

When a curve shifts, the equilibrium moves. A useful three-step method: (1) decide whether the event shifts demand, supply, or both; (2) decide the direction of the shift; (3) read the new equilibrium. For example, a heat wave raises demand for air conditioners, shifting demand right and raising both the equilibrium price and quantity. A new manufacturing technique lowers production costs, shifting supply right and lowering price while raising quantity. When both curves shift at once, we can usually predict the change in either price or quantity but not both without knowing the relative sizes of the shifts.

Efficiency, Surplus, and Price Controls

Competitive markets tend to be efficient: they channel goods to the buyers who value them most and direct production to the sellers who can produce at lowest cost. The gains buyers receive above what they pay (consumer surplus) and the gains sellers receive above their costs (producer surplus) are together maximized at the competitive equilibrium.

This benchmark helps us see the cost of interfering with prices. A price ceiling set below equilibrium (such as rent control) creates a persistent shortage; a price floor set above equilibrium (such as a minimum wage or agricultural support price) creates a persistent surplus. In both cases the market is pushed away from the efficient equilibrium, illustrating why prices are such a powerful coordinating device.

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Next chapter: markets are powerful but not perfect. We turn next to the situations in which they fail to allocate resources efficiently.

Further Listening & Reading

• 🎥 Supply and Demand Terminology (MRU) — the core vocabulary of the model.
• 🎥 The Equilibrium Price and Quantity (MRU) — a worked example of finding equilibrium.
• 🎥 Equilibrium, Surplus, and Consumer/Producer Surplus (MRU) — why gains from trade are maximized at equilibrium.