Market Equilibrium and Market Adjustment

The Competitive-Market Setting

The basic supply-and-demand model works best for a competitive market. Such a market has many buyers and sellers, no single participant controls the market price, the goods are reasonably comparable, and people can respond when prices change.

Competition does not require every real market to be perfect. The model is a useful starting point when individual buyers and sellers must take account of the market price instead of simply choosing it. Later chapters study monopoly, product differences, government rules, and other settings where the basic model needs to be changed or expanded.

The market for one patented medicine does not fit this model as well as the market for wheat sold by many farms. A negotiation over one used house may also depend on bargaining between two parties. Choosing the model requires judgment about the market being studied.

For now, assume a competitive market where prices can adjust. We begin with a situation in which buyers’ and sellers’ plans fit together.

Equilibrium: Where Plans Fit Together

At every possible price, the demand curve shows quantity demanded and the supply curve shows quantity supplied. Most prices produce different buyer and seller plans. One price makes them match.

Market equilibrium occurs when quantity demanded equals quantity supplied. The equilibrium price, written \(P^*\), is the price at which the plans match. The equilibrium quantity, written \(Q^*\), is the amount buyers plan to buy and sellers plan to sell at that price.

Finding the intersection may seem almost too easy: two lines cross, so X marks the spot. But what makes that point economically special? It is the only price at which quantity demanded equals quantity supplied. At every other price, buyers’ and sellers’ plans conflict. Because the equilibrium price makes those plans fit together, it is also called the market-clearing price.

The snow-shovel schedule gives us a numerical example before we turn to the graph.

The numerical equilibrium price is therefore $30, and the equilibrium quantity is 60 shovels per day. In symbols, \(P^*=\$30\) and \(Q^*=60\).

The word equilibrium can sound as though nothing ever changes. That is not what it means. The weather can change, technology can improve, and buyer preferences can shift. Equilibrium describes a market situation in which the current plans fit together. When an outside event changes those plans, the equilibrium changes too.

Market clearing does not mean everyone receives everything desired. Buyers who are unwilling or unable to pay the equilibrium price do not purchase the good, and sellers who require a higher price do not sell it. It means that the quantities planned at the current price are equal.

To understand why the market tends toward the intersection, consider prices away from it.

A Low Price Creates A Shortage

Suppose the price is $25, which is below the $30 equilibrium price. The schedule shows that buyers want 80 shovels per day while sellers provide only 40. The result is a shortage:

\[ Q_d - Q_s = 80 - 40 = 40. \]

The shortage is not just a gap on paper. Some buyers arrive after the shovels are gone. Others call several stores, wait in line, travel farther, or offer more money. Sellers see that the available units can be sold easily and have an incentive to raise prices, bring inventory out sooner, order more, or expand future production.

As price rises, buyers move upward along the demand curve and reduce quantity demanded. Sellers move upward along the supply curve and increase quantity supplied. The shortage narrows until the two quantities match.

Every good is scarce, including goods that are easy to find on store shelves. A shortage depends on the price. A shovel market can have no shortage at one price and a shortage at a lower price.

A High Price Creates A Surplus

Now suppose the price is $35, which is above equilibrium. Sellers plan to offer 80 shovels per day while buyers plan to purchase only 40. The result is a surplus:

\[ Q_s - Q_d = 80 - 40 = 40. \]

Unsold goods give sellers a reason to adjust. A store may advertise a sale, bargain with buyers, reduce future orders, store inventory, or leave the market. As price falls, buyers move down along demand and purchase more, while sellers move down along supply and offer less. The surplus narrows.

The law of supply and demand summarizes both cases. When a competitive market is out of equilibrium, price tends to move in the direction that brings quantity demanded and quantity supplied together.

The word tends matters here. Adjustment takes time. Posted prices may change slowly. Contracts may hold prices fixed. Sellers may run out of inventory before new production arrives. Buyers and sellers may also adjust through waiting, search, quality, timing, entry, and exit. The model identifies the direction of pressure; it does not claim that every market reaches equilibrium instantly or without cost.

The shortage and surplus diagrams held demand and supply fixed. We now return to the snowstorm, where an outside event changes one of the curves.

Analyzing A Market Change

When market conditions change, do not guess what happens to price and quantity. Follow a repeatable sequence.

Economists sometimes call this before-and-after method comparative statics. The technical name is less important than the reasoning. For the first examples, we will also explain what happens between the old equilibrium and the new one.

One rule deserves special attention: when one curve shifts, the resulting price change causes movement along the other curve. The other curve does not shift. In each of the next four figures, notice that both equilibrium points lie on the unchanged curve. Tracing between those points shows the movement along it.

Changes In Supply

A supply shift changes sellers’ plans. Buyers then respond to the resulting price change by moving along the existing demand curve.

When Demand And Supply Both Shift

Sometimes one event changes both curves, or two events occur at once. The safest method is to analyze each shift separately before combining the predictions.

Suppose a major event brings thousands of visitors to a city just as several hotels close for repairs.

• More visitors increase demand for hotel rooms. By itself, this raises equilibrium price and quantity.
• Repair closures decrease the supply of hotel rooms. By itself, this raises equilibrium price and lowers quantity.

Both shifts push price upward, so the price of hotel rooms rises. The demand shift pushes quantity up, while the supply shift pushes quantity down. Without knowing which shift is larger, the change in quantity is ambiguous: the model does not give a definite direction.

Do not settle the question by drawing one combined graph with convenient shift sizes. The drawing would silently assume which shift is larger. When the sizes are unknown, separate reasoning protects us from claiming more than the model shows.

This is not a failure of economics. “Ambiguous without more information” is often the correct answer. The model tells us exactly which additional fact we need: the relative sizes of the two shifts.

The Supply and Demand Shift Practice applet provides additional single- and simultaneous-shift cases. For each one, explain the arrows before accepting the result.

Prices Coordinate Decentralized Choices

The snow-shovel example began with thousands of separate plans. Buyers knew their own needs, travel options, budgets, and weather concerns. Store managers knew their inventories, suppliers, and local customers. Manufacturers knew something about production capacity and input costs. No one knew all of it.

A rising shovel price gives those people a reason to adjust even if they do not know every cause of the shortage. Buyers conserve, search for substitutes, or decide a shovel is not worth the higher price. Sellers release inventory, arrange deliveries, or order more. Producers consider extra shifts or greater future output.

This is the core of the price system as a method of coordination. A central planner trying to produce the same result would need timely information about what buyers value, what sellers can produce, what resources cost in other uses, and how all of those facts are changing. In a competitive market, price changes allow people to respond using the local knowledge they already possess.

Friedrich Hayek emphasized this knowledge problem: much of the information needed for economic coordination is spread across many people and cannot simply be handed to one decision-maker in complete form.¹ Chapter 12 will examine that argument more fully. Here, the lesson is narrower. Prices are signals that help people revise their plans without first learning the entire story behind a market change.

This coordination is powerful, but not magical. Prices may be slow to change. People may lack information, face market power, or be unable to respond quickly. A high price can also create serious hardship. The model explains adjustment; it does not settle every question about the outcome.

Markets Need Rules

Supply and demand curves do not operate by themselves. People must know what can be sold, who owns it, whether payment will be honored, and what happens if a promise is broken.

Property rights identify who may use, transfer, or exclude others from an asset. Contract enforcement makes agreements more dependable. Informal rules, reputation, and basic trust also reduce uncertainty. Douglass North described institutions broadly as the formal rules and informal limits that structure interaction.²

If ownership is unclear, a buyer may fear purchasing stolen land or goods. If sellers can accept payment and refuse delivery without consequence, buyers hesitate to pay. If corruption makes recorded claims unreliable, a posted price cannot solve the deeper problem. Markets need supporting institutions before price adjustment can coordinate much exchange.

These institutions do not have to take exactly the same form everywhere. Rights may be individual, shared, customary, or communal. What matters is that people understand the claims well enough to make plans and have credible ways to resolve disputes.

The digital-land example reveals why “use markets” is not a complete policy instruction. Before exchange can work well, people need an institutional answer to what is owned, whose claims count, and how agreements will be enforced.

What Equilibrium Does And Does Not Tell Us

Equilibrium answers a coordination question: at what price do planned purchases equal planned sales? It does not yet answer whether the outcome creates the greatest possible gains, whether those gains are distributed fairly, or whether important costs fall on people outside the market.

A market can be in equilibrium even when one seller has market power. A polluting market can be in equilibrium even when nearby residents bear costs that buyers and sellers ignore. A housing market can be in equilibrium while many families cannot afford the available homes.

Chapter 6 will explain when a competitive equilibrium provides a useful standard for economic efficiency. Later chapters examine market power, external costs, information problems, and government policies. Keeping these questions separate prevents a common mistake: treating the fact that a market has settled at a price as proof that every important problem has been solved.

Before turning to welfare, Chapter 5 asks how strongly buyers and sellers respond. Supply and demand tell us the direction in which price and quantity move. Elasticity helps measure the size of those responses.

Chapter Study Map

Review Questions

1. What features make the basic supply-and-demand model a good fit for a market?
2. Define market equilibrium, equilibrium price, and equilibrium quantity.
3. What does it mean for a market to clear?
4. Why does equilibrium not prove that an outcome is fair or efficient?
5. Define shortage and write its quantity relationship.
6. Why does a shortage create upward pressure on price?
7. Why is a shortage different from scarcity?
8. Define surplus and write its quantity relationship.
9. Why does a surplus create downward pressure on price?
10. State the law of supply and demand.
11. List the four steps for analyzing a market change.
12. After demand increases, why does supply not also shift merely because sellers offer more?
13. What happens to equilibrium price and quantity when demand decreases?
14. What happens to equilibrium price and quantity when supply increases?
15. Trace a supply decrease from the initial event through the old-price imbalance to the new equilibrium.
16. How should simultaneous shifts be analyzed when their sizes are unknown?
17. What does it mean for two effects to reinforce or offset one another?
18. How do prices help coordinate people who possess different information?
19. Why do markets require property rights, contract enforcement, and trust?
20. Why can a digital record help establish a market without being sufficient to create an enforceable property right?

Economic Reasoning Questions

1. A market price is below equilibrium. State the relationship between \(Q_d\) and \(Q_s\), then name three ways buyers or sellers might adjust.
2. A store has unsold inventory at the current price. Use demand and supply to explain the direction of price pressure.
3. A medical study increases demand for blueberries. Trace the complete adjustment from the old equilibrium price to the new equilibrium.
4. A cheaper manufacturing method increases the supply of batteries. Explain why the resulting increase in quantity demanded is movement along demand rather than a demand shift.
5. A disease destroys part of the orange crop. Predict the changes in equilibrium price and quantity and explain each step.
6. Household income falls and restaurant meals are a normal good. Identify the curve and direction, then predict the new equilibrium price and quantity.
7. Demand for electric vehicles rises while battery technology also lowers production costs. Which equilibrium variable has a clear direction, and which is ambiguous? Explain without drawing one combined graph.
8. Demand for hotel rooms rises during a major event while several hotels close for repairs. Analyze the two shifts separately and combine the results.
9. A friend says, “Demand increased, so supply increased too.” Rewrite the statement using the correct distinction between a shift and movement along a curve.
10. Give an example of a price that adjusts slowly because of contracts, posted prices, inventories, or another barrier. Explain which other adjustment margins might appear first.
11. A digital map identifies the physical boundaries of a farm, but two families claim the right to use it. Explain why mapping and record keeping do not settle the economic problem.
12. Describe one market that fits the competitive model reasonably well and one that does not. Identify the feature responsible for the difference.

Source Notes