Demand, Supply, and Market Equilibrium

How Demand Describes Buyers’ Decisions

Markets coordinate the plans of buyers and sellers through prices. Buyers decide how much they are willing and able to purchase, while sellers decide how much they are willing and able to offer. The central distinction is between an entire relationship and one quantity at one price.

Demand describes the complete relationship between price and consumers’ purchasing plans. A demand schedule lists prices and quantities in a table, and a demand curve displays the same relationship graphically. Quantity demanded is one specific amount at one particular price.

The law of demand states that, ceteris paribus, a higher price leads to a lower quantity demanded and a lower price leads to a higher quantity demanded. Thus, a typical demand curve slopes downward. Three ideas help explain this pattern:

• The substitution effect encourages consumers to choose relatively cheaper alternatives when a good becomes more expensive.

• The income effect means that a higher price reduces consumers’ purchasing power.

• Diminishing marginal benefit means that additional units often provide less additional satisfaction, so consumers generally buy more units only at lower prices.

A change in the good’s own price causes a movement along a curve. For example, if the price of apples falls, consumers move to a point with a higher quantity demanded on the same demand curve. This is not an increase in demand.

Takeaway: Demand is the whole price–purchase relationship; quantity demanded is one point on it, and the good’s own price causes movement along the demand curve.

What Shifts Demand

A demand curve shifts when consumers want to buy a different quantity at every possible price. A rightward shift represents an increase in demand; a leftward shift represents a decrease in demand.

Important determinants include:

• Tastes and preferences: Greater popularity increases demand.

• Number of buyers: A larger market increases demand.

• Income: Demand for a normal good rises when income rises. Demand for an inferior good falls when income rises.

• Prices of related goods: A higher price for a substitute can increase demand for the good being studied. A lower price for a complement can also increase demand.

• Expectations: If buyers expect higher future prices or income, they may purchase more now.

For example, suppose a popular actor releases a new film while movie-ticket prices remain unchanged. Consumers may want to see more movies at every ticket price, so the demand curve for movie tickets shifts right. If supply is unchanged, the new equilibrium price and quantity usually rise.

Use this test: if the event changes buyers’ willingness to purchase at every price, shift the demand curve. If it changes only the good’s own price, move along the existing curve.

Takeaway: Demand shifts come from nonprice determinants such as preferences, income, related-good prices, market size, and expectations.

How Supply Describes Sellers’ Decisions

Supply describes the complete relationship between price and producers’ selling plans. A supply schedule lists the relationship in a table, while a supply curve shows it graphically. Quantity supplied is one specific amount at one particular price.

The law of supply states that, holding other factors constant, a higher price leads to a higher quantity supplied and a lower price leads to a lower quantity supplied. A typical supply curve slopes upward because higher prices can make production more profitable and can encourage firms to enter or expand in the market.

A change in the good’s own price causes a movement along the supply curve. If the price of a product rises, quantity supplied increases along the existing curve. The supply curve itself shifts only when a nonprice determinant changes.

Key supply determinants include:

• Input prices, such as labor, materials, and energy

• Technology and production methods

• Natural conditions, such as weather or disease

• Taxes, regulations, and subsidies

• The number of sellers

• Producers’ expectations about future prices

For example, a new machine that lowers the cost of producing smartphones allows firms to supply more smartphones at every price. Supply shifts right. With demand unchanged, equilibrium price usually falls and equilibrium quantity rises.

Takeaway: Supply is the entire seller–price relationship; quantity supplied is one point on it, and nonprice production conditions shift the supply curve.

Finding Market Equilibrium

Equilibrium occurs where the plans of buyers and sellers match:

On a graph, equilibrium is the intersection of the demand curve and the supply curve. The equilibrium price is the price at that intersection, and the equilibrium quantity is the amount bought and sold there.

To find equilibrium from a table, locate the row where quantity demanded equals quantity supplied. For example, if at a price of $\$6$, quantity demanded is $50$ units and quantity supplied is $50$ units, then the equilibrium price is $\$6$ and the equilibrium quantity is $50$ units.

To find equilibrium from equations, set the two quantities equal. Suppose:

Set them equal and solve:

Substitute $P = 10$ into either equation:

The equilibrium price is $\$10$, and the equilibrium quantity is $50$ units.

Takeaway: Equilibrium is found by matching quantity demanded with quantity supplied, whether the information comes from a graph, table, or equations.

Diagnosing Shortages and Surpluses

A market is not at equilibrium when the current price causes buyers’ and sellers’ plans to differ.

A shortage occurs when quantity demanded exceeds quantity supplied:

Shortages occur at prices below equilibrium. For example, if quantity demanded is $70$ units and quantity supplied is $50$ units, the shortage is $20$ units. Buyers compete for limited goods, creating upward pressure on price.

A surplus occurs when quantity supplied exceeds quantity demanded:

Surpluses occur at prices above equilibrium. If quantity supplied is $70$ units and quantity demanded is $30$ units, the surplus is $40$ units. Unsold inventory creates downward pressure on price.

The relationships can be summarized as follows:

• Below equilibrium price: $Q_D > Q_S$, so there is a shortage and upward pressure on price.

• At equilibrium price: $Q_D = Q_S$, so there is no shortage or surplus pressure.

• Above equilibrium price: $Q_S > Q_D$, so there is a surplus and downward pressure on price.

Takeaway: Compare quantity demanded and quantity supplied directly, then calculate the difference using the quantity associated with the larger side minus the quantity associated with the smaller side.

Analyzing Market Changes Step by Step

A reliable market-graph analysis follows four steps.

1. Identify the market. State the good or service being analyzed.

2. Identify the affected curve. Decide whether the event changes buyers’ willingness to purchase or sellers’ willingness to produce.

3. Determine the direction of the shift. A demand increase shifts demand right; a demand decrease shifts demand left. A supply increase shifts supply right; a supply decrease shifts supply left.

4. Compare the new equilibrium with the original equilibrium. State what happens to equilibrium price and equilibrium quantity.

Use the following examples:

• A health study reports that blueberries improve memory. Demand for blueberries shifts right, so equilibrium price and quantity rise if supply is unchanged.

• A drought destroys part of the wheat crop. Supply shifts left, so equilibrium price rises and equilibrium quantity falls if demand is unchanged.

• The price of movie tickets rises. Because movie tickets and popcorn are complements, fewer movie tickets may be purchased, reducing demand for popcorn. This shifts the popcorn demand curve left; it is not a movement along the popcorn demand curve caused by the price of popcorn.

• A new production process lowers the cost of solar panels. Supply shifts right, so equilibrium price falls and equilibrium quantity rises if demand is unchanged.

When drawing a graph, label the vertical axis Price and the horizontal axis Quantity. Draw a downward-sloping demand curve labeled $D$, an upward-sloping supply curve labeled $S$, and label the original and new equilibria. A price line below equilibrium illustrates a shortage; a price line above equilibrium illustrates a surplus.

Takeaway: Always identify the market, affected curve, shift direction, and changes in equilibrium price and quantity before explaining a scenario.