The Joy of Economics:  Making Sense out of Life
 Robert J. Stonebraker, Winthrop University
 

Demand and Supply: An Overview

 

 

          If you can't pay for a thing, don't buy it. If you can't get paid for it, don't sell it.
                                                                                    ...Benjamin Franklin

 

          In most economies, prices determine what, how, and for whom goods are produced.  Firms will produce whatever goods can be sold at a profitable price and will choose resources on the basis of what prices must be paid to employ them.  The consumers willing to pay the price will be the ones for whom goods are produced.  Given the importance of prices, we need to know how they are determined.  Why are some high and others low?  Why do some rise while others fall?  It’s all demand and supply.

Demand

      Economists use the term demand to indicate willingness to buy.  While the demand for a product depends upon many different factors, one obvious determinant is price.  Price has a negative effect on willingness to buy.  All else equal, as the price of a product falls, the quantity demanded will rise.

          It is often useful to illustrate these relationships graphically.   We draw a demand curve to show the relationship between the price of the good and the quantity that consumers are willing to buy. For example, suppose people are willing to buy 20 pounds of strawberries at a price of $2, but are willing to buy 30 pounds if the price falls to $1.  The relevant demand curve is drawn below.   Because a change in price will always push the quantity demanded in the opposite direction, all demand curves will have a negative slope.

 

 

          The price of good is not the only factor that impacts willingness to buy.  Other important factors include:

 

            1.         consumer tastes and preferences (or the perceived value of the product)

            2.         consumer income

            3.         prices of substitute and complementary goods

Note: Substitute goods can be used in place of one another (like corn and green beans); complementary goods are used together (like cars and gasoline).

            4.         consumer expectations

            5.         number of potential consumers in the market or population

 

          These factors often are called demand shifters.  If any of them changes, the entire demand curve will move or shift.   For example, suppose new research indicates that eating strawberries will make you more attractive to members of the opposite sex.  Will consumers react to this news?  Of course.  It will change the perceived value of strawberries and increase the quantity of strawberries people are willing to buy at every price.  Using the above numbers, suppose this new research triples the quantities people are willing to buy at each price.  In other words, consumers are now willing to buy 60 pounds (rather than 20) at the $2 price and 90 pounds (rather than 30) at the $1 price.  The demand curve will shift to the right.  As shown below, the original demand curve (D1) has shifted to become a new demand curve (D2).

          Changes in other demand shifters can have similar effects.  The following will cause the demand curve to shift to the right (i.e. larger quantities will be demanded at each price).

            1.         an increase in perceived value of the good

            2.         an increase in consumer income

Note: This is true only for normal goods. Increased income will lower the demand for what we call inferior goods.  Inferior goods are those we would buy less of if our income rose.  Low-quality (but inexpensive) cuts of meat might be an example.  Can you think of others?

            3.         an increase in the price of a substitute good

            4.         a decrease in the price of a complementary good

            5.         an increase in the number of potential consumers in the market

 

          Opposite changes in the above factors will cause the demand curve to shift down or to the left (i.e. less will be demanded at each price than before).

          Be careful not to confuse a movement along a demand curve with a shift to a new demand curve. It is a common mistake.  Remember that the demand curve already shows the negative effect of price on quantity demanded.  If the price of the good changes, we simply move to a new point along the existing demand curve.  We call this a change in quantity demanded.  If there is a change in a factor influencing demand, other than the price of the good, the entire demand curve moves or shifts.  We term this a change in demand.

Supply

           Supply indicates willingness to sell.  Like demand, the supply of a product depends upon many different factors and, like demand, one obvious factor is price.  However, while high prices discourage buyers, they are likely to encourage sellers.  Price has a positive effect on willingness to sell.  All else equal, as the price of a product rises, the quantity firms are willing to sell will rise as well. 

           A supply curve illustrates the relationship between the price of the good and the quantity that firms are willing to sell.  For example, firms might be willing to sell 600 bushels of wheat at a price of $3, but be willing to sell 900 bushels at a price of $4.  The relevant supply curve is drawn below.   Because a change in price will push the quantity supplied in the same direction, supply curves will have a positive slope.

          Price is not the only factor that impacts willingness to sell. Other important factors include:

            1.         cost of inputs

            2.         available technology

            3.         profitability of other goods

            4.         number of sellers in the market

            5.         producer expectations

 

          These factors are supply shifters.  If any of them changes, the entire supply curve will move or shift.   For example, suppose new technology lowers the cost of growing wheat.  How will farmers react?   The new technology increases the profitability, and therefore the willingness to sell at every price.  Suppose the new technology doubles the quantities people are willing to sell at each price.  In other words, firms are now willing to sell 1200 bushels (rather than 600) at the $3 price and 1800 pounds (rather than 900) at the $4 price.  The supply curve will shift to the right.  As shown below, the original supply curve (S1) has shifted to become a new supply curve (S2).

          Changes in other supply shifters can have similar effects.  The following will cause the supply curve to shift to the right (i.e. larger quantities will be supplied at each price)

            1.         a decrease in the price of inputs

            2.         a increase in technological efficiency

            3.         a decrease in the profitability of producing other goods

            4.         an increase in the number of sellers in the market

 

           Opposite changes in the above factors will cause the supply curve to shift to the left (i.e. less will be supplied at each price than before).

           As above, be careful not to confuse a movement along a curve and a shift to a new curve.  Remember that the supply curve already shows the positive effect of price on quantity supplied.  If the price of the good changes, we simply move to a new point along the existing supply curve. We call this a change in quantity supplied.  If there is a change in a factor influencing supply, other than the price of the good, the entire supply curve moves or shifts.  We term this a change in supply.

 Equilibrium

           At last we return to the initial question: how does a market economy determine prices? The answer is that every market has a stable equilibrium where the quantities supplied and demanded are equal.  Use your common sense for a minute.  If you have supplied 100 pounds of strawberries, but consumers are willing to buy only 70, what will happen?  How do real-world firms react when they are faced with products sitting on their shelves that no one wants to buy?  They have a sale.  They lower the price.  After all, it’s better to sell products at a reduced price than to not sell them at all.  In other words, if the quantity supplied exceeds the quantity demanded (a surplus), prices will fall.

          Change the example.  This time, suppose consumers are clamoring to buy 100 pounds of strawberries, but you have only 70 to sell.  Might you raise the price?  In fact, consumers probably will offer a higher price.  If you were one of the 100 potential customers, how could you make sure that the firm sold the scarce strawberries to you rather than someone else?  Offer to pay a higher price!  In other words, when the quantity demanded exceeds the quantity supplied (a shortage), prices will rise.  Prices will be stable or in equilibrium only if the quantities supplied and demanded are equal.

          No doubt you are eagerly awaiting a graphical illustration. Since both supply and demand curves are drawn with price on the vertical axis and quantity on the horizontal axis, we can put both curves on the same graph.  Pretty exciting, right?  The point at which the curves cross or intersect is the equilibrium. 

          In the example below, P1 is the equilibrium price and Q1 is the equilibrium quantity.  Any price above P1 (such as P2) will create an excess supply or surplus.  The quantity supplied (as shown by the supply curve) will exceed the quantity demanded (shown by the demand curve).  In light of the surplus, firms will lower price to P1 to sell their extra goods

          Any price below P1 (such as P3) will create an excess demand or shortage.  The quantity demanded will exceed the quantity supplied.  Because of the shortage, firms will soon discover that they can sell all they have even at a higher price.  As a result, the price will rise to P1.  In the long run, the price always moves to the equilibrium.

Changing equilibriums

            In the real world, prices change every day.  Why?  Perhaps it is because of a shift either in demand or supply.  If either the demand or supply curves shifts or moves, the equilibrium price and quantity will move as well.  As an example, suppose there is an increase in the costs of inputs needed to produce a good.  This lowers the profitability of production and causes a decrease in supply (i.e. the supply curve shifts to the left showing that less will be supplied at each price).

          The initial equilibrium price is P1, and the initial equilibrium quantity is Q1.  When the supply curve shifts left from S1 to S2, consumers still want to buy Q1 but, given the new supply curve, firms are only interested in selling Q3. The gap between Q1 and Q3 represents a temporary shortage of the good.  The shortage causes the price to rise to the new equilibrium with price P2 and quantity Q2.

          To test yourself, try shifting a demand curve.  Can you trace out what happens to equilibrium price and quantity?  Can you explain the underlying economic logic?

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Testing Yourself

 

To test your understanding of the major concepts in this reading, try answering the following:

 

   1.      Define demand; draw a demand curve and explain its shape.

   2.      Distinguish between substitute and complementary goods, between normal and inferior goods, and give examples.

   3.      Identify factors that will cause demand curves to shift.  Illustrate graphically and explain.

   4.      Distinguish between a change in demand (a shift in the curve) and a change in the quantity demanded (a movement along the curve) and give examples.

   5.      Define supply; draw a supply curve and explain its shape.

   6.      Identify factors that will cause supply curves to shift.  Illustrate graphically and explain.

   7.      Distinguish between a change in supply (a shift in the curve) and a change in the quantity supplied (a movement along the curve) and give examples.

   8.      Explain why prices above or below the equilibrium level are not stable in the long run.

   9.      Illustrate how shifts in supply and demand curves will affect equilibrium prices and quantities and explain.

 


Permission to reproduce or copy all or parts of this material for non-profit use is granted on the condition that the author and source are credited.  Suggestions and comments are welcomed.

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Last modified 07/29/13