Key points

  • Demand curves can shift. Changes in factors like average income and preferences can cause an entire demand curve to shift right or left. This causes a higher or lower quantity to be demanded at a given price.
  • Ceteris paribus assumption. Demand curves relate the prices and quantities demanded assuming no other factors change. This is called the ceteris paribus assumption. This article talks about what happens when other factors aren't held constant.

What factors affect demand?

We defined demand as the amount of some product a consumer is willing and able to purchase at each price. That suggests at least two factors in addition to price that affect demand.
Willingness to purchase suggests a desire, based on what economists call tastes and preferences. If you neither need nor want something, you will not buy it. Ability to purchase suggests that income is important. Professors are usually able to afford better housing and transportation than students because they have more income.
Prices of related goods can affect demand also. If you need a new car, the price of a Honda may affect your demand for a Ford. Finally, the size or composition of the population can affect demand. The more children a family has, the greater their demand for clothing. The more driving-age children a family has, the greater their demand for car insurance, and the less for diapers and baby formula.

The ceteris paribus assumption

A demand curve or a supply curve is a relationship between two, and only two, variables: quantity on the horizontal axis and price on the vertical axis. The assumption behind a demand curve or a supply curve is that no relevant economic factors, other than the product’s price, are changing. Economists call this assumption ceteris paribus, a Latin phrase meaning “other things being equal”. If all else is not held equal, then the laws of supply and demand will not necessarily hold. The rest of this article explores what happens when other factors aren't held constant.

How does income affect demand?

Say we have an initial demand curve for a certain kind of car. Now imagine that the economy expands in a way that raises the incomes of many people, making cars more affordable and that people generally see cars as a desirable thing to own. This will cause the demand curve to shift.
 

Normal and inferior goods

A product whose demand rises when income rises, and vice versa, is called a normal good. A few exceptions to this pattern do exist, though. As incomes rise, many people will buy fewer generic-brand groceries and more name-brand groceries. They are less likely to buy used cars and more likely to buy new cars. They will be less likely to rent an apartment and more likely to own a home, and so on. A product whose demand falls when income rises, and vice versa, is called an inferior good. In other words, when income increases, the demand curve for an inferior good shifts to the left.

Other factors that shift demand curves

Income is not the only factor that causes a shift in demand. Other things that change demand include tastes and preferences, the composition or size of the population, the prices of related goods, and even expectations. A change in any one of the underlying factors that determine what quantity people are willing to buy at a given price will cause a shift in demand.
Graphically, the new demand curve lies either to the right (an increase) or to the left (a decrease) of the original demand curve. Let’s look at these factors.

Changing tastes or preferences

From 1980 to 2014, the per-person consumption of chicken by Americans rose from 48 pounds per year to 85 pounds per year, and consumption of beef fell from 77 pounds per year to 54 pounds per year, according to the U.S. Department of Agriculture (USDA). Changes like these are largely due to movements in taste, which change the quantity of a good demanded at every price—that is, they shift the demand curve for that good, rightward for chicken and leftward for beef.

Changes in the composition of the population

The proportion of elderly citizens in the United States population is rising. It rose from 9.8% in 1970 to 12.6% in 2000 and is projected by the U.S. Census Bureau to be 20% of the population by 2030.
A society with relatively more children, like the United States in the 1960s, will have a greater demand for goods and services like tricycles and daycare facilities. A society with relatively more elderly persons, as the United States is projected to have by 2030, has a higher demand for nursing homes and hearing aids. Similarly, changes in the size of the population can affect the demand for housing and many other goods. Each of these changes in demand will be shown as a shift in the demand curve.

Related goods

The demand for a product can also be affected by changes in the prices of related goods such as substitutes or complements. A substitute is a good or service that can be used in place of another good or service. As electronic resources, like the one you are reading now, become more available, you would expect to see a decrease in demand for traditional printed books. A lower price for a substitute decreases demand for the other product.
For example, in recent years as the price of tablet computers has fallen, the quantity demanded has increased because of the law of demand. Since people are purchasing tablets, there has been a decrease in demand for laptops, which can be shown graphically as a leftward shift in the demand curve for laptops. A higher price for a substitute good has the reverse effect.
Other goods are complements for each other, meaning that the goods are often used together because consumption of one good tends to enhance consumption of the other. Examples include breakfast cereal and milk; notebooks and pens or pencils; golf balls and golf clubs; gasoline and sport utility vehicles; and the five-way combination of bacon, lettuce, tomato, mayonnaise, and bread. If the price of golf clubs rises, the quantity demanded of golf clubs falls because of the law of demand, and demand for a complement good like golf balls decreases along with it. Similarly, a higher price for skis would shift the demand curve for a complement good like ski resort trips to the left, while a lower price for a complement has the reverse effect.

Changes in expectations about future prices or other factors that affect demand

While it is clear that the price of a good affects the quantity demanded, it is also true that expectations about the future price—or expectations about tastes and preferences, income, and so on—can affect demand. For example, if people hear that a hurricane is coming, they may rush to the store to buy flashlight batteries and bottled water. If people learn that the price of a good like coffee is likely to rise in the future, they may head for the store to stock up on coffee now. These changes in demand are shown as shifts in the curve. Therefore, a shift in demand happens when a change in some economic factor other than price causes a different quantity to be demanded at every price.

Summing up factors that change demand

Six factors that can shift demand curves are summarized in the graph below. Notice that a change in the price of the good or service itself is not listed among the factors that can shift a demand curve. A change in the price of a good or service causes a movement along a specific demand curve, and it typically leads to some change in the quantity demanded, but it does not shift the demand curve.