Let's use income as an example of how factors other than price affect demand. Figure 1 shows the initial demand for automobiles as D0. At point Q, for example, if the price is $20,000 per car, the quantity of cars demanded is 18 million. D0 also shows how the quantity of cars demanded would change as a result of a higher or lower price. For example, if the price of a car rose to $22,000, the quantity demanded would decrease to 17 million, at point R.
Figure 1. Shifts in Demand: A Car Example.
The original demand curve D0, like every demand curve, is based on the ceteris paribus assumption that no other economically relevant factors change. Now imagine that the economy expands in a way that raises the incomes of many people, making cars more affordable. How will this affect demand? How can we show this graphically?
Return to Figure 1. The price of cars is still $20,000, but with higher incomes, the quantity demanded has now increased to 20 million cars, shown at point S. As a result of the higher income levels, the demand curve shifts to the right to the new demand curve D1, indicating an increase in demand. Table 1, below, shows clearly that this increased demand would occur at every price, not just the original one.
|Price||Decrease to D2||Original Quantity Demanded D0||Increase to D1|
|$16,000||17.6 million||22.0 million||24.0 million|
|$18,000||16.0 million||20.0 million||22.0 million|
|$20,000||14.4 million||18.0 million||20.0 million|
|$22,000||13.6 million||17.0 million||19.0 million|
|$24,000||13.2 million||16.5 million||18.5 million|
|$26,000||12.8 million||16.0 million||18.0 million|
Figure 2. A New Shopping Trip. When this man got a raise, he shopped at an expensive organic grocery store instead of buying generic groceries. The generic groceries are an example of an inferior good.
When a demand curve shifts, it does not mean that the quantity demanded by every individual buyer changes by the same amount. In this example, not everyone would have higher or lower income and not everyone would buy or not buy an additional car. Instead, a shift in a demand curve captures a pattern for the market as a whole: Increased demand means that at every given price, the quantity demanded is higher, so that the demand curve shifts to the right from D0 to D1. And, decreased demand means that at every given price, the quantity demanded is lower, so that the demand curve shifts to the left from D0 to D2.
We just argued that higher income causes greater demand at every price. This is true for most goods and services. For some – luxury cars, vacations in Europe, and fine jewelry – the effect of a rise in income can be especially pronounced. A product whose demand rises when income rises, and vice versa, is called a normal good. A few exceptions to this pattern do exist, however. 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 shifts to the left.