Figure 3-4 illustrates the two basic ways in which demand curves can shift. When economists talk about an “increase in demand,” they mean a rightward shift of the demand curve: at any given price, consumers demand a larger quantity of the good or service than before. This is shown by the rightward shift of the original demand curve D₁ to D₂. And when economists talk about a “decrease in demand,” they mean a leftward shift of the demand curve: at any given price, consumers demand a smaller quantity of the good or service than before. This is shown by the leftward shift of the original demand curve D₁ to D₃.

What caused the demand curve for natural gas to shift? As we mentioned earlier, the reason was the stronger U.S. economy in 2006 compared to 2002. If you think about it, you can come up with other factors that would be likely to shift the demand curve for natural gas. For example, suppose that the price of heating oil rises. This will induce some consumers, who heat their homes and businesses in winter with heating oil, to switch to natural gas instead, increasing the demand for natural gas.

Economists believe that there are five principal factors that shift the demand curve for a good or service:

Although this is not an exhaustive list, it contains the five most important factors that can shift demand curves. So when we say that the quantity of a good or service demanded falls as its price rises, other things equal, we are in fact stating that the factors that shift demand are remaining unchanged. Let’s now explore, in more detail, how those factors shift the demand curve.

Changes in the Prices of Related Goods or Services Heating oil is what economists call a substitute for natural gas. A pair of goods are substitutes if a rise in the price of one good (heating oil) makes consumers more likely to buy the other good (natural gas). Substitutes are usually goods that in some way serve a similar function: coffee and tea, muffins and doughnuts, train rides and air flights. A rise in the price of the alternative good induces some consumers to purchase the original good instead of it, shifting demand for the original good to the right.

But sometimes a rise in the price of one good makes consumers less willing to buy another good. Such pairs of goods are known as complements. Complements are usually goods that in some sense are consumed together: computers and software, cappuccinos and cookies, cars and gasoline. Because consumers like to consume a good and its complement together, a change in the price of one of the goods will affect the demand for its complement. In particular, when the price of one good rises, the demand for its complement decreases, shifting the demand curve for the complement to the left. So, for example, when the price of gasoline began to rise in 2009 from under $3 per gallon to close to $4 per gallon in 2011, the demand for gas-guzzling cars fell.

Changes in Income Why did the stronger economy in 2006 lead to an increase in the demand for natural gas compared to the demand during the weak economy of 2002? Because with the stronger economy, Americans had more income, making them more likely to purchase more of most goods and services at any given price. For example, with a higher income you are likely to keep your house warmer in the winter than if your income is low.

And, the demand for natural gas, a major source of fuel for electricity-generating power plants, is tied to the demand for other goods and services. For example, businesses must consume power in order to provide goods and services to households. So when the economy is strong and household incomes are high, businesses will consume more electricity and, indirectly, more natural gas.

Why do we say that people are likely to purchase more of “most goods,” not “all goods”? Most goods are normal goods—the demand for them increases when consumer income rises. However, the demand for some products falls when income rises. Goods for which demand decreases when income rises are known as inferior goods. Usually an inferior good is one that is considered less desirable than more expensive alternatives—such as a bus ride versus a taxi ride. When they can afford to, people stop buying an inferior good and switch their consumption to the preferred, more expensive alternative. So when a good is inferior, a rise in income shifts the demand curve to the left. And, not surprisingly, a fall in income shifts the demand curve to the right.

One example of the distinction between normal and inferior goods that has drawn considerable attention in the business press is the difference between so-called casual-dining restaurants such as Applebee’s or Olive Garden and fast-food chains such as McDonald’s and KFC. When their incomes rise, Americans tend to eat out more at casual-dining restaurants. However, some of this increased dining out comes at the expense of fast-food venues—to some extent, people visit McDonald’s less once they can afford to move upscale. So casual dining is a normal good, whereas fast-food consumption appears to be an inferior good.

Changes in Tastes Why do people want what they want? Fortunately, we don’t need to answer that question—we just need to acknowledge that people have certain preferences, or tastes, that determine what they choose to consume and that these tastes can change. Economists usually lump together changes in demand due to fads, beliefs, cultural shifts, and so on under the heading of changes in tastes or preferences.

For example, once upon a time men wore hats. Up until around World War II, a respectable man wasn’t fully dressed unless he wore a dignified hat along with his suit. But the returning GIs adopted a more informal style, perhaps due to the rigors of the war. And President Eisenhower, who had been supreme commander of Allied Forces before becoming president, often went hatless. After World War II, it was clear that the demand curve for hats had shifted leftward, reflecting a decrease in the demand for hats.

Economists have relatively little to say about the forces that influence consumers’ tastes. (Although marketers and advertisers have plenty to say about them!) However, a change in tastes has a predictable impact on demand. When tastes change in favor of a good, more people want to buy it at any given price, so the demand curve shifts to the right. When tastes change against a good, fewer people want to buy it at any given price, so the demand curve shifts to the left.

Changes in Expectations When consumers have some choice about when to make a purchase, current demand for a good is often affected by expectations about its future price. For example, savvy shoppers often wait for seasonal sales—say, buying next year’s holiday gifts during the post-holiday markdowns. In this case, expectations of a future drop in price lead to a decrease in demand today. Alternatively, expectations of a future rise in price are likely to cause an increase in demand today.

For example, the fall in gas prices in recent years to around $2 per BTU has spurred more consumers to switch to natural gas from other fuel types than when natural gas fell to $2 per BTU in 2002. But why are consumers more willing to switch now? Because in 2002, consumers didn’t expect the fall in the price of natural gas to last—and they were right.

In 2002, natural gas prices fell because of the weak economy. That situation changed in 2006 when the economy came roaring back and the price of natural gas rose dramatically. In contrast, consumers have come to expect that the more recent fall in the price of natural gas will not be temporary because it is based on a permanent change: the ability to tap much larger deposits of natural gas.

Expected changes in future income can also lead to changes in demand: if you expect your income to rise in the future, you will typically borrow today and increase your demand for certain goods; if you expect your income to fall in the future, you are likely to save today and reduce your demand for some goods.

Changes in the Number of Consumers Another factor that can cause a change in demand is a change in the number of consumers of a good or service. For example, population growth in the United States eventually leads to higher demand for natural gas as more homes and businesses need to be heated in the winter and cooled in the summer.

Let’s introduce a new concept: the individual demand curve, which shows the relationship between quantity demanded and price for an individual consumer. For example, suppose that the Gonzalez family is a consumer of natural gas for heating and cooling their home. Panel (a) of Figure 3-5 shows how many BTUs of natural gas they will buy per year at any given price. The Gonzalez family’s individual demand curve is D_Gonzalez.

The market demand curve shows how the combined quantity demanded by all consumers depends on the market price of the good. (Most of the time when economists refer to the demand curve they mean the market demand curve.) The market demand curve is the horizontal sum of the individual demand curves of all consumers in that market. To see what we mean by the term horizontal sum, assume for a moment that there are only two consumers of natural gas, the Gonzalez family and the Murray family. The Murray family consumes natural gas to fuel their natural gas–powered car. The Murray family’s individual demand curve, D_Murray, is shown in panel (b). Panel (c) shows the market demand curve. At any given price, the quantity demanded by the market is the sum of the quantities demanded by the Gonzalez family and the Murray family. For example, at a price of $5 per BTU, the Gonzalez family demands 30 BTUs of natural gas per year and the Murray family demands 20 BTUs per year. So the quantity demanded by the market is 50 BTUs per year, as seen on the market demand curve, D_Market.

Clearly, the quantity demanded by the market at any given price is larger with the Murray family present than it would be if the Gonzalez family were the only consumer. The quantity demanded at any given price would be even larger if we added a third consumer, then a fourth, and so on. So an increase in the number of consumers leads to an increase in demand.

For a review of the factors that shift demand, see Table 3-1.