Law of Demand: Definition, Explained, Examples

The Law of Demand Explained Using Examples in the U.S. Economy

Law of demand
When the price of groceries rise, you buy less. Photo: Tetra Images/Getty Images

Definition: The law of demand states that, all other things being equal, the quantity bought of a good or service is a function of price. As long as nothing else changes, people will buy less of something when its price rises. They'll buy more when its price falls. 

The demand schedule tells you the exact quantity that will be purchased at any given price. Here's a real-life example of how this works in the demand schedule for beef in 2014.

The demand curve plots those numbers on a chart. The quantity is on the horizontal or x-axis, and the price is on the vertical or y-axis.  

If the amount bought changes a lot when the price does, then it's called elastic demand. An example of this is ice cream. You can easily get a different dessert if the price rises too high. 

If the quantity doesn't change much when the price does, that's called inelastic demand. An example of this is gasoline. You need to buy enough to get to work regardless of the price. 

This relationship holds true as long as "all other things remain equal." That part is so important that economists use a Latin term to describe it -- ceteris paribus. The "all other things" that need to be equal under ceteris paribus are the other determinants of demand. These are prices of related goods or services, income, tastes or preferences, and expectations. For aggregate demand, the number of buyers in the market is also a determinant.

 

If the other determinants change, then consumers will buy more or less of the product even though the price remains the same. That's called a shift in the demand curve.

Law of Demand Explained

For example, airlines want to lower costs when oil prices rise to remain profitable. They also don't want to cut flights.

Instead, they buy more fuel-efficient planes, fill all seats, and change operations to improve efficiency. As a result, they've raised seat-miles per gallon from 55 in 2005 to 60 in 2011. The law of demand would describe this as the quantity of fuel required by the airlines dropped as the price rose. 

Of course, all other things were not equal during this period. In fact, demand for jet fuel was further lessened because airlines' income also dropped at the same time. The 2008 global financial crisis meant that travelers cut back on their demand for air travel. The airlines' expectations about the price of jet fuel also changed. They realized it would probably continue to rise over the long term. The other two determinants of airline's demand for jet fuel stayed the same. They couldn't switch to another fuel, and their tastes or desire to use jet fuel didn't change. (Source: "High Airline Jet Fuel Costs Prompt Cost-saving Measures," EIA.)

Retailers use the law of demand every time they offer a sale. In the short-term, all other things are equal. That's why sales are usually very successful in driving demand. Shoppers respond immediately to the advertised price drop. It works especially well during massive holiday sales, such as Black Friday and Cyber Monday.

The Law of Demand and the Business Cycle

Politicians and central bankers understand the law of demand very well. The Federal Reserve's mandate is to prevent inflation while reducing unemployment. During the expansion phase of the business cycle, the Fed tries to reduce demand for all goods and services by raising the price of everything. It does this with contractionary monetary policy. It raised the Fed funds rate, which increases interest rates on loans and mortgages.  That has the same effect as raising prices, first on loans, then on everything bought with loans, and finally everything else.

Of course, when prices go up, so does inflation. That's not always a bad thing. The Fed has an inflation target of 2 percent. That sets an expectation that prices will increase 2 percent a year. It increases demand because people know that things will only cost more next year. Therefore, they may as well buy it now ceteris paribus.

During a recession, or the contraction phase of the business cycle, policy makers have a worse problem. They've got to stimulate demand when workers are losing jobs and homes, and they have less income and wealth. Expansionary monetary policy lowers interest rates, thereby reducing the price of everything. If the recession is bad enough, it doesn't reduce the price enough to offset the lower income. 

In that case, fiscal policy is needed. The federal government usually starts spending to create public works jobs, extending unemployment benefits, and cutting taxes. That increases the deficit, since the government's income through taxes is usually lower. Once confidence and demand are restored, the deficit will fall as tax receipts increase.