What factors change demand?

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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.