EBF 200
Introduction to Energy and Earth Sciences Economics


Causes of Market Dynamics


Reading Assignment

In Chapter 7 ("Consumer Choice and Elasticity"), there is coverage of material concerning complements, substitutes, and income elasticities. You may wish to refer back to this chapter to complement the content here.

In the previous section, we examined what happens to the market equilibrium when the supply and/or demand curves move. Because markets are dynamic things, that is, they are always changing with time, the market equilibrium is always moving. From the previous section, you should understand what happens to a market when the demand and supply curves move up or down (or in or out.) Now we want to consider why these curves move.

It is important to remember that the supply and demand diagram is a static object, but the economy is not static, and things are changing all the time. A supply and demand diagram is only a snapshot of a market at some fixed point in time. We need to understand what causes changes, and what results from these changes.

Causes of Demand Curve Movements

When thinking of things like this, I always like to go back to “first principles.” Or what are sometimes called “fundamentals.” That is, when trying to understand why something happens, try to go back to the underlying root causes. To do that in this instance, we first have to understand what a demand curve is. You should be able to tell me at this point: it is a functional relationship that describes the quantity of goods that consumers in a market will want to purchase at any given price. Digging a bit deeper into the fundamentals, we understand where the demand curve comes from: marginal utility, or how much happiness the consumers obtain from consuming the good.

So, if the demand curve comes from the amount of utility a consumer will get from consuming the good, then the demand curve can only change if the consumers get a different amount of utility from the good in question.

That’s a bit of a mouthful. I’ll try to make it simpler: demand curves change because people change their willingness to pay. They want to buy more or less of the good. The next question then arises: what causes this change in utility?

There are several factors that can cause the demand curve to shift. If the curve shifts upwards (or outwards, away from the origin), then more of a good is demanded by the consumers at a given price. Or, looked at another way, for a fixed quantity, the price will be higher. This means that people derive more utility from a unit of the good. If the reverse happens, and the curve shifts down (or inwards, toward the origin) then less is demanded at a given price, or a lower price will be offered for a certain quantity, and this happens because something has caused the consumer to derive less utility from consumption of the good.

Some Causes of Demand Shifts

Cause #1 – Population

This one is pretty trivial. As we know, a market demand curve is simply an aggregation of every consumer’s individual demand curve. So, it is a matter of arithmetic to understand that if there are more consumers, then there are more individual demand curves to add together, and therefore, the demand curve will be further to the right. There are many examples of this. For example, when Penn State was a much smaller school, in the 1960s and 1970s, there were far fewer apartment complexes in State College. We now have many more apartments than we had in 1970 because there are far more students, and not because students today want to consume more apartments than they did 40 years ago.

Cause #2 – Income

As a person makes more money, their ability to consume more goods increases. The “willingness to pay” increases because the consumer has more money to spend. For this reason, for a lot of goods, as a person makes more money, the individual demand curve shifts to the right. In a community, the wealthier the community, the more the aggregate demand curve moves outwards. This explains why stores that sell luxury goods are usually in well-to-do suburbs, and not poor, inner-city neighborhoods.

To describe this situation, we define something called the “income elasticity of demand.” This is written as follows:

η(I) = % delta Q/% delta I   (where “I” is the symbol for income.)

Spelled out, this means “the percent change in the quantity demanded for a given percent change in income.” If your consumption of sushi goes from 2 times a month to 3 when your salary goes up 10%, then your income elasticity of demand for sushi is 50%/10% = 5.

The income elasticity can either be positive or negative. If it is positive, then the quantity demanded increases as income increases (a positive number divided by a positive number). Goods that have positive income elasticities are usually referred to as “normal” goods by economists. Luxury cars have positive income elasticities.

It is also possible for a good to have a negative income elasticity. This means that as I increases, Q decreases (a negative number divided by a positive number = a negative number). What does this mean? It means that as a person makes more money, their marginal utility from consuming a certain good declines. I mentioned above that while luxury cars have positive income elasticities, we might say that used cars, or economy cars, have negative elasticities: as people in a society make more money, they are less likely to consume a good. I know that as I have gone through life, my willingness to buy new cars has increased, and my desire to purchase used cars has declined.

Another example might be ramen noodles: students typically do not have a lot of money, so they buy a lot of cheap food, and ramen noodles are about as cheap as it comes. However, when students graduate and get jobs, they can afford to eat better, more satisfying meals, and given that most of us eat a fixed amount of food, this means that the quantity of ramen noodles decreases as income rises. (For the record, I still like a brick of noodles every once in a while, but I do not eat them nearly as much as when I was a student.)

Goods that have negative income elasticities are referred to by economists as “inferior” goods. An inferior good is one that we consume less of as we become wealthier. As we have more money, we can substitute for the inferior good with something that is more expensive, but more enjoyable. We will talk more about substitutes in a minute.

Cause #3 – The Price of Other Goods

The willingness to pay for a good is always relative to the willingness to pay for any and all other goods. The price of some other good can have an effect on our consumption choices.

When we think of how the price of one good can affect the demand curve for another good, we have to define two categories of goods: substitutes and complements.

A substitute is a good that you would consume instead of the good in question. As mentioned above, ramen noodles and steaks can be thought of as substitutes: if you are eating a lot of one, you are likely not eating a lot of the other. Life is full of substitution options: working instead of going to school, taking the bus instead of driving, renting a house instead of buying, taking an expensive vacation versus buying football tickets, going to a movie instead of going to a nightclub, and so on.

A complement is a good that you consume in addition to the good in question, with the condition that without one, you would not consume the other. For example, cars and gasoline (and tires) are all complements. On their own, each of these goods is fairly useless. But use them together, and they suddenly have more value. And, as you consume more of one, you are likely to consume more of another. Think of DVDs and DVD players, or iPods and earbuds, or shoes and shoelaces.

Now, we have to think about how the price of one good can affect the price of another. For this, we define the term “cross-elasticity of demand”. This is defined as follows:

η(XY) = % delta Q(X)/% delta P(Y)    (where X and Y are subscripts denoting the two goods in question.)

Spelled out, this statement reads: the cross-elasticity of goods X and Y is the percent change in the quantity of good X demanded that corresponds to a percent change in the price of good Y.

The cross-elasticity can be either positive or negative, and the sign will tell us if goods are substitutes or complements.

Let’s think of two common substitutes: chicken and beef. It is not hard to understand that if the price of beef goes up while the price of chicken stays the same, then people will tend to substitute chicken for beef. So, as the price of beef increases, the quantity of chicken demanded increases. The cross-elasticity in this case is a positive number divided by a positive number (or a negative divided by a negative), which gives us a positive number. Therefore, substitute goods have a positive cross-elasticity.

Now, let us think about complements. In the 1980s, CD players came on to the market. At first, CD players were very expensive, and very few people had them. Correspondingly, there were fewer music CDs sold. Over time, the price of CD players came down, and as a result, the quantity of CDs sold increased dramatically (even though the price did not change much for many years). It is easy to see that CD players and CDs are complements: one of the two is of little use without the other. So, when we look at our formula for the cross-elasticity, a decrease in the price of CD players led to an increase in the quantity of CDs demanded. A positive number (delta QX) is divided by a negative number (delta PY). Thus, the cross-elasticity is negative.

This leads to the definition of a handy rule: if the cross-elasticity of two goods can be shown to have a consistently positive value, then the goods are substitutes. If the cross-elasticity is shown to be consistently negative, the goods are complements. If the cross elasticity is either zero, or inconsistent, then it is likely that the goods are neither complements nor substitutes, but unrelated. Obviously, in our complicated economy, everything is related to everything else - the price of jet planes in Europe probably has some effect on the price of corn in Illinois, but the effect of one on the other is so dispersed as to be unobservable in any meaningful manner.

Cause #4 – Expectations

This is similar to income, but instead refers to future changes that a buyer expects to happen in the near future. For example, if you expect gasoline to be more expensive next week, you are more likely to fill up this week. This will cause the equilibrium for gasoline to shift today.

Cause #5 – Taste (or Fashion)

Some things, such as music styles, car models, clothing or house furnishings change in style over time. As such, desires for certain styles can change. What is fashionable, and popular one day may be out-of-date and unwanted the next. For example, why is it that skinny ties or Britney Spears CDs sell millions in one year, but are not popular a couple of years later? This is a point where economics starts to approach psychology, which is an area where I have little expertise. At this point, we need simply be content with the fact that at different points in time, people will want to buy different quantities of things for reasons that do not have any direct economic causes.

Cause #6 – Information

Going back to fundamentals, the demand for a good is based upon the utility, or happiness, one gets from consuming it. Sometimes, new information comes onto the market that leads to a change in the happiness that someone derives from consuming a good. The most obvious example of this has to do with health information. We often learn that consumption of a good may have beneficial or detrimental effects on one’s health. A few years ago, the “Atkins” diet was popular, in which it was believed that people could lose weight by eating a diet that was high in proteins and low in carbohydrates. A lot of people adopted this diet, so much so that many bakeries went out of business because bread sales declined a great deal. A little while later, some detrimental health effects of the Atkins diet were publicized, and this caused some people to abandon this diet. Fewer people smoke today than in the past, because we have better information about the long-term effects of smoking on health.

Health information is not the only form of information that can move demand curves. For example, when some people publicized what they felt were poor working conditions in clothing factories in Asia, a number of people in the US decided against purchasing goods made in those factories. In other cases, people fall out of favor. There are probably not too many Brett Favre football jerseys being sold in Green Bay these days, even though he was a hero in that city for many years. Deciding to play for a rival team in Minnesota greatly reduced the demand for clothing with his name and picture on it in Green Bay.

Causes of Movements of the Supply Curve

Unlike changes in demand, changes in supply are usually simpler to explain. A downward shift of the supply curve, which means that more goods are supplied at the same price, usually results from a lowering of the cost of production. This cost reduction could be because of a new, more efficient technology, or could be because of lowered taxes, or cheaper labor costs.

An upward shift of the supply curve (less being offered at the same price) is usually the result of some disturbance in the market. The most common example is when crops are damaged by weather conditions: hurricanes, unexpected cold, not enough rain, and so on. If prices for materials or labor of energy, any of the things that go into making a good, have increased in price, then the supply curve shifts upwards. If a tax on a good is increased, then the supply curve will shift upwards, as a tax is a cost that has to be paid on the good, not to the seller of the good or to the sellers of the factors of production, but to government. We will talk more about the incidence and effects of taxes later in the course.

Sample Question

Let us suppose that the following two events happen simultaneously:

  1. The Food and Drug Administration releases a report showing that drinking orange juice causes bald men to grow hair.
  2. A giant freeze destroys the orange crop in Florida, meaning that we have to import all of our oranges from Brazil for a year.

What do you expect to happen to the equilibrium price and quantity of orange juice?


The first statement will likely cause many bald men to start drinking more orange juice. This will cause an outward shift of the demand curve.

The second statement means that oranges will likely be more expensive to produce, which corresponding to an upward shift of the supply curve.

The demand curve movement will increase the equilibrium price and quantity. The supply curve movement will increase the equilibrium price, but reduce the equilibrium quantity.

Therefore, the net result will be an increase in the price of orange juice (both curve movements cause price to increase), but the change in quantity sold is unknown from the information at hand. One movement increases quantity sold, the other decreases it. Without measurement, we do not know which effect will dominate the other.


When prices change, they change because either the supply curve or demand curve (or both) has moved. Whenever a price changes, to understand why we want to figure out which of the underlying curves has moved, and why. After working through this lesson, you should be able to explain what happens when supply and demand curves move, and what some of the common causes of such movements are.

Take Aways

After working through the material on this page and reading the associated textbook content, you should be able to confidently:

  • explain the basic causes of the movements of the demand curve;
    • changes in tastes
    • the prices of other goods
    • changes in income
    • new information
    • expectations
    • population changes
  • understand the concepts of complementary and substitute goods:
    • explain how cross-elasticity is linked to the definition of substitutes and complements
    • understand what the sign of the cross-elasticity implies
  • understand the concepts of normal and inferior goods;
    • explain how income-elasticity is linked to the definition of substitutes and complements
    • understand what the sign of the income elasticity implies
  • explain the basic causes of upwards and downwards shifts of the supply curve;
  • understand that the supply curve is strongly related to the cost of producing goods.