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3.1 Theory of the Consumer

Back in Chapter 2 "Key Measures and Relationships", we used a demand curve to represent the relationship between the price charged for ice cream bars and the maximum number of ice cream bars that customers would purchase. We will address how to create a demand curve later in this chapter, but we will begin our discussion with a brief review of microeconomic theory that endeavors to explain how consumers behave.

A consumer is someone who makes consumption decisions for herself or for her household unit. In a modern society, consumption is largely facilitated by purchases for goods and services. Some of these goods and services are essential to a consumer’s livelihood, but others are discretionary, perhaps even a luxury. Consumers are limited in how much they can consume by their wealth. A consumer’s wealth will change over time due to income and expenditures. She might be able to borrow against future income so as to increase her capacity to purchase now in exchange for diminished wealth and consumption later. Similarly, she may retain some of her current wealth as savings toward increased future consumption. Consumption decisions may be planned into the future, taking account of the expected changes in wealth over time.

The theory of the consumerIndividuals plan their purchases, the timing of those purchases, and borrowing and saving so as to maximize the satisfaction that they and their household units will experience from consumption of goods and services. posits that a consumer plans her purchases, the timing of those purchases, and borrowing and saving so as maximize the satisfaction she and her household unit will experience from consumption of goods and services. In this theory, consumers are able to compare any two patterns of consumption, borrowing, and saving and deem that either one is preferred to the second or they are indifferent between the two patterns. Based on the ability to do these comparisons, consumers look at the prices charged for various services now, and what they expect prices to be for goods and services in the future, and select the pattern of consumption, borrowing, and saving that generates the greatest satisfaction over their lifetime within the constraint of their wealth and expected future income.

Although the consumers may anticipate changes in prices over time, they may find that their guesses about future prices are incorrect. When this happens, the theory states that they will adjust their consumption, borrowing, and saving to restore the optimality under the newly revealed prices. In fact, the theory identifies two effects of price changes: the substitution effect and the income effect.

The substitution effect is based on an argument employing marginal reasoning like the marginal analysis discussed in Chapter 2 "Key Measures and Relationships". Economists often use the term utilityA hypothetical quantitative value for satisfaction that a consumer receives from a pattern of consumption. as a hypothetical quantitative value for satisfaction that a consumer receives from a pattern of consumption. If a consumer were to receive one more unit of some good or service, the resulting increase in their utility is called the marginal utility of the goodThe increase in satisfaction that results from a consumer receiving one more unit of some good or service.. As a consequence of maximizing their overall satisfaction from consumption, or equivalently maximizing their utility, it will be the case that if you take the marginal utility of one good or service and divide it by its price, you should get the same ratio for any other good or service. If they were not roughly equal, the consumer would be able to swap consumption of one good or service for another, keep within their wealth constraint, and have higher utility. The substitution effectThe consumer's response to a change in the price of a good that restores the ratios of marginal utility to price for two goods to a state of balance. is the consumer’s response to a changing price to restore balance in the ratios of marginal utility to price.

Just as a simple illustration, suppose a consumer likes bananas and peaches as a treat. For the sake of the illustration, let’s suppose an additional banana has a marginal utility of 2 and a peach has a marginal utility of 3. If a banana costs $0.20 and a peach costs $0.30, bananas and peaches have a ratio of the marginal utility to its price equal to 10. If the peach price increases to $0.40, the ratio will become lower for peaches and the consumer may substitute some purchases of peaches with purchases of more bananas.

As the result of price changes and substitution, the consumer’s overall utility may increase or decrease. Consequently, the consumer may experience the equivalent of an increase or decrease in wealth, in the sense that it would have required a different level of wealth to just barely afford the new consumption pattern under the previous set of prices. This equivalent change in purchasing power is called the income effectThe change in the mix of goods and services that consumers can afford (that is, the change in wealth or purchasing power) when the price of a good changes..

Economists have precise techniques for separating the response to a price change into a substitution effect and an income effect.See Varian (1993) for a discussion of the substitution effect, income effect, and Giffen goods. This is beyond the scope of this text. For our purposes, it is sufficient to appreciate that price changes will affect the mix of goods and services that is best and change the consumer’s overall level of satisfaction.

In most cases, the primary response to a price change is a substitution effect, with a relatively modest income effect. However, for goods and services that a consumer cannot substitute easily, a sizeable price change may have a significant income effect. For example, when gasoline prices jumped dramatically in the United States, consumers may have reduced their driving somewhat but were unable to find a substitute for the essential needs served by driving their cars. As a result, consumers experienced a dramatic drop in wealth available for other goods and services and consumed generally less of all of those to compensate for the greater expenditure on gasoline.

Normally, price increases result in less consumption of the associated good or service, whereas price decreases results in more consumption. This typical pattern is usually supported by both the substitution effect and the income effect. An interesting exception is the case of Giffen goodsA good or service for which consumption may increase in response to a price increase or decrease in response to a price decrease., which is a situation where consumption of a good or service may increase in response to a price increase or decrease in response to a price decrease. This anomaly is explained by a strong income effect. An economist named Robert Giffen discovered that Irish consumers increased the use of potatoes in their diet during the Irish Potato Famine of the 1840s, even though the price of potatoes rose dramatically. Basically, because potatoes were a staple of the Irish diet, when the potato price increased, the wealth available to purchase other food items diminished and Irish consumers wanted to purchase more potatoes to compensate for the diminished purchases of other food items.