3.1 Utility Theory

Learning Objectives

  • In this section we discuss economists’ utility theory.
  • You will learn about assumptions that underlie individual preferences, which can then be mapped onto a utility “function,” reflecting the satisfaction level associated with individuals’ preferences.
  • Further, we will explore how individuals maximize utility (or satisfaction).

Utility theoryA theory postulated in economics to explain behavior of individuals based on the premise people can consistently order rank their choices depending upon their preferences. bases its beliefs upon individuals’ preferences. It is a theory postulated in economics to explain behavior of individuals based on the premise people can consistently rank order their choices depending upon their preferences. Each individual will show different preferences, which appear to be hard-wired within each individual. We can thus state that individuals’ preferences are intrinsic. Any theory, which proposes to capture preferences, is, by necessity, abstraction based on certain assumptions. Utility theory is a positive theoryTheory that seeks to explain an individual’s observed behavior and choices. that seeks to explain the individuals’ observed behavior and choices.The distinction between normative and positive aspects of a theory is very important in the discipline of economics. Some people argue that economic theories should be normative, which means they should be prescriptive and tell people what to do. Others argue, often successfully, that economic theories are designed to be explanations of observed behavior of agents in the market, hence positive in that sense. This contrasts with a normative theoryTheory that dictates that people should behave in the manner prescribed by it., one that dictates that people should behave in the manner prescribed by it. Instead, it is only since the theory itself is positive, after observing the choices that individuals make, we can draw inferences about their preferences. When we place certain restrictions on those preferences, we can represent them analytically using a utility functionA mathematical formulation that ranks the preferences of the individual in terms of satisfaction different consumption bundles provide.—a mathematical formulation that ranks the preferences of the individual in terms of satisfaction different consumption bundles provide. Thus, under the assumptions of utility theory, we can assume that people behaved as if they had a utility function and acted according to it. Therefore, the fact that a person does not know his/her utility function, or even denies its existence, does not contradict the theory. Economists have used experiments to decipher individuals’ utility functions and the behavior that underlies individuals’ utility.

To begin, assume that an individual faces a set of consumption “bundles.” We assume that individuals have clear preferences that enable them to “rank order” all bundles based on desirability, that is, the level of satisfaction each bundle shall provide to each individual. This rank ordering based on preferences tells us the theory itself has ordinal utilityUtility that can only represent relative levels of satisfaction between two or more alternatives, that is, rank orders them.—it is designed to study relative satisfaction levels. As we noted earlier, absolute satisfaction depends upon conditions; thus, the theory by default cannot have cardinal utilityUtility that can represent the absolute level of satisfaction., or utility that can represent the absolute level of satisfaction. To make this theory concrete, imagine that consumption bundles comprise food and clothing for a week in all different combinations, that is, food for half a week, clothing for half a week, and all other possible combinations.

The utility theory then makes the following assumptions:

  1. Completeness: Individuals can rank order all possible bundles. Rank ordering implies that the theory assumes that, no matter how many combinations of consumption bundles are placed in front of the individual, each individual can always rank them in some order based on preferences. This, in turn, means that individuals can somehow compare any bundle with any other bundle and rank them in order of the satisfaction each bundle provides. So in our example, half a week of food and clothing can be compared to one week of food alone, one week of clothing alone, or any such combination. Mathematically, this property wherein an individual’s preferences enable him or her to compare any given bundle with any other bundle is called the completenessProperty in which an individual’s preferences enable him/her to compare any given consumption bundle with any other bundle. property of preferences.
  2. More-is-better: Assume an individual prefers consumption of bundle A of goods to bundle B. Then he is offered another bundle, which contains more of everything in bundle A, that is, the new bundle is represented by αA where α = 1. The more-is-better assumption says that individuals prefer αA to A, which in turn is preferred to B, but also A itself. For our example, if one week of food is preferred to one week of clothing, then two weeks of food is a preferred package to one week of food. Mathematically, the more-is-better assumption is called the monotonicity assumptionThe assumption that more consumption is always better. on preferences. One can always argue that this assumption breaks down frequently. It is not difficult to imagine that a person whose stomach is full would turn down additional food. However, this situation is easily resolved. Suppose the individual is given the option of disposing of the additional food to another person or charity of his or her choice. In this case, the person will still prefer more food even if he or she has eaten enough. Thus under the monotonicity assumption, a hidden property allows costless disposal of excess quantities of any bundle.
  3. Mix-is-better: Suppose an individual is indifferent to the choice between one week of clothing alone and one week of food. Thus, either choice by itself is not preferred over the other. The “mix-is-better” assumptionThe assumption that a mix of consumption bundles is always better than stand-alone choices. about preferences says that a mix of the two, say half-week of food mixed with half-week of clothing, will be preferred to both stand-alone choices. Thus, a glass of milk mixed with Milo (Nestlè’s drink mix), will be preferred to milk or Milo alone. The mix-is-better assumption is called the “convexity” assumption on preferences, that is, preferences are convex.
  4. Rationality: This is the most important and controversial assumption that underlies all of utility theory. Under the assumption of rationalityThe assumption that individuals’ preferences avoid any kind of circularity., individuals’ preferences avoid any kind of circularity; that is, if bundle A is preferred to B, and bundle B is preferred to C, then A is also preferred to C. Under no circumstances will the individual prefer C to A. You can likely see why this assumption is controversial. It assumes that the innate preferences (rank orderings of bundles of goods) are fixed, regardless of the context and time.

If one thinks of preference orderings as comparative relationships, then it becomes simpler to construct examples where this assumption is violated. So, in “beats”—as in A beat B in college football. These are relationships that are easy to see. For example, if University of Florida beats Ohio State, and Ohio State beats Georgia Tech, it does not mean that Florida beats Georgia Tech. Despite the restrictive nature of the assumption, it is a critical one. In mathematics, it is called the assumption of transitivity of preferences.

Whenever these four assumptions are satisfied, then the preferences of the individual can be represented by a well-behaved utility functionA representation of the preferences of the individual that satisfies the assumptions of completeness, monotonicity, mix-is-better, and rationality..The assumption of convexity of preferences is not required for a utility function representation of an individual’s preferences to exist. But it is necessary if we want that function to be well behaved. Note that the assumptions lead to “a” function, not “the” function. Therefore, the way that individuals represent preferences under a particular utility function may not be unique. Well-behaved utility functions explain why any comparison of individual people’s utility functions may be a futile exercise (and the notion of cardinal utility misleading). Nonetheless, utility functions are valuable tools for representing the preferences of an individual, provided the four assumptions stated above are satisfied. For the remainder of the chapter we will assume that preferences of any individual can always be represented by a well-behaved utility function. As we mentioned earlier, well-behaved utility depends upon the amount of wealth the person owns.

Utility theory rests upon the idea that people behave as if they make decisions by assigning imaginary utility values to the original monetary values. The decision maker sees different levels of monetary values, translates these values into different, hypothetical terms (“utils”), processes the decision in utility terms (not in wealth terms), and translates the result back to monetary terms. So while we observe inputs to and results of the decision in monetary terms, the decision itself is made in utility terms. And given that utility denotes levels of satisfaction, individuals behave as if they maximize the utility, not the level of observed dollar amounts.

While this may seem counterintuitive, let’s look at an example that will enable us to appreciate this distinction better. More importantly, it demonstrates why utility maximization, rather than wealth maximization, is a viable objective. The example is called the “St. Petersburg paradox.” But before we turn to that example, we need to review some preliminaries of uncertainty: probability and statistics.

Key Takeaways

  • In economics, utility theory governs individual decision making. The student must understand an intuitive explanation for the assumptions: completeness, monotonicity, mix-is-better, and rationality (also called transitivity).
  • Finally, students should be able to discuss and distinguish between the various assumptions underlying the utility function.

Discussion Questions

  1. Utility theory is a preference-based approach that provides a rank ordering of choices. Explain this statement.
  2. List and describe in your own words the four axioms/assumptions that lead to the existence of a utility function.
  3. What is a “util” and what does it measure?