The Utility-Function Approach to Consumer Choice

Finding the highest attainable indifference curve on a budget constraint is just one way that economists have analyzed the consumer choice problem. For many applications, a second approach has also proved useful. In this approach we represent the consumer's preferences not with an indifference map but with a utility function (a formula that yields a number representing the satisfaction provided by a bundle of goods.)

A utility function is simply a formula that, for each possible bundle of goods, yields a number that represents the amount of satisfaction provided by that bundle. Suppose, for example, that Tom consumes only food and shelter and that his utility function is given by U(F, S) = FS, where F denotes the number of pounds of food, S the number of square yards of shelter he consumes per week, and U his satisfaction, measured in "utils" per week.¹ If F = 4 lb/wk. and S = 3 sq yd/wk, Tom will receive 12 utils/wk of utility, just as he would if he consumed 3 lb/wk of food and 4 sq yd/wk of shelter. By contrast, if he consumed 8 lb/wk of food and 6 sq yd/wk of shelter, he would receive 48 utils/wk.

The utility function is analogous to an indifference map in that both provide a complete description of the consumer's preference ordering. In the indifferencecurve framework, we can rank any two bundles by seeing which one lies on a higher indifference curve. In the utility-function framework, we can compare any two bundles by seeing which one yields a greater number of utils. Indeed, as the following example illustrates, it is straightforward to use the utility function to construct an indifference map.
Example A.3-1

If Tom's utility function is given by U(F, S) = FS, graph the indifference curves that correspond to 1, 2, 3, and 4 utils, respectively. 

(A.3.1)

which is the difference curve labeled U = 1 in Figure A.3-1. The indifference curve that corresponds to 2 units of utility is generated by solving FS= 2 to get S = 2/F, and it is shown by the curve labeled U = 2 in Figure A.3-1. In similar fashion, we generate the indifference curves to U = 3 and U = 4, which are correspondingly labeled in the diagram. More generally, we get the indifference curve corresponding to a utility level of U₀ by solving FS = U₀ to get S = U₀/F.

In the indifference-curve framework, the best attainable bundle is the bundle on the budget constraint that lies on the highest indifference curve. Analogously, the best attainable bundle in the utility-function framework is the bundle on the budget constraint that provides the highest level of utility. In the indifferencecurve framework, the best attainable bundle occurs at a point of tangency between an indifference curve and the budget constraint. At the optimal bundle, the slope of the indifference curve, or MRS, equals the slope of the budget constraint. Suppose food and shelter are again our two goods, and P_F and P_S are their respective prices. If DS/DF denotes the slope of the highest attainable indifference curve at the optimal bundle, the tangency condition says that DS/DF = P_F/P_S What is the analogous condition in the utility-function framework?

To answer this question, we must introduce the concept of marginal utility (the marginal utility of a good is the rate at which total utility changes with consumption of the good) which is the rate at which total utility changes as the quantities of food and shelter change. More specifically, let MU_F denote the number of additional utils we get for each additional unit of food and MU_S denote the number of additional utils we get for each additional unit of shelter. In Figure 3-2, note that bundle K has DF fewer units of food and DS more units of shelter than bundle L. Thus, if we move from bundle K to bundle L, we gain MU_F DFutils from having more food, but we lose MU_SDS utils from having less shelter.

Because K and L both lie on the same indifference curve, we know that both bundles provide the same level of utility. Thus the utility we lose from having less shelter must be exactly offset by the utility we gain from having more food. This tells us that
 

MUFDF = MUSDS.

(A.3.2)

Cross-multiplying terms in Equation A.3.2 gives
 

.

(A.3.3)

Suppose that the optimal bundle lies between K and L, which are very close together, so that DF and DS are both very small. As K and L move closer to the optimal bundle, the ratio DS/DF becomes equal to the slope of the indifference curve at that bundle, which Equation A.3.3 tells us is equal to the ratio of the marginal utilities of the two goods. And since the slope of the indifference curve at the optimal bundle is the same as that of the budget constraint, the following condition must also hold for the optimal bundle:
 

.

(A.3.4)

Equation A.3.4 is the condition in the utility-function framework that is analogous to the MRS = P_F/P_S condition in the indifference-curve framework.

If we cross-multiply terms in Equation A.3.4, we get an equivalent condition that has a very straightforward intuitive interpretation:
 

.

(A.3.5)

In words, Equation A.3.5 tells us that the ratio of marginal utility to price must be the same for all goods at the optimal bundle. The following examples illustrate why this condition must be satisfied if the consumer has allocated his budget optimally.
Example A.3-2

Suppose that the marginal utility of the last dollar John spends on food is greater than the marginal utility of the last dollar he spends on shelter. For example, suppose the prices of food and shelter are $1/lb and $2/sq yd, respectively, and that the corresponding marginal utilities are 6 and 4. Show that John cannot possibly be maximizing his utility. 

Abstracting from the special case of corner solutions, a necessary condition for optimal budget allocation is that the last dollar spent on each commodity yield the same increment in utility.
Example A.3-3

Mary has a weekly allowance of $10, all of which she spends on newspapers (N.) and magazines (M), whose respective prices are $1 and $2. Her utility from these purchases is given by U(N) + V(M). If the values of U(N) and V(M) are as shown in the table, is Mary a utility maximizer if she buys 4 magazines and 2 newspapers each week? Of not, how should she reallocate her allowance?

For Mary to be a utility maximizer, extra utility per dollar must be the same for both the last newspaper and the last magazine she purchased. But since the second newspaper provided 8 additional utils per dollar spent, which is four times the 2 utils per dollar she got from the fourth magazine (4 extra utils at a cost of $2), Mary is not a utility maximizer.

To see clearly how she should reallocate her purchases, let us rewrite the table to include the relevant information on marginal utilities.
 

From this table, we see that there are several bundles for which MU(N)/P_N = MU(M)/P_M—namely, 3 newspapers and 2 magazines; or 4 newspapers and 3 magazines; or 5 newspapers and 4 magazines. The last of these bundles yields the highest total utility but costs $13, and is hence beyond Mary's budget constraint. The first, which costs only $7, is affordable, but so is the second, which costs exactly $10 and yields higher total utility than the first. With 4 newspapers and 3 magazines, Mary gets 4 utils per dollar from her last purchase in each category. Her total utility is 70 utils, which is 6 more than she got from the original bundle.

In Example A.3-3, note that if all Mary's utility values were doubled, or cut by half, she would still do best to buy 4 newspapers and 3 magazines each week. This illustrates the claim, that consumer choice depends not on the absolute number of utils associated with different bundles, but instead on the ordinal ranking of the utility levels associated with different bundles. If we double all the utils associated with various bundles, or cut them by half, the ordinal ranking of the bundles will be preserved, and thus the optimal bundle will remain the same. This will also be true if we take the logarithm of the utility function, the square root of it, or add 5 to it, or transform it in any other way that preserves the ordinal ranking of different bundles.

Cardinal versus Ordinal Utility

In our discussion about how to represent consumer preferences, we assumed that people are able to rank each possible bundle in order of preference. This is called the ordinal utility approach to the consumer budgeting problem. It does not require that people be able to make quantitative statements about how much they like various bundles. Thus it assumes that a consumer will always be able to say whether he prefers A to B, but that he may not be able to make such statements as "A is 6.43 times as good as B."

In the nineteenth century, economists commonly assumed that people could make such statements. Today we call theirs the cardinal utility approach to the consumer choice problem. In the two-good case, it assumes that the satisfaction provided by any bundle can be assigned a numerical, or cardinal, value by a utility function of the form
 

U = U(X, Y).

(A.3.6)

In three dimensions, the graph of such a utility function will look something like the one shown in Figure A.3-3. It resembles a mountain, but because of the more-is-better assumption, it is a mountain without a summit. The value on the U axis measures the height of the mountain, which continues to increase the more we have of X or Y.

Suppose in Figure A.3-3 we were to fix utility at some constant amount, say, U0. That is, suppose we cut the utility mountain with a plane parallel to the XY plane, U0 units above it. The line labeled JK in Figure A.3-3 represents the intersection of that plane and the surface of the utility mountain. All the bundles of goods that lie on JK provide a utility level of U0. If we then project the line JK downward onto the XY plane, we have what amounts to the U0 indifference curve, shown in Figure A.3-3.

Suppose we then intersect the utility mountain with another plane, this time U1 units above the XY plane. In Figure A.3-3, this second plane intersects the utility mountain along the line labeled LN. It represents the set of all bundles that confer the utility level U1. Projecting LN down onto the XY plane, we thus get the indifference curve labeled U1 in Figure A.3-3. In like fashion, we can generate an entire indifference map corresponding to the cardinal utility function U(X, Y).

Thus we see that it is possible to start with any cardinal utility function and end up with a unique indifference map. But it is not possible to go in the other direction! That is, it is not possible to start with an indifference map and work backward to a unique cardinal utility function. The reason is that there will always be infinitely many such utility functions that give rise to precisely the same indifference map.

To see why, just imagine that we took the utility function in Equation A.3.4 and doubled it, so that utility is now given by V = 2U(X, Y). When we graph V as a function of X and Y. the shape of the resulting utility mountain will be much the same as before. The difference will be that the altitude at any X, Y point will be twice what it was before. If we pass a plane 2 U0 units above the XY plane, it would intersect the new utility mountain in precisely the same manner as the plane U0 units high did originally. If we then project the resulting intersection down onto the XY plane, it will coincide perfectly with the original U0 indifference curve.

All we do when we multiply (divide, add to, or subtract from) a cardinal utility function is to relabel the indifference curves to which it gives rise. Indeed, we can make an even more general statement: If U(X, Y) is any cardinal utility function and if V is any increasing function, then U = U(X, Y) and V = V[U(X, Y)] will give rise to precisely the same indifference maps. The special property of an increasing function is that it preserves the rank ordering of the values of the original function. That is, if U(X1, Y1) > U(X2, Y2), the fact that V is an increasing function assures that V[U(X1, Y1)] will be greater than V[U(X2, Y2)]. And as long as that requirement is met, the two functions will give rise to exactly the same indifference curves.

The concept of the indifference map was first discussed by Francis Edgeworth, who derived it from a cardinal utility function in the manner described above. It took the combined insights of Vilfredo Pareto, Irving Fisher, and John Hicks to establish that Edgeworth's apparatus was not uniquely dependent on a supporting cardinal utility function. As we have seen, the only aspect of a consumer's preferences that matters in the standard budget allocation problem is the shape and location of his indifference curves. Consumer choice turns out to be completely independent of the labels we assign to these indifference curves, provided only that higher curves correspond to higher levels of utility.

Modern economists prefer the ordinal approach because it rests on much weaker assumptions than the cardinal approach. That is, it is much easier to imagine that people can rank different bundles than to suppose that they can make precise quantitative statements about how much satisfaction each provides.

Generating Indifference Curves Algebraically

Even if we assume that consumers have only ordinal preference rankings, it will often be convenient to represent those preferences with a cardinal utility index. The advantage is that this procedure provides a compact algebraic way of summarizing all the information that is implicit in the graphical representation of preferences.

Suppose, for example, that Tom's utility function is given by U(X, Y) = XY, and we want to use this information to graph Tom's indifference map. In the language of utility functions, an indifference curve is all combinations of X and Y that yield the same level of utility. Suppose we look at the indifference curve that corresponds to 1 unit of utility—that is, the combinations of bundles for which XY = 1. Solving this equation for Y, we have
 

.

(A.3.7)

which is the indifference curve labeled U = 1 in Figure A.3-5. The indifference curve that corresponds to 2 units of utility is generated by solving XY = 2 to get Y = 2/X and it is shown by the curve labeled U = 2 in Figure A.3-5. In similar fashion, we generate the indifference curves for U = 3 and U = 4, which are correspondingly labeled in the diagram. More generally, we get the indifference curve corresponding to a utility level of U0 by solving XY = U0to get Y = U0/X.

Consider another illustration, this time with U(X, Y) = (²/₃)X + 2Y. The bundles of X and Y that yield a utility level of U0 are again found by solving U(X, Y) = U0 for Y. This time we get Y = (U0/2) - (¹/₃)X. The indifference curves corresponding to U = 1, U = 2, and U = 3 are shown in Figure A.3-6. Note that they are all linear, which tells us that this particular utility function describes a preference ordering in which X and Y are perfect substitutes.

Problems

1. Tom spends all his $100 weekly income on two goods, X and Y. His utility function is given by U(X, Y) = XY. If P_X = 4 and P_Y = 10, how much of each good should he buy?
2. Same as Problem 1, except now Tom's utility function is given by U(X, Y) = X^l/2Y^l/2
3. Note the relationship between your answers in Problems 1 and 2. What accounts for this relationship?
4. Sue consumes only two goods, food and clothing. The marginal utility of the last dollar she spends on food is 12, and the marginal utility of the last dollar she spends on clothing is 9. The price of food is $1.20/unit, and the price of clothing is $0.90/unit. Is Sue maximizing her utility?
5. Albert has a weekly allowance of $17, all of which he spends on used CDs (C) and movie rentals (M), whose respective prices are $4 and $3. His utility from these purchases is given by U(C) + V(M). If the values of U(C) and V(M) are as shown in the table, is Albert a utility maximizer if he buys 2 CDs and rents 3 movies each week? If not, how should be reallocate his allowance?

¹The term "utils" represents an arbitrary unit. As we will see, what is important for consumer choice is not the actual number of utils various bundles provide, but the rankings of the bundles based on their associated utilities.

²Assuming that the second-order conditions for a local maximum are also met.

³Here, the second-order condition for a local maximum is that d ²U/dX ² < O.

⁴Again, an increasing function is one for which V(X₁) > V(X₂) whenever X_l > X₂.