1. Consumer Preferences chapter 4
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2. Learning Objectives
Explain the Ranking Principle and the Choice Principle.
Illustrate consumers’ preferences for consumption bundles graphically through indifference curves.
Understand the properties and functions of indifference curves.
Determine a consumer’s willingness to trade one good for another by examining indifference curves.
Explain the concept of utility and compare consumption bundles by calculating the numerical values of a given utility function.
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3. Overview
Rational consumers follow two basic decision-making principles
Ranking principle
Choice principle
Consumer preferences can be graphically represented through indifference curves
Preferences and indifference curves real a consumer’s willingness to substitute one good for another
Economists use the utility function to describe consumer preferences
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4. Principles of Consumer Choice
Preferences tell us about a consumer’s likes and dislikes
Ranking principle: a consumer can rank, in order of preference (though possibly with ties), all potentially available preferences
Choice principle: among the available alternatives, the consumer selects the one that he ranks the highest
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5. Ranking Principle Implications
Preferences are complete: between any pair of alternatives, a consumer either prefers one to the other or is indifferent between them.
A consumer is indifferent between two alternatives if he likes (or dislikes) them equally
Preferences are transitive: if the consumer prefers one alternative to a second, and prefers the second alternative to a third, then he also prefers the first alternative to the third.
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6. Consumption Bundles
A consumption bundle is the collection of goods that an individual consumes over a given period
Consumer choices reflect how a consumer feels about various consumption bundles, rather than how the consumer feels about one good in isolation
More-is-better principle: when one consumption bundle contains more of every good than a second bundle, a consumer prefers the first bundle to the second.
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7. Ranking Consumption Bundles – Example
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8. Indifference Curves
An indifference curve shows all consumption bundles that a consumer likes equally well
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9. Properties of Indifference Curves
Indifference curves are thin
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10. Properties of Indifference Curves
Indifference curves are thin
ICs do not slope upward
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11. Properties of Indifference Curves
Better-than-A bundles
Indifference curves are thin
IC do not slope upward
The IC that runs through consumption bundle A separates all the better-than-A bundles from the worse-than-A bundles
Worse-than-A bundles
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12. Families of Indifference Curves
A family of indifference curves is a collection of indifference curves that represent one individual’s preferences
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13. Properties of Indifference Curves
Indifference curves are thin
IC do not slope upward
The IC that runs through consumption bundle A separates all the better-than-A bundles from the worse-than-A bundles
IC curves from the same family do not cross
By the transitive property of the ranking principle, the consumer should be indifferent between B and D
By being on the same indifference curve, the consumer is indifferent between A and B.
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14. Properties of Indifference Curves
Indifference curves are thin
IC do not slope upward
The IC that runs through consumption bundle A separates all the better-than-A bundles from the worse-than-A bundles
IC curves from the same family do not cross
Consumer prefers bundles on IC furthest from the origin
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15. Plotting Indifference Curves from a Formula
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16. Goods vs. Bads
A bad is an object, condition, or activity that makes a consumer worse off
We can think of a bad as the absence of a good
For example, we can think of a student choosing leisure time (a good) instead of study time (a bad)
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17. Substitution between Goods
A consumer can substitute one good for another so that she is indifferent between the two bundles
For example, in moving from bundle A to bundle B, Madeline loses 1 pint of yogurt and gains 2 ounces of pizza
The rate at which Madeline is willing to substitute for yogurt with pizza is 2 ounces per pint
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18. Substitution between Goods
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19. Indifference Curves and the MRS
The marginal rate of substitution for frozen yogurt with pizza at bundle A equals the slope of the line drawn tangent to the indifference curve running through point A.
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20. What Determines Rates of Substitution?
Within the same indifference curve, as pizza becomes more scarce and yogurt becomes more plentiful, the MRS for yogurt with pizza falls.
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21. What Determines Rates of Substitution?
Angie likes yogurt relative to pizza more than Marcus
To keep Angie indifferent after sacrificing one pint of yogurt, she needs three additional ounces of pizza
Compare with Marcus, who would need just ½ ounce of pizza to compensate for one less pint of yogurt
Angie’s MRS for yogurt with pizza is higher than Marcus’ MRS
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22. Why Are Rates of Substitution Important?
Mutually beneficial trade depends on the marginal rates of substitution of the individual potentially involved in a trading relationship.
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23. Special Case – Perfect Substitutes
Two products are perfect substitutes when their functions are identical, so that a consumer is willing to swap one for the other at a fixed rate.
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24. Special Case – Perfect Complements
Two products are perfect complements if they are valuable only when used together in fixed proportions.
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25. Utility
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26. From Indifference Curves to Utility Function
Same utility value to all points on a single indifference curve.
Higher utility values for indifference curves that correspond to higher levels of well-being (further from the origin).
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27. From Utility Function to Indifference Curves
The indifference curve passing through A consists of all the bundles for which the height of the utility function is the same.
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28. Ordinal vs. Cardinal Utility
Ordinal information allows us to determine only whether one alternative is better or worse than another.
Cardinal information tells us something about the intensity of those preferences.
In modern microeconomic theory, utility functions are only intended to summarize ordinal information, rejecting old cardinal interpretations of utility.
Example: if one consumption bundle has a utility value of 10 and a second has a utility value of 5, we know the consumer prefers the first to the second, but it does not necessarily make him twice as happy.
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29. Utility Functions and the MRS
Marginal utility is the change in the consumer’s utility resulting from the addition of a very small amount of some good, divided by the amount added.
Moving along the same indifference curve, for small changes in X and Y…
Replacing (1) into (2)…
From the definition of MRS…
Replacing (3) into (4)…
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30. Utility Functions and the MRS
The marginal utility associated with a particular good is meaningless, since its unit has no practical meaning.
On the other hand, the marginal rate of substitution is the ratio of marginal utilities, and thus it is unit free. If we change the units of the utility function, the MRS remains constant.
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31. Some Special Utility Functions
Cobb-Douglas utility functions
Quasi-linear utility functions
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32. Cobb-Douglas Utility Functions
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33. Cobb-Douglas Utility Functions
Two families of indifference curves for Cobb-Douglas utility functions
Light red: a = b = 1
Dark red: a = 3 and b = 1
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34. Quasi-Linear Utility Functions
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35. Review
Consumer theory assumes that consumers rank available bundles and choose the one they rank highest (ranking and choice principles).
Consumer preferences can be graphically represented with indifference curves.
The marginal rate of substitution varies across consumers according to the relative importance the consumer attaches to the goods in question.
Usually the marginal rate of substitution for X with Y declines as X becomes more plentiful and Y becomes more scarce (declining MRS).
The concept of utility summarizes everything that is known about a consumer’s preferences.
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36. Looking Forward
Next we will introduce the budget line, and how it interacts with the indifference curves to determine the consumer’s best choice and yield the consumer’s demand.
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