0. 16 Understanding Consumer Behavior
This long chapter is a survey of the most prominent work on consumption since Keynes. It is particularly useful to students who expect to continue with graduate studies in economics.
After reviewing the Keynesian consumption function and its implications, the chapter presents Irving Fisher’s theory of intertemporal choice, the basis for much subsequent work on consumption. This section of the chapter uses indifference curves and budget constraints. The chapter and this PowerPoint presentation do not require or assume that students know these tools. But if they do not, this section of the chapter is the most difficult.
The chapter then presents the Life-Cycle and Permanent Income Hypotheses, and discusses Hall’s Random Walk Hypothesis.
Finally, there is a brief discussion of some very recent work by Laibson and others on psychology and economics, in particular how the pull of instant gratification can cause consumers to deviate from perfect rationality.

1. IN THIS CHAPTER, YOU WILL LEARN:
an introduction to the most prominent work on consumption, including:
John Maynard Keynes: consumption and current income
Irving Fisher: intertemporal choice
Franco Modigliani: the life-cycle hypothesis
Milton Friedman: the permanent income hypothesis
Robert Hall: the random-walk hypothesis
David Laibson: the pull of instant gratification 1

2. Keynes’s conjectures 1. 0 < MPC < 1
2. Average propensity to consume (APC ) falls as income rises. (APC = C/Y )
3. Income is the main determinant of consumption.
The MPC was defined in Chapter 3 and used in various chapters since.

3. The Keynesian consumption function
c = MPC
= slope of the consumption function c
Interpretations of C-bar:
autonomous consumption: the portion of consumption that does not depend on income
the value of consumption if income were zero.
a shift parameter 1

4. The Keynesian consumption function
As income rises, consumers save a bigger fraction of their income, so APC falls. C Y
Pick a point on the consumption function; that point represents a particular combination of consumption and income.
Now draw a ray from the origin to that point. The slope of that ray equals the average propensity to consume at that point.
(Why? The slope equals the rise over the run. The rise from zero to that point equals the value of C at that point. The run from zero to that point equals the value of Y at that point. Hence, the rise over the run equals C/Y, or the APC.)
At higher values of Y, the APC (or the slope of the ray from the origin) is smaller. This is what Keynes conjectured: at higher values of income, people spend a smaller fraction of their income.
slope = APC

5. Early empirical successes: Results from early studies
Households with higher incomes:
consume more, so MPC > 0
save more, so MPC < 1
save a larger fraction of their income, so APC i as Y h
Very strong correlation between income and consumption: income seemed to be the main determinant of consumption

6. Problems for the Keynesian consumption function
Based on the Keynesian consumption function, economists predicted that C would grow more slowly than Y over time.
This prediction did not come true:
As incomes grew, APC did not fall, and C grew at the same rate as income.
Simon Kuznets showed that C/Y was very stable from decade to decade.

7. The Consumption Puzzle
Consumption function from long time-series data (constant APC ) C Y
Consumption function from cross-sectional household data
(falling APC )
Figure 16-2, p.469.

8. Irving Fisher and Intertemporal Choice
The basis for much subsequent work on consumption.
Assumes consumer is forward-looking and chooses consumption for the present and future to maximize lifetime satisfaction.
Consumer’s choices are subject to an intertemporal budget constraint, a measure of the total resources available for present and future consumption.

9. The basic two-period model
Period 1: the present Period 2: the future
Notation
Y1, Y2 = income in period 1, 2
C1, C2 = consumption in period 1, 2
S = Y1 - C1 = saving in period 1
(S < 0 if the consumer borrows in period 1)
Note: There is no saving in period 2. Period 2 is the final period, and there are no bequests, so saving in period 2 would only reduce lifetime consumption and therefore lifetime utility/satisfaction.

10. Deriving the intertemporal budget constraint
Period 2 budget constraint:
Rearrange terms:
Explain the intuition/interpretation of the period 2 budget constraint. If students understand it, then everything else follows nicely.
Divide through by (1+r ) to get…

11. The intertemporal budget constraint
present value of lifetime consumption
present value of lifetime income
If your students are not familiar with the present value concept, it is explained in a very nice FYI box on p.472.

12. The intertemporal budget constraint
Saving
Consump = income in both periods
The budget constraint shows all combinations of C1 and C2 that just exhaust the consumer’s resources.
The point (Y1, Y2) is always on the budget line because C1=Y1, C2=Y2 is always possible, regardless of the real interest rate or the existence of borrowing constraints.
To obtain the expression for the horizontal intercept, set C2=0 in the equation for the intertemporal budget constraint and solve for C1. Similarly, the expression for the vertical intercept is the value of C2 when C1=0. There is intuition for these expressions. Take the vertical intercept, for example. If the consumer sets C1=0, then he will be saving all of his first-period income. In the second period, he gets to consume this saving plus interest earned, (1+r)Y1, as well as his second-period income.
If the consumer chooses C1<Y1, then the consumer will be saving, so his C2 will exceed his Y2.
Conversely, if consumer chooses C1>Y1, then consumer is borrowing, so his second-period consumption will fall short of his second-period income (he must use some of the second-period income to repay the loan).
Borrowing Y1 Y2

13. The intertemporal budget constraint
The slope of the budget line equals -(1+r ) 1
(1+r )
The slope of the budget line equals -(1+r):
To increase C1 by one unit, the consumer must sacrifice (1+r) units of C2. Y1 Y2 C1

14. Consumer preferences
Higher indifference curves represent higher levels of happiness. C1 C2
An indifference curve shows all combinations of C1 and C2 that make the consumer equally happy.
IC2
IC1

15. Consumer preferences
The slope of an indifference curve at any point equals the MRS at that point. C1 C2
Marginal rate of substitution (MRS ): the amount of C2 the consumer would be willing to substitute for one unit of C1.
IC1 1
MRS

16. Optimization C1 C2
The optimal (C1,C2) is where the budget line just touches the highest indifference curve.
At the optimal point, MRS = 1+r
All points along the budget line are affordable, including the two points where the orange indifference curve intersects the budget line. However, the consumer prefers (and can afford) point O to these points, because O is on a higher indifference curve.
At the optimal point, the slope of the indifference curve (MRS) equals the slope of the budget line (1+r). O

17. How C responds to changes in Y
An increase in Y1 or Y2 shifts the budget line outward.
Results: Provided they are both normal goods, C1 and C2 both increase,
…whether the income increase occurs in period 1 or period 2.

18. Keynes vs. Fisher
Keynes: Current consumption depends only on current income.
Fisher: Current consumption depends only on the present value of lifetime income. The timing of income is irrelevant because the consumer can borrow or lend between periods.
Fisher’s point about the irrelevance of the timing of income is the basis of Ricardian equivalence, covered in Chapter 19.

19. How C responds to changes in r
An increase in r pivots the budget line around the point (Y1,Y2 ).
As depicted here, C1 falls and C2 rises.
However, it could turn out differently… A A B Y1 Y2

20. How C responds to changes in r
income effect: If consumer is a saver, the rise in r makes him better off, which tends to increase consumption in both periods.
substitution effect: The rise in r increases the opportunity cost of current consumption, which tends to reduce C1 and increase C2.
Both effects imply ΔC2.
Whether C1 rises or falls depends on the relative size of the income & substitution effects.
Note: Keynes conjectured that the interest rate matters for consumption only in theory. In Fisher’s theory, the interest rate doesn’t affect current consumption if the income and substitution effects are of equal magnitude.
After you have shown and explained this slide, it would be useful to pause for a moment and ask your students (perhaps working in pairs) to do the analysis of an increase in the interest rate on the consumption choices of a borrower. In that case, the income effect tends to reduce both current and future consumption, because the interest rate hike makes the borrower worse off. The substitution effect still tends to increase future consumption while reducing current consumption. In the end, current consumption falls unambiguously; future consumption falls if the income effect dominates the substitution effect, and rises if the reverse occurs.

21. Constraints on borrowing
In Fisher’s theory, the timing of income is irrelevant: Consumer can borrow and lend across periods.
Example: If consumer learns that her future income will increase, she can spread the extra consumption over both periods by borrowing in the current period.
However, if consumer faces borrowing constraints (a.k.a. liquidity constraints), then she may not be able to increase current consumption
…and her consumption may behave as in the Keynesian theory even though she is rational & forward-looking.

22. Constraints on borrowing
The budget line with no borrowing constraints Y2 Y1

23. Constraints on borrowing
The borrowing constraint takes the form:
C1 ≤ Y1
The budget line with a borrowing constraint
Similar to Figure 17-8 on p. 508. Y2 Y1

24. Consumer optimization when the borrowing constraint is not binding
The borrowing constraint is not binding if the consumer’s optimal C1 is less than Y1.
Figure 16-9, panel (a), on p.479
In this case, the consumer optimally was not going to borrow, so his inability to borrow has no impact on his choices. Y1

25. Consumer optimization when the borrowing constraint is binding
The optimal choice is at point D.
But since the consumer cannot borrow, the best he can do is point E.
Figure 16-9, panel (b), on p.479
In this case, the consumer would like to borrow to achieve his optimal consumption at point D. If he faces a borrowing constraint, though, then the best he can do is at point E.
If you have a few minutes of class time available, have your students do the following experiment:
Suppose Y1 is increased by $1000 while Y2 is reduced by $1000(1+r), so that the present value of lifetime income is unchanged. Determine the impact on C1:
- when consumer does not face a binding borrowing constraint - when consumer does face a binding borrowing constraint
Note that the intertemporal redistribution of income in this exercise could be achieved by a debt-financed tax cut in period 1, followed by a tax increase in period 2 that is just sufficient to retire the debt. E D Y1

26. The Life-Cycle Hypothesis
due to Franco Modigliani (1950s)
Fisher’s model says that consumption depends on lifetime income, and people try to achieve smooth consumption.
The LCH says that income varies systematically over the phases of the consumer’s life cycle,
and saving allows the consumer to achieve smooth consumption.

27. The Life-Cycle Hypothesis
The basic model:
W = initial wealth
Y = annual income until retirement (assumed constant)
R = number of years until retirement
T = lifetime in years
Assumptions:
zero real interest rate (for simplicity)
consumption smoothing is optimal
The consumer’s initial wealth could be zero, or could be a gift or inheritance from her parents.

28. The Life-Cycle Hypothesis
Lifetime resources = W + RY
To achieve smooth consumption, consumer divides her resources equally over time:
C = (W + RY )/T , or
C = aW + bY
where
a = (1/T ) is the marginal propensity to consume out of wealth
b = (R/T ) is the marginal propensity to consume out of income

29. Implications of the Life-Cycle Hypothesis
The LCH can solve the consumption puzzle:
The life-cycle consumption function implies APC = C/Y = a(W/Y ) + b
Across households, income varies more than wealth, so high-income households should have a lower APC than low-income households.
Over time, aggregate wealth and income grow together, causing APC to remain stable.

30. Implications of the Life-Cycle Hypothesis$
LCH implies that saving varies systematically over a person’s lifetime.
Wealth
Income
Figure 16-12, p.482.
Saving
Consumption
Dissaving
Retirement begins
End of life

31. The Permanent Income Hypothesis
due to Milton Friedman (1957)
Y = Y P + Y T
where
Y = current income
Y P = permanent income average income, which people expect to persist into the future
Y T = transitory income temporary deviations from average income
The middle of page 484 gives two hypothetical examples that help students understand the concepts of permanent and transitory income.

32. The Permanent Income Hypothesis
Consumers use saving & borrowing to smooth consumption in response to transitory changes in income.
The PIH consumption function:
C = a Y P
where a is the fraction of permanent income that people consume per year.

33. The Permanent Income Hypothesis
The PIH can solve the consumption puzzle:
The PIH implies APC = C / Y = a Y P/ Y
If high-income households have higher transitory income than low-income households, APC is lower in high-income households.
Over the long run, income variation is due mainly (if not solely) to variation in permanent income, which implies a stable APC.

34. PIH vs. LCH
Both: people try to smooth their consumption in the face of changing current income.
LCH: current income changes systematically as people move through their life cycle.
PIH: current income is subject to random, transitory fluctuations.
Both can explain the consumption puzzle.

35. The Random-Walk Hypothesis
due to Robert Hall (1978)
based on Fisher’s model & PIH, in which forward-looking consumers base consumption on expected future income
Hall adds the assumption of rational expectations, that people use all available information to forecast future variables like income.

36. The Random-Walk Hypothesis
If PIH is correct and consumers have rational expectations, then consumption should follow a random walk: changes in consumption should be unpredictable.
A change in income or wealth that was anticipated has already been factored into expected permanent income, so it will not change consumption.
Only unanticipated changes in income or wealth that alter expected permanent income will change consumption.

37. Implication of the R-W Hypothesis
If consumers obey the PIH and have rational expectations, then policy changes will affect consumption only if they are unanticipated.
This result is important because many policies affect the economy by influencing consumption and saving. For example, a tax cut to stimulate aggregate demand only works if consumers respond to the tax cut by increasing spending. The R-W Hypothesis implies that consumption will respond only if consumers had not anticipated the tax cut.
This result also implies that consumption will respond immediately to news about future changes in income. Students connect with the following example: Suppose a student is job-hunting in her senior year for a job that will begin after graduation. If the student secures a job with a higher salary than she had expected, she is likely to start spending more now in anticipation of the higher-than-expected permanent income.

38. The Psychology of Instant Gratification
Theories from Fisher to Hall assume that consumers are rational and act to maximize lifetime utility.
Recent studies by David Laibson and others consider the psychology of consumers.

39. The Psychology of Instant Gratification
Consumers consider themselves to be imperfect decision makers.
In one survey, 76% said they were not saving enough for retirement.
Laibson: The “pull of instant gratification” explains why people don’t save as much as a perfectly rational lifetime utility maximizer would save.

40. Two questions and time inconsistency
1. Would you prefer (A) a candy today, or (B) two candies tomorrow?
2. Would you prefer (A) a candy in 100 days, or (B) two candies in 101 days?
In studies, most people answered (A) to 1 and (B) to 2.
A person confronted with question 2 may choose (B).
But in 100 days, when confronted with question 1, the pull of instant gratification may induce her to change her answer to (A).
Discussed further in Chapter 18, the text introduces time inconsistency here.
Simple definition (from p.490): A person’s preferences are time-inconsistent if they alter their decisions simply because time passes.

41. Summing up Keynes: consumption depends primarily on current income.
Recent work: consumption also depends on
expected future income
wealth
interest rates
Economists disagree over the relative importance of these factors, borrowing constraints, and psychological factors.

42. CHAPTER SUMMARY 1. Keynesian consumption theory Keynes’s conjectures
MPC is between 0 and 1
APC falls as income rises
current income is the main determinant of current consumption
Empirical studies
in household data & short time series: confirmation of Keynes’s conjectures
in long-time series data: APC does not fall as income rises 42

43. CHAPTER SUMMARY 2. Fisher’s theory of intertemporal choice
Consumer chooses current & future consumption to maximize lifetime satisfaction of subject to an intertemporal budget constraint.
Current consumption depends on lifetime income, not current income, provided consumer can borrow & save. 43

44. CHAPTER SUMMARY 3. Modigliani’s life-cycle hypothesis
Income varies systematically over a lifetime.
Consumers use saving & borrowing to smooth consumption.
Consumption depends on income & wealth. 44

45. CHAPTER SUMMARY 4. Friedman’s permanent-income hypothesis
Consumption depends mainly on permanent income.
Consumers use saving & borrowing to smooth consumption in the face of transitory fluctuations in income. 45

46. CHAPTER SUMMARY 5. Hall’s random-walk hypothesis
Combines PIH with rational expectations.
Main result: changes in consumption are unpredictable, occur only in response to unanticipated changes in expected permanent income. 46

47. CHAPTER SUMMARY 6. Laibson and the pull of instant gratification
Uses psychology to understand consumer behavior.
The desire for instant gratification causes people to save less than they rationally know they should. 47