16. Understanding Consumer Behavior. 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
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Understanding Consumer Behavior
an introduction to the most prominent work on consumption, including:
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.
income seemed to be the main determinant of consumption
Consumption function from cross-sectional household data
(falling APC )
YThe Consumption Puzzle
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)
present value of lifetime consumption
present value of lifetime income
Consump = income in both periods
Y1The intertemporal budget constraint
Higher indifference curves represent higher levels of happiness.
The slope of an indifference curve at any point equals the MRSat that point.
At the optimal point, MRS = 1+r
C1How C responds to changes in Y
Results: Provided they are both normal goods, C1 and C2 both increase,
…whether the income increase occurs in period 1 or period 2.
Y1How C responds to changes in r
As depicted here, C1 falls and C2 rises.
However, it could turn out differently…
Whether C1 rises or falls depends on the relative size of the income & substitution effects.
…and her consumption may behave as in the Keynesian theory even though she is rational & forward-looking.
C1Consumer optimization when the borrowing constraint is not binding
But since the consumer cannot borrow, the best he can do is point E.
C1Consumer optimization when the borrowing constraint is binding
and saving allows the consumer to achieve smooth consumption.
W = initial wealth
Y = annual income until retirement (assumed constant)
R = number of years until retirement
T = lifetime in years
C = (W + RY)/T , or
C = aW +bY
a = (1/T) is the marginal propensity to consume out of wealth
b = (R/T) is the marginal propensity to consume out of income
The LCH can solve the consumption puzzle:
End of lifeImplications of the Life-Cycle Hypothesis
Y = current income
YP = permanent incomeaverage income, which people expect to persist into the future
YT = transitory incometemporary deviations from average income
C = aYP
where a is the fraction of permanent income that people consume per year.
The PIH can solve the consumption puzzle:
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).
1. Keynesian consumption theory
2. Fisher’s theory of intertemporal choice
3. Modigliani’s life-cycle hypothesis
4. Friedman’s permanent-income hypothesis
5. Hall’s random-walk hypothesis
6. Laibson and the pull of instant gratification