微观经济学
(平狄克，鲁宾费尔德，第四版)
LECTURE 3 消费者理论
一、消费者行为
基本概念
市场篮子、无差异曲线、无差异曲线图、边际替代率、效用、基数效用函数和序数效用函数、预算约束、预算线、拐点解、显示性偏好、边际效用、消费者物价指数（CPI）、拉氏指数、派氏指数、连锁权重物价指数
消费者行为研究的三个步骤：
消费者偏好-预算约束-结合前面二者确定消费者选择
消费者偏好
基本假设：
偏好的完全性、可传递性，以及“多总比少好”。
无差异曲线的特征：
向下倾斜
其右上和左下分别表示偏好于和不偏好于
每条无差异曲线绝不相交
边际替代率（无差异曲线的斜率）递减（无差异曲线是凸的）。
完全替代品和完全互补品，图PP5，6。
思考：BADS，无差异曲线将是怎么样的？
序数排列就足以解释大多数消费者决策是如何做出的。
Consumer Preferences
Orange Juice
(glasses)
Apple
Juice
(glasses)
2
3
4
1
1
2
3
4
0
Perfect
Substitutes
Consumer Preferences
Right Shoes
Left
Shoes
2
3
4
1
1
2
3
4
0
Perfect
Complements
预算约束
预算线的斜率反映了两种商品的相对价格，是两种商品价格比取负；截距表示使用已有收入能够购买的最多单种商品。
收入变化：
收入增加/减少使预算线向外/内平移。图PP8；
价格变化：
一种商品价格变化，预算线以另一种商品的截距点为原点旋转。图：PP9；
两种商品价格发生同比变化，预算线斜率不变，向外或向内平移。
Budget Constraints
Food
(units per week)
Clothing
(units
per week)
80
120
160
40
20
40
60
80
0
A increase in
income shifts
the budget line
outward
(I = $160)
L2
(I = $80)
L1
L3
(I =
$40)
A decrease in
income shifts
the budget line
inward
Budget Constraints
Food
(units per week)
Clothing
(units
per week)
80
120
160
40
40
(PF = 1)
L1
An increase in the
price of food to
$ changes
the slope of the
budget line and
rotates it inward.
L3
(PF = 2)
(PF = 1/2)
L2
A decrease in the
price of food to
$.50 changes
the slope of the
budget line and
rotates it outward.
消费者选择
最大化消费者满足的市场篮子必须具备两个条件：
应在预算线上，
应给予消费者最受偏好的商品和劳务组合。
根据无差异曲线的斜率MRS=-C/F，预算线斜率为-Pf/Pc，最大化条件是MRS=Pf/Pc
因此，当边际替代率等于两种商品价格比时，消费者获得最大满足。图 PP11。
拐点解：
发生在无差异曲线与横轴或纵轴相切时，此时MRSPa/Pb。图 PP12。
Consumer Choice
U2
Pc = $2 Pf = $1 I = $80
Budget Line
A
At market basket A
the budget line and the
indifference curve are
tangent and no higher
level of satisfaction
can be attained.
At A:
MRS =Pf/Pc = .5
Food (units per week)
Clothing
(units per
week)
40
80
20
20
30
40
0
A Corner Solution
Ice Cream (cup/month)
Frozen
Yogurt
(cups
monthly)
B
A
U2
U3
U1
A corner solution
exists at point B.
边际效用与消费者选择
边际效用递减法则
相等边际原则：
边际效用与无差异曲线：
又，在无差异曲线上：MRS=-C/F
因此
又根据消费者最大化其满足感时有MRS=Pf/Pc
因此有效用最大化条件（相等边际原则）：
生活成本指数
拉氏指数VS派氏指数：
两种指数都涉及基期和当前的价格水平对比；
拉氏指数立足于基期消费构成，派氏指数立足于当前消费构成；
拉氏指数倾向于高估生活成本，派氏指数倾向于低估生活成本。
二、个体需求和市场需求
个体需求
需求曲线的两个特点：
沿曲线移动时可获得的效用发生变化；
曲线上的每点消费者都是通过实现两种商品的边际替代率等于其价格比来实现效用最大化的。
收入-消费曲线
描画出与每种收入水平相联系的效用最大化商品组合。
收入增/减将使预算线右/左移，并使消费点沿收入-消费曲线右/左移；同时使需求曲线右/左移。
当收入-消费曲线有正的斜率时，需求随收入增加而增加，需求的收入弹性为正，此种商品为正常商品；
当收入-消费曲线有负的斜率时，需求随收入增加而减少，需求的收入弹性为负，此种商品为劣质商品。
An Inferior Good
Hamburger
(units per month)
Steak
(units per
month)
15
30
U3
C
Income-Consumption
Curve
…but hamburger
becomes an inferior
good when the income
consumption curve
bends backward
between B and C.
10
5
20
5
10
A
U1
B
U2
Both hamburger
and steak behave
as a normal good,
between A and B...
个体需求
恩格尔曲线：
将商品的消费量与收入联系起来。
正常商品的恩格尔曲线向上倾斜;
劣质商品的恩格尔曲线则向下倾斜。
Engel Curves
Engel curves slope
backward bending
for inferior goods.
Inferior
Normal
Food (units
per month)
30
4
8
12
10
Income
($ per
month)
20
16
0
个体需求
替代品VS互补品
一种商品价格上涨/下跌导致另一种商品需求上升/下降，这两种商品是替代品；
一种商品价格上涨/下跌导致另一种商品需求下降/上升，这两种商品是互补品。
价格消费曲线如向下倾斜，则两种商品被视为替代品；该曲线向上倾斜，则两种商品被视为互补品；
值得注意的是，两种商品可能在一定条件下是前者，一旦条件改变（如收入变化）则变成后者。
替代品VS互补品
Price-Consumption Curve
Food (units
per month)
Clothing
(units per
month)
4
5
6
U2
U3
A
B
D
U1
4
12
20
A-B,替代品；B-D,互补品
个体需求
替代效应与收入效应:
替代效应：在效用保持恒定的条件下因一种商品价格变化而改变对该种商品的消费数量，一般是价格下跌，消费数量增加。
收入效应：在价格水平保持恒定的条件下因购买力变化而改变对该种商品的消费数量，收入增加，对某种商品的消费可能上升或下降。
即便是劣质商品，收入效应也很少有可能大于替代效应。
吉芬商品：收入效应大于替代效应，导致对某种商品的需求曲线向上倾斜。
Income and Substitution Effects: Normal Good
Food (units
per month)
O
Clothing
(units per
month)
R
F1
S
C1
A
U1
The income effect, EF2,
( from D to B) keeps relative
prices constant but
increases purchasing power.
Income Effect
C2
F2
T
U2
B
When the price of food falls, consumption increases by F1F2 as the consumer moves from A to B.
E
Total Effect
Substitution
Effect
D
The substitution effect,F1E,
(from point A to D), changes the
relative prices but keeps real income
(satisfaction) constant.
Income and Substitution Effects: Inferior Good
Food (units
per month)
O
R
Clothing
(units per
month)
F1
S
F2
T
A
U1
E
Substitution
Effect
D
Total Effect
Since food is an
inferior good, the
income effect is
negative. However,
the substitution effect
is larger than the
income effect.
B
Income Effect
U2
市场需求
市场需求曲线：
更多消费者进入市场将使市场需求曲线右移；影响众多消费者需求的因素也将影响市场需求。
需求的价格弹性：
需求的点弹性：
需求的弧弹性：
消费者剩余：图PP26
Consumer Surplus
Demand Curve
Consumer
Surplus
Actual
Expenditure
Consumer Surplus
for the Market Demand
Rock Concert Tickets
Price
($ per
ticket)
2
3
4
5
6
13
0
1
14
15
16
17
18
19
20
Market Price
市场需求
连带外部效应:
攀比效应:
原因：互相攀比，赶时髦。
图 PP28-31.
虚荣效应:
原因：物以稀为贵。
图 PP32-33.
Positive Network
Externality: Bandwagon Effect
Quantity
(thousands per month)
Price
($ per
unit)
D20
20
40
When consumers believe more
people have purchased the
product, the demand curve shifts
further to the the right .
D40
60
D60
80
D80
100
D100
Positive Network
Externality: Bandwagon Effect
Demand
Quantity
(thousands per month)
Price
($ per
unit)
D20
20
40
60
80
100
D40
D60
D80
D100
The market demand
curve is found by joining
the points on the individual
demand curves. It is relatively
more elastic.
Positive Network
Externality: Bandwagon Effect
Demand
Quantity
(thousands per month)
Price
($ per
unit)
D20
20
40
60
80
100
D40
D60
D80
D100
Pure Price
Effect
48
Suppose the price falls
from $30 to $20. If there
were no bandwagon effect,
quantity demanded would
only increase to 48,000
$20
$30
Positive Network
Externality: Bandwagon Effect
Demand
Quantity
(thousands per month)
Price
($ per
unit)
D20
20
40
60
80
100
D40
D60
D80
D100
Pure Price
Effect
$20
48
Bandwagon
Effect
But as more people buy
the good, it becomes
stylish to own it and
the quantity demanded
increases further.
$30
Negative Network
Externality: Snob Effect
Quantity (thousands
per month)
Price
($ per
unit)
Demand
2
D2
$30,000
$15,000
14
Pure Price Effect
Originally demand is D2,
when consumers think 2000
people have bought a good.
4
6
8
D4
D6
D8
However, if consumers think 6,000
people have bought the good,
demand shifts from D2 to D6 and its
snob value has been reduced.
Negative Network
Externality: Snob Effect
Quantity (thousands
per month)
2
4
6
8
The demand is less elastic and
as a snob good its value is greatly
reduced if more people own
it. Sales decrease as a result.
Examples: Rolex watches and long
lines at the ski lift.
Price
($ per
unit)
D2
$30,000
$15,000
14
D4
D6
D8
Demand
Pure Price Effect
Snob Effect
Net Effect
三、不确定条件下的选择
描绘风险
正确计量风险必须了解：
行为可能导致的所有结果，
每种结果发生的概率。
主观判断VS客观判断
几个基本概念:
期望值:
方差
标准差：
风险的偏好
预期效用：
是各种结果根据发生概率进行加权的平均。
风险规避VS风险中性VS风险偏好
风险规避:
收入带来的边际效用递减的个体是风险规避型的；
对保险的运用体现了风险规避行为;
例子:PP37-42
风险中性： 图PP43;
风险偏好: 图PP44.
一个例子
A Scenario
A person can have a $20,000 job with 100% probability and receive a utility level of 16.
The person could have a job with a .5 chance of earning $30,000 and a .5 chance of earning $10,000.
Risk Averse
一个例子
Expected Income = ()($30,000) + ()($10,000) = $20,000
Risk Averse
一个例子
Expected income from both jobs is the same -- risk averse may choose current job
Risk Averse
一个例子
The expected utility from the new job is found:
E(u) = (1/2)u ($10,000) + (1/2)u ($30,000)
E(u) = ()(10) + ()(18) = 14
E(u) of Job 1 is 16 which is greater than the E(u) of Job 2 which is 14.
Risk Averse
一个例子
This individual would keep their present job since it provides them with more utility than the risky job.
They are said to be risk averse.
Risk Averse
一个例子
Income ($1,000)
Utility
The consumer is risk
averse because she
would prefer a certain
income of $20,000 to a
gamble with a .5 probability
of $10,000 and a .5
probability of $30,000.
E
10
10
15
20
13
14
16
18
0
16
30
A
B
C
D
Risk Neutral
Income ($1,000)
10
20
Utility
0
30
6
A
E
C
12
18
The consumer is risk
neutral and is indifferent
between certain events
and uncertain events
with the same
expected income.
Risk Loving
Income ($1,000)
Utility
0
3
10
20
30
A
E
C
8
18
The consumer is risk
loving because she
would prefer the gamble
to a certain income.
风险的偏好
风险溢价：
结果可能的变动越大，风险就越大，风险溢价也越大。
例:PP46-51.
风险溢价举例
A Scenario
The person has a .5 probability of earning $30,000 and a .5 probability of earning $10,000 (expected income = $20,000).
The expected utility of these two outcomes can be found:
E(u) = .5(18) + .5(10) = 14
Question
How much would the person pay to avoid risk?
风险溢价举例
Income ($1,000)
Utility
0
10
16
Here , the risk premium
is $4,000 because a
certain income of $16,000
gives the person the same
expected utility as the
uncertain income that
has an expected value
of $20,000.
10
18
30
40
20
14
A
C
E
G
20
F
Risk Premium
风险溢价举例(续)
Variability in potential payoffs increase the risk premium.
Example:
A job has a .5 probability of paying $40,000 (utility of 20) and a .5 chance of paying 0 (utility of 0).
风险溢价举例(续)
Example:
The expected income is still $20,000, but the expected utility falls to 10.
Expected utility = .5u($) + .5u($40,000)
= 0 + .5(20) = 10
风险溢价举例(续)
Example:
The certain income of $20,000 has a utility of 16.
If the person is required to take the new position, their utility will fall by 6.
风险溢价举例(续)
Example:
The risk premium is $10,000 (. they would be willing to give up $10,000 of the $20,000 and have the same E(u) as the risky job.
风险的偏好
将预期收益和收益的标准差作为两轴可画出无差异曲线。
风险的规避程度可通过图形直观观察:图 PP53-54.
Risk Aversion and indifference Curves
Standard Deviation of Income
Expected
Income
Highly Risk Averse:An
increase in standard
deviation requires a
large increase in
income to maintain
satisfaction.
U1
U2
U3
Risk Aversion and Indifference Curves
Standard Deviation of Income
Expected
Income
Slightly Risk Averse:
A large increase in standard
deviation requires only a
small increase in income
to maintain satisfaction.
U1
U2
U3
降低风险
降低风险的三种方式：
多样化、保险、信息搜集
多样化：
不要把鸡蛋放在一个篮子里，例：课本
保险：
购买保险通过放弃一定的预期收益来增加确定性 ,
完全保险：保险成本等于预期损失。
单起事件是随机发生和不可预测的，但众多相似事件的平均结果则可根据大数法则进行预测。
信息的价值：
信息是否完全决策的预期结果将大相径庭。例：课本
38
40
61
42
65
42
78
84
4
40
40
40
43
44
46
49
52
54
57
58
58
59
59
