外国语考研汇总微观经济week five

2021/6/121Week

FiveChapter

6The

Consumption

Decision2021L/6e/1a2

rn

consumer

surplus.2ObjectivesLearn

the

basic

problem

facing

all

consumers

of

choosingthe

best

point

given

limited

e.Understand

the e

elasticity

of

demand.Explain

how

changes

in

prices

and e

change

the

positionand

slope

of

the

budget

constraint.Explain

how

relative

prices

relate

to

opportunity

cost

andefficient

choice.Understand

the e

and

substitution

effects

of

a

change

inprices.Understand

how

the

willingness

to

pay

can

be

used

as

ameasure

of

marginal

utility.2021/6/123AgendaThe

basic

problem

of

consumer

choiceA

closer

look

at

the

demandcurveUtility

and

preferencesCritical

review

of

the

basic

model

ofthetheory

of

consumer

choice2021/6/124The

basic

problem

of

consumer

choiceThe

budget

constraint:

The

consumer

haslimited e

to

spend

on

many

goods.What

happens

to

consumption

when

echanges: e

elasticityChoosing

a

point

on

the

budget

constraint:Individual

preferences2021/6/125Assumption

of

RationalityThere

are

over

100

million

households

in

theUnitedStates

choosing

millions

of

different

types

and

brandsof

goodsdaily.Economists

assume

all

of

them

are

rational!This

is

not

so

farfetched.Rationality

means

not

wasting

resources.The

Budget

ConstraintA

consumer

has

only

so

much e

tospend.A

budget

constraint

gives

all

of

the

combinations

oftwo

goods

the

consumer

can

buy

if

she

spends

all

ofher

e.The

budget

constraint

shows

the

opportunity

set

forthe

consumer

given

the

prices

and

theconsumer's

e.2021/6/126Intercepts

and

the

Slope

of

the

BudgetConstraint

(a)Interceptsx-intercept

=

M/pX,

the

most

X

a

consumer

can

buy.y-intercept

=

M/pY,

the

most

Ya

consumer

can

buy.The

slope

of

the

budget

constraint

=

-pX/pY.2021/6/1278Intercepts

and

the

Slope

of

the

BudgetConstraint

(b)The

slope

of

the

budget

constraint

shows

the

trade-off

a

consumerfaces.

The

trade-off

is

the

relative

prices

of

the

goods.

If

pX

=

$2

and

pY

=

$1,

then

2

Ys

trade

for

1

X;

theslope

of

the

budget

constraint

is-$2/$1

=

-2,

whichis

the

trade-off.The

ratio

of

the

relative

prices,

the

trade-off,

and

the

slope

are

all

thesame.This

analysis

applies

to

examples

with

any

number

of

products.Economists

get

around

the

need

for

higher-dimensional

graphs

byhaving

one

good

on

one

axis

and

the

dollar

amount

of

all

other

goods2021o/6n/12the

other.What

Happens

to

Consumption

When

eChanges

(a)Changes

in e

shift

the

budget

constraint

parallel

tothe

original,

because

the

slope,

which

is

the

price

ratio,is

unchanged.2021/6/129What

Happens

to

Consumption

When

eChanges

(a)

(cont.)If e

increases,

the

budget

constraint

shifts

outfromthe

origin,parallel

to

the

old

budget

constraint.The

increasein e

makes

consumers

betteroff.

Their

opportunityset

expands

and

they

can

buy

moregoods

If e

decreases,

the

budget

constraint

shifts

in

towardthe

origin,parallel

to

the

old

budget

constraint.2021/6/12

102021/6/1211What

Happens

to

Consumption

When

eChanges

(b)When e

increases,

consumers

choose

a

new

point

on

a

new

budgetconstraint

farther

not

from

the

origin.The

point

they

choose

depends

on

their

own

tastes

or

preferences.When e

rises,

the

consumption

of

most

goods

rises:

normal

goods.When e

rises,

consumption

of

some

goods

falls:

inferior

goods.e

elasticity

of

demand:

sensitivity

of

demand

to

changes

in

e.e

elasticity

of

demand

=%

Qd/%

e.e

elasticity

for

normal

goods>0.（正常商品）e

elasticity

for

inferior

goods<0.（劣质商品）2021/6/1212What

Happens

to

Consumption

When

eChanges

(c)The e

elasticity

of

luxuries

(movies,

restaurant

meals)

is

high.The e

elasticity

of

necessities

(car

repairs,

clothing,

furniture)is

low.Engel’s

law(恩格尔定律）：The

ratio

of

expenditure

onfood

to

total

expenditure

,can

reflect

the

extent

of

poverty

orrichness

of

a family

or

country.

The

larger

the

ratio,

the

lessrich;

vice

versa.The

Engel

coefficient(恩格尔系数，expenditure

onfood/total

expenditure)in

China

is

0.447

on

average.2S02o1u/r6c/e1:2H.S.

Houthakker

and

Lester

D.Taylor,

Consumer

Demand

in

the

United

States

(Cambrid1g3e,

Mass.:

Harvard

University

Press,

1970).Table

8.1SOME

INCOME

ELASTICITIES

OF

DEMANDElastic(longrun)Inelastic(longrun)Motion

pictures3.41Carrepairs.90Drugs

and

medicines3.04Tobacco

products.86Owner-occupied

housing2.45China,

glassware,

and

utensils.77Nondurable

toys2.01Shoe

repairs.72Electricity1.94Alcoholic

beverages.62Restaurantmeals1.61Water.59Local

buses

and

trains1.38Furniture.53Gasoline

and

oil1.36Clothing.51Carinsurance1.26Physicians'

services1.15Carpurchases1.07Choosing

a

Point

on

the

BudgetConstraint:

Individual

Preferences

(a)

At

whichpoint

on

thebudget

constraint

willanindividual

choose

to

buy?

Depends

on

the

individual's

preferences

for

the

goods.Few

people

choose

extreme

pointsOnly

pizza

and

no

shoes,

or

only

shoes

and

no

pizza.If

we

get

too

much

of

one

thing,

we

get

a

little

bored

with

it.We

like

variety

and

diversity.Most

consumers

choose

a

point

on

the

budget

constraint2021/6/12between

the

x-intercept

and

the

y-intercept.

14shoesshoes,shoes15Choosing

a

Point

on

the

Budget

Constraint:Individual

Preferences(b)An

individual's

choice

depends

on

how

he

values

the

twogoods.When

making

decisions

people

look

at

the

marginal

value.Pizza

and

shoesThe

consumer

considers

the

value

of

an

additional

pair

of

shoes.He

compares

this

to

the

opportunity

cost

of

an

additional

pair

ofshoes:

if

he

buys

one

more

pair

of

shoes,

how

many

pizzas

must

hedo

without?If

the

value

of

an

additional

pair

of

shoes

>

the

cost

of

an

additional2021/6/12pairof

shoes,

he

buys

that

pair

of

shoes.2021/6/1216A

closer

look

at

the

demandcurveDeriving

demand

curvesThe

importance

of

distinguishingbetween e

andsubstitutioneffectsDeriving

Demand

CurvespricUse

the

budget

constraint

to

derive

a

consumer'sdemand

curve.Suppose

the e

of

good

X

rises:pX.Geometrically

there

are

two

effects:The

budget

constraint

is

steeper:

slope

-pX/pY.The

budget

constraint

rotates

toward

the

origin,

so

theconsumer

loses

some

area

of

the

opportunity

set.2021/6/12172021/6/1218

This

effect

is

represented

by

the

steepness

of

the

new

budget

constraint.The

Importance

of

Distinguishing

between eandSubstitution

Effects（收入和替代效应）The e

effect（收入效应）:

pX ,

so

the

consumer

loses

purchasingpower,

real

e.The

opportunity

set

shrinks;

The

consumer

is

poorer.Because

she

is

poorer,

she

will

change

her

consumption

of

good

X.If

X

is

a

normal

good,

she

buys

less

X.If

X

is

an

inferior

good,

since

she

has

been

made

poorer

by

the

priceincrease,

she

buys

more

X.The

substitution

effect（替代效应)

:

pX ,

so

good

X

relatively

moreexpensive

compared

to

good

Y.

The

consumer

substitutes

away

fromgood

X

toward

good

Y.2021/6/1219Normal

Goods（正常商品）When

pX ,

total

changein

demandfor

X

=fromthe

substitution

effect+ fromthe eeffect.If

X

is

a

normal

good,

the

total

demand

for

Xfalls.The

substitution

effect:

The

consumer

substitutesaway

from

the

relatively

more

expensiveX.The e

effect:

The

price

rise

reduces

real e,

so

theconsumer

demands

fewer

normal

goods

such

as

X.Substitution

and e

effect

ofnormalgoods(px

decrease)yxAFx1

x22021/6/1220GB’acBbI1Ox3I2X

is

an

inferior

good,

and

the

total

demand

for

X

falls.The

substitution

effect:

The

consumer

substitutes

away

from

therelatively

more

expensive

X.The e

effect:

The

price

rise

reduces

real e,

so

the

consumer

demandsmore

of

an

inferior

good

such

as

X.The e

effect

offsets

(partially)

the

substitution

effect.However,

the e

effect

of

an

inferior

good

is

small.The

substitution

effect

dominates,

so

total

demand

for

good

X

fallswhen

pX

rises

even

if

X

is

an

inferior

good.A

good

may

be

so

inferior

that

the e

effect

outweighs

the

substitution2021e/6f/f1e2ct

and

total

demand

increases

when

the

price

rises:

This

is

a

Giffen21good(吉芬商品）

and

isvery

rare.Inferior

Goods

When

PX

RisesSubstitution

and e

effect

ofinferiorgoods

(px

decrease)GOI2B’BAFxy•abcI1x1

x2x32021/6/1222Substitution

and e

effect

of

Giffen

goods(吉芬商品）x2021/6/1223yOAF2bI2acI1B

E1x

x

x3B’Substitution

effect

and e

effect

of

completelycomplement

goods

(

px

decrease)x2021/6/1224yo1x

xy2y12Only e

effect,No

substitution

effect.

i.e.eEffect

=total

effect.Substitution

effect

and e

effect

of

completelysubstitute

goods

(

px

decrease)Budget

constraint

lineIndifference

curve2021/6/1225xyOOnly

substitution

effect;No e

effect.

eEffect

=total

effect.Substitution

effect

and e

effect

of

quasi-linearutilityx2021/6/1226yO

x1

x2utility

function:u(

x,

y)=v(

x

)+yonly

substitutioneffect,

no

eeffect.•••I2I3I12021/6/1227Utility

and

the

description

of

preferences2021/6/1228Cardinal

utility(基数效用）2021/6/1229UtilityThe

benefits

derived

from

consuming

are

calledutility

(happiness).Understanding

the

happiness:what

is

happiness?WantsCannot

compare

the

utilities

of

different

people.Cannotsay

"You

are

happier

than

I

am"Can

say

"You

would

be

willing

to

pay

more

than

I

amfor

some

good.“Happines

=

Utility幸福是什么，怎样增加幸福幸福=效用/欲望用公式表示就是：对上面的公式两边分别取自

然对数，然后对时间求导数，得到：根据此公式，想要增加幸福，理论上可以有以下几种途径：1、效用不变时，欲望减少；就是我们经常说的“知足者常乐”。2、欲望不变时，效用增加；3、效用的增加快于欲望的增加；4、效用的减少慢于欲望的减少；5、欲望减少时效用增加。

h

wuh

=

u

-

wU

WH

=2021/6/1230

Total

score

2021/6/1231Wrap-up:

measure

your

happinessHow

happy

are

you?

Sure,

you

may

think

you

know,

but

this

little

testwill

help

youkeep

score.

The

satisfaction

with

Life

Scale

was

devised

in

1980

by

University

ofIllinois

psychologist

Edward

Diener,

a

founding

father

of

happiness

research.

Sincethen

the

scale

has

been

used

by

researcher

around

the

world.Reading

the

following

five

statements.

Then

use

a

1-to-7

scale

to

rate

yourlevelofagreement.Notat

all

true moderately

true absolutely

true①

In

most

ways

my

life

is

close

to

my

ideal.②

The

conditions

of

my

life

are

excellent.③

I

am

satisfied

with

my

life.④

So

far

I

have

gotten

the

important

things

I

want

in

life.⑤

If

I

could

live

my

life

over,

I

would

change

almost

nothing.12345672021/6/1232Scoring:

•31

to

35:

you

are

extremely

satisfied

with

your

life26

to

30:

very

satisfied

•21

to

25:

slightly

satisfied

•20

is

theneutral

point

•15

to

10:

slightly

dissatisfied

•10

to

14:dissatisfied

•5

to

9:

extremely

dissatisfied2021/6/1233Eight

Steps

Toward

a

More

Satisfying

LifeWant

to

lift

your

level

of

happiness?

Here

are

some

practical

suggestionsfrom

University

of

California

psychologist

Sonja

Lyubomirsky,

based

onresearch

findings

by

her

and

others.

Satisfaction

(

at

least

a

temporaryboost)

guaranteed1.

Countyour

blessings.One

way

to

do

this

is

with

a

“gratitudejournal”

in

which

you

write

down

three

to

five

things

for

which

you

arethankful---from

the

mundane(

your

peonies

are

in

bloom)

to

themagnificent(

a

child’s

first

steps).

Do

this

once

a

week,

say,

on

Sundaynight.

Keep

it fresh

by

varying

your

entries

as

much

as

possible.2.

Practice

acts

of

kindness.

These

should

be

both

random(

let

thatharried

mom

go

aheadof

you

in

the

checkout

line)

and

systematic(bring

Sunday

supper

to

an

elderly

neighbor).

Being

kind

to

others,whether

friends

or

strangers,

triggers

a

cascade

of

positive

effects---itmakes

you

feel

generous

and

capable,

gives

you

a

greater

sense

ofconnection

with

others

and

wins

you

smiles,

approval

and

reciprocatedkindness----allhappinessboosters.2021/6/1234Eight

Steps

Toward

a

More

Satisfying

Life(

cont.)3.

Savor

life’s

joys.

Pay

close

attention

to

momentary

pleasure

andwonders.

Focus

on

the

sweetness

of

aripe strawberry

or

the

warmthof

the

sun

whenyou

step

out

from

the

shade.

Somepsychologistssuggest

taking

“mental

photographs”

of

pleasurable

moments

to

reviewin

less

happy

times.4.

Thank

a

mentor.

If

there’s

someone

whom

you

owe

a

debtofgratitude

for

guiding

you

at

one

of

life’s

crossroads,

don’t

waittoexpress

yourappreciation---in

detail

and

,

if

possible

,in

person.5.Learn

to forgive.

Let

go

of

anger

and

resentment

by

writing

a

letterof

forgiveness

to

a

person

whohas

hurt

or

wrongedyou.

Inability

toforgive

is

associated

with

persistent

rumination

or

dwelling

on

revenge,while

forgiving

bolsters

positive

feeling

about

your

past

and

gives

youpeace

of

mind.2021/6/1235Eight

Steps

Toward

a

More

Satisfying

Life(

cont.)6.

Invest

time

and

energy

in

friends

and

family.

Where

you

live,howmuch

money

you

make,

your

job

title

and

even

your

health

havesurprisingly

small

effects

on

your

satisfaction

with

life.

Thebiggestfactor

appears

to

be

strong

personal

relationships.7.

Take

care

of

your

body.

Getting

plenty

of

sleep,

exercising,stretching,

smiling

and

laughing

can

all

enhance

your

mood

in

the

shortterm.

Practiced

regularly,

they

can

help

make

your

daily

lifemoresatisfying.8.

Develop

strategies

for

coping

with

stress

and

hardships.

Thereis

no

avoiding

hard

times.

Religious

faith

has

been

shown

to

helppeople

cope,

but

so

do

the

secular

beliefs

enshrined

in

axioms

like“This

too

small

pass”

and

“That

which

doesn’t

kill

me

make

mestronger.”

The

trick

is

that

you

have

to

believethem.Material

resources:

《Times》February

28.200536Willingness

to

PayEconomists

use

a

willingness

to

pay

criterion

as

ameasure

of

utility.Marginal

utility:

how

much

extra

utility

or

happinessaconsumer

receives

from

one

additional

unit

ofthegood.Willingness

to

pay

=

how

much

the

consumer

will

payfor

the

nextunit.Willingness

to

pay

is

a

way

of

measuring,

in

dollars,2021/6/12marginal

utility.2021/6/12

37Willingness

to

Pay

(cont.)SWEATERSSWEATERSDiminishing

marginal

utility

law边际效用递减规律：在其他商品的消费

保持不变的情况下，随着消费者对某种

物品的消费量的增加，他从连续增加的

消费增量当中得到的满足程度是递减的。2021/6/1238Utility

table

of

one

goods

λ=2quantity

of

goodstotal

utiltymarginalutilityPrice

ofgoods001101052188432463428425302163000728-22021/6/1239TUTUQMUQMUthe

relation

between

total

utilityand

marginal

utilitySummingthe

marginalutilityequalstotalutilityf

(x)dx

=

F

(x)baMu(x)dx

=

Tu(x)baTu

(x)

=

Mu(x)Mu(x)MuOxTu(x)2021/6/1240Maximizing

Utility

(a)This

says

the

consumer

receives

more

utility

perdollar

from

consuming

good

X

than

good

Y.She

should

buy

more

X

and

less

Y.If

she

buys

more

X,

MUX

falls

due

to

diminishing

marginalutility.Ifshe

buys

less

Y,

MUY

increases.2021/6/1241Maximizing

Utility

(b)

Since

MU

=

p

for

all

goods,

it

must

be

the

case

thatfor

all

goods,MU

/p

=

MU

/p

=

MU

/p

.X

X

Y

Y

z

zThese

ratios

say

that

if

the

consumer

is

maximizing

herutility

then

the

extra

utility

per

dollar

must

be

equal

for

allgoods.

To

see

why

this

must

be

the

case,

suppose

that

theseequalities

do

nothold:2021/6/12MUX/pX

>

MUY/pY.

42This

says

the

consumer

receives

more

utility

per

dollar等边际原理2021/6/1243An

exampleSome

consumer

wants

to

buy

two

kinds

ofgoods。His e

：

I

= 8

yuanPrices

of

goods：P1

=

P2

=

1

yuanThe

consumer’s

consumption

sets

are

asfollowsWhat

is

the

consumer’s

optimal

consumptionset？Quantity（Q）2021/6/124412345678marginal

utility

of

goodsoneMu11110987654marginal

utility

ofgoods

two

MU2191715131210892021/6/1245Consumptionsetstotal

utilityQ1=1,Q2

=711+19+17+15+13+12+10+8105Q1=2,Q2

=611+10+19+17+15+13+12+10107Q1=3,Q2

=511+10+9+19+17+15+13+12106Q1=4,Q2

=411+10+9+8+19+17+15+13102……2021/6/1246Utility

table

of

one

goodsλ(

marginal

utility

of

money)=2quantityTotal

utilityMarginalutilityprice001101052188432463428425302163000728-22021/6/1247Deriving

the

demand

curveAccording

to

equal

marginal

theoremMU(Q1) /

P

1=

MU(Q2) /P

2

=2demand

function

on

goods

1

of

individualconsumer

：MU(Q1) /P

1=

2P2021/6/1248QIndividual

consumer’s

demand

curve54321Demand

curve•••••123452021/6/1249Ordinal

utility(序数效用）Indifference

curve（无差异曲线）Indifference

curves

give

the

combinations

of

goods

amongwhich

an

individual

is

indifferent

or

which

yield

the

same

level

ofutility.Anexample:combination

ofgoodsquantity

offoodquantity

ofclothA2030B1050C4020D3040E1020F10402021/6/125010

20

30

40foodclothA•B•C•D•E502021/6/125140302010OIndifference

curve

cannot

cross•A2021/6/1252•B•CfoodclothOIf

two

indifference

curves

crossed

,alogical

contradiction

would

occur.

Ifcurves

crossed

at

point

A,

then

onewould

be

indifference

between

A

andB,

between

A

and

C,

and

thereforebetween

B

and

C.

But

since

Binvolves

higher

consumption

of

bothgoods

than

C,

B

is

clearly

preferredto

C.Indifference

curve

of

completely

complement

goods1234Right

shoeLeft

shoeA•C•DE

•B4321O••

•Utility

function:U(x1.x2)=Min(ax1,bx2)2021/6/1253Indifference

curve

of

completely

substitutegoodsred

pencilblue

pencilOUtility

function

:U(x1,x2)=ax1+bx22021/6/1254Indifference

curve

of

quasi-linear

utilityx12021/6/1255x2Outility

function:u(x1,x2)=v(x1)+x22021/6/1256Marginal

rate

of

substitution（边际替代率）The

marginal

rate

of

substitution

tells

us

how

much

ofone

good

an

individual

is

willing

to

give

up

in

return

forone

more

unit

of

another.根据上面的公式，边际替代率是无差异曲线在某一点切线的斜率dXDX

DXDX

fi

0MRS

=-

DY

=

lim-

DY

=-

dY2021/6/1257Marginal

rate

of

substitutionclothfoodx1•A•B△x△yx2y1y22021/6/1258foodcloth10

20

30

40•A•B•CD△y1△y2△y3Diminishing

marginal

rate

ofsubstitution:

As

a

result

of

theprinciple

of

diminishing

marginalrate

of

substitution,

the

slope

of

theindifference

curve es

flatter

aswe

move

from

left

to

right

along

thecurve.Consumer’s

choice(consumer

equilibrium消费者均衡)YXOEABI1I3I2X1Y1•A•B2021/6/12592021/6/1260Consumer

equilibrium’s

conditionWhen

the

consumer

maximizes

the

utility

subject

tohis

budget

constraint,

his

budget

line

tangents

to

thehighest

indifference

curve.

At

the

tangent

point

,the

slope

of

budget

line

equalsto

the

slope

of

indifference

curve.i.e.

At

the

tangent

point

,MRS

=the

relative

price

ofgoodsPyMRS

=

PxCalculus

analysisMax

u(x,

y)subject

to M

=Px

x+Py

yL

=u(x,

y)+l(M

-Px

x-Py

y)(1)(

2

)y

=

0

(3)x

-

Py=

M

-

P¶M¶y¶y

¶L

¶L

=Px

=

0Py

=

0¶x¶x¶L

=¶u

-

l¶u

-

lxxy2021/6/1261=

Mux

=

PxMuy

Py=

MRS¶u¶y(2)

(1)

=

¶u

¶x2021/6/1262Deriving

the

demand

curvePrice

–consumption

curve

and

demand

curve2021/6/1263XYAPrice-consumptioncurveI3I1I2X1XPxP1P2P3•A•B•C1

2X

X

XX2

X33Demand

curve基数效用和序数效用的一致性在cardinal

utility

theory理论下，消费者达到效用最大化的条件是在ordinal

utility

theory

理论下，消费者达到效用最其中，所以，，推导如下，Px

Py

PzMUx=MUy=MUzPy大化的条件是MRS

=

Px

,MUyMRS

=

MUx

MUx

=

MUyPx

PyMUy为什么MRS

=

MUxdx

+

¶u

dy

=

0,

dy

=

-

¶u

/

¶u

=

-

MUx¶y

dx

¶x

¶y

MUy2021/6/1264¶u¶xTu(x,y)=u

,在公式两边求全微分，可以得到2021/6/1265Consumer

Surplus

(a)Consumer

surplus

is

a

measure

of

consumers'

totalhappiness

in

dollar

terms.Use

the

willingness

to

pay

criteria.Consumer

surplus

is

the

difference

between

what

theconsumer

is

willing

to

pay

and

what

she

actually

pays.Consumer

surplus

represents

the

"savings"

consumers

receivebecause

the

market

price

is

lower

than

what

they

are

willing

to

pay.Consumer

surplus

represents

the

total

bargain

consumers

receive.Consumer

Surplus

(b)Consumersurplus

isthe

areaunder

the

demandcurve

out

to

the

equilibriumquantity

and

above

theprice

consumers

actuallypay.2021/6/1