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“The Economic Way of Thinking”12th Edition

Chapter 3

Substitutes Everywhere: The Concept of Demand

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Chapter Outline Introduction On the Notion of “Needs” Marginal Values Everyday Choices – Marginal Choices The Demand Curve The Law of Demand Demand and Quantity Demanded Demand Itself Can Change Everything Depends Upon Everything Else

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Chapter Outline Misperceptions Caused by Inflation Time Is on Our Side Price Elasticity of Demand Elasticity and Total Receipts The Myth of Vertical Demand All Scarce Goods Must Be Rationed – Someh

ow Is Money All That Matters? Money Costs, Oth

er Costs and Economic Calculation

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Introduction

Most goods are scarce. Sacrifice is necessary

There are substitutes for everything Intelligent choice entails trade-offs. Market price signals encourage buyers to

economize

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On the Notion of “Needs”（需要） What is the relationship between “trade-off

s” and “needs”? Consider these four statements…

The average person needs eight glasses of water per day to maintain good health.

All citizens should be able to obtain the medical care they need regardless of their ability to pay.

A diabetic needs insulin. （糖尿病人需要胰岛素。 ）

You need to read your economics textbook.

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On the Notion of Needs

这 4 句话均有可能产生严重的误导！

Higher prices (sacrifices) lead people to seek substitutes （替代品） .

The fact that goods and services are scarce entails trade-offs, i.e., the sacrifice of other goods and services we value.

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Marginal Values Questions

Which is more valuable, water or diamonds? Which is more valuable, a glass of water or a glas

s of diamonds? Answers

The values that matter are marginal values （。要紧的价值是边际价值） .

Marginal means “additional,” so in economics we make decisions based on expected marginal benefits versus marginal cost.

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Marginal Values 经济学理论就是边际分析，因为理论假设了人

们的决策是通过比较预期的额外收益和预期的额外成本做出的，收益和成本都是站在决策者所处的边界上来衡量的。

在经济决策当中，除了边际收益和边际成本，其他的都不重要。

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Everyday Choices and Marginal Choices 通常，当我们被要求做出选择时，我们会发现，从我

们的境况出发，情况往往是此多彼少或者此少彼多。 The Economic Way of Thinking…

Rejects the all-or-nothing approach （非此即彼） Favors attention to

Marginal benefits Marginal costs

More of A and less of B versus more of B and less of A （经济学的思维方式拒绝用非此即彼的方式想问题，

而是注意边际收益和边际成本。）

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The Demand Curve “ 需要”这一概念鼓励了非此即彼的思维方

式，却不欣赏边际思维。 人们确实有需要。然而，在一个充满了稀缺性

的世界里，人们要权衡利弊——少选择一点甲，是为了多选择一点乙。

Thus, economists developed the idea of demand.

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The Demand Curve 需求这个概念把人们想获得某种东西的数量与他们为了得到这些数量要付出的代价联系在一起。

这一概念是边际分析的深入且极其重要的应 用。

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The Demand CurveMaking it Graphic Graphs can be used to illustrate relationships. Demand Curves （需求曲线）

Illustrate the relationship between price and quantity demanded.

例如，在一个小镇上，人们的计划用水量因价格不同而不同，如表 3-1 所示：

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表 3-1

The Demand CurveConsumption Table

$.07 23

.04 40

.02 80

.01 160

.005 320

Price per Gallon Gallons per Day

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The Demand Curve

Vertical Axis – possible prices that might be charged.

Horizontal Axis – quantity purchased at those prices.

Economists call that a Demand Curve.

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The Demand Curve

GALLONS OF WATER PER DAY

0 40 80 120 200 280 320 360 400160 240.00

.02

.03

.04

.05

.06

$.07

PR

ICE

PE

R G

AL

LO

N

.01

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The Demand Curve

A demand curve illustrates the amount of a good that consumers plan to purchase at any given price.

Read a demand curve by taking a price and finding corresponding quantity.

That is the Quantity Demanded. 当水价上升时，作为节约者，人们一般不会做

出非此即彼的选择，而是作出边际调整。

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The Law of Demand

“Law of Demand”（需求法则）： A negative relationship exists between the a

mount of anything that people want to purchase and the price they must pay.

当其他条件不变时，物品价格上涨，则需求量下降；类似地，当其他条件不变时，物品价格下跌，则需求量上升。

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Demand and Quantity Demanded（需求和需求量） 在使用需求概念时，必须注意，除了价格变动

以外，有没有其他因素变动的可能性。最好的办法就是掌握需求和需求量的区别。

“A change in demand” is not the same thing as a “change in quantity demanded.”

Demand is a relationship between two specific variables. It is a schedule or a curve.

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Demand and Quantity Demanded

Change in Quantity demanded:Change in Quantity demanded:

D

Q

P

Change in demand::Change in demand::

D1Q

P

D2

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Demand and Quantity Demanded A change in quantity demanded is a

movement from one point on a curve to another point on the same curve.

A change in demand results from some other factor that makes households buy more or less at each price. The demand curve shifts.

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Demand Itself Can Change 需求法则是说，当其他条件不变时，如果一种

物品的价格变化，那么对这种物品的需求量也会随之变化。这里的关键之处在于“其他条件不变”这个短语。

价格对我们的选择有重要影响，但是，除了价格本身，还有其他影响因素在鼓励人们增加或是减少对商品或服务的消费。

需求曲线本身能移动。对某一特定物品的需求能增加也能减少。

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Demand Itself Can Change

Price per Gallon Original Gallons/Day New Gallons/Day

$.07 40 15

.04 60 25

.02 140 55

.01 240 100

.005 400 200

表 3-2

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Demand Itself Can Change

GALLONS OF WATER PER DAY

0 40 80 120 200 280 320 360 400160 240.00

.02

.03

.04

.05

.06

$.07

PR

ICE

PE

R G

AL

LO

N

.01

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Everything Depends Upon Everything Else 下面我们明确分离出导致需求变动的一些影响

因素，也就是能 " 导致需求曲线移动 " 的因素。 Influences that can cause a change in deman

d for a good: # of customers Change in customer tastes Change in income 【经济学家区别正常品和低档品

的唯一方法，就是看消费者对收入变化的反应如何。 " 低档 " 这一标签和质量并不存在必然联系。】

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Everything Depends Upon Everything Else

Change in price of a substitute 【当消费者面临不同的选择时，他们也会比较其他物品的价格。其他物品的价格变化很容易引起对消费者正在考虑的物品的需求发生变化。总之，任何单个物品的价格只有在消费者将其同大量其他物品和服务的价格相比的时候才有意义。 】

Change in the price of a complementary good Change in the expected price of a good

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Misperceptions Caused by Inflation 在通货膨胀严重的时期，很多价格表面上确实

上涨了，但根本不是真正的价格上涨。通货膨胀严重扭曲了我们对相对价格和成本变动的感知。

Inflation is an increase in the average money price of goods.

If the money price of all goods (including labor) increases, then no good (except money) will have changed in real price.

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Misperceptions Caused by Inflation 事实上，由于通货膨胀的作用，不是所有东西

的价格都同比例地增加，这正是通货膨胀造成诸多问题的原因之一。

但是，各种价格还是倾向于一起变动的。所以，如果我们想要检验某一种商品价格上涨的后果，我们首先得剔除价格普遍上涨的因素。

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Time Is On Our Side

Changes in the quantity demanded take time. These changes will be greater for any price

change the longer the time period allowed for adjustment. It takes time for customers to find and begin to

use substitutes. It takes time for producers to devise, produce, and

publicize substitutes.

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Price Elasticity of Demand

Price elasticity of demand is the percentage change in the quantity demanded divided by the percentage change in price.

it measures consumer responsiveness to price changes.

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Price Elasticity of Demand

Price elasticity of demand = % change in Q

% change in P

EP .

..

5

22 5

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Price Elasticity of Demand

Example Price increases by 20% Quantity demanded decreases by 50%

Consumers are relatively responsive - Elastic

Example Price decreases by 20% Quantity demanded increases by 15%

Consumers are relatively unresponsive - Inelastic

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Price Elasticity of Demand

Elastic Demand Price elasticity > 1

Inelastic Demand Price elasticity < 1

Unit Elastic Price elasticity = 1

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Price Elasticity of Demand

…. Elasticity is influenced by: Time

人们对价格变化调整的时间越长，需求弹性就越大。

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Price Elasticity of Demand

Availability of substitutes 消费者在面临更高的价格时，通过寻求替代品

以达到节约的目的。任何东西都有替代品，但有些东西比另一些东西有更多的已知的替代品。替代品越多，需求弹性越大。替代品越少，需求弹性越小。

我们常常需要时间来考虑和发现适当的替代品。

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Price Elasticity of Demand

Proportion of one’s budget spent on a good 预算中被用于某种东西的比例越小，消费者对

价格变化的敏感度就越低。需求就越缺乏弹性。 如果预算中被用于某种东西的比例较大，消费

者就会变得更细心，精挑细选，他们对价格变化的敏感度就越高，因此需求就更有弹性。

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Elasticity and Total Receipts

“The university’s total receipts from tuition would actually increase if tuition rates were cut by 20 percent.”

The 20 percent cut in prices must cause quantity demanded to increase by more than 20 percent. Elastic demand

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Elasticity and Total Receipts

Elastic Demand Prices and total receipts move in opposite directio

ns. Inelastic Demand

Prices and total receipts move in the same direction.

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Elasticity and Total Receipts

P

Q

D

Elastic demandbetween C and Esince OBCG < OAEF.

BC

G

AE

FO

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Elasticity and Total Receipts

P

Q

D

A

Inelastic demandbetween A and B.

B

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Elasticity and Total Receipts

“It’s odd but true. Wheat farmers would gross more money if they all got together and burned one-quarter of this year’s crop.”

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Elasticity and Total Receipts

P

Q

Demand for wheat

The market pricewhen entire crop is sold

The price when1/4 of crop is burned.

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Elasticity and Total Receipts 不要认为大学降低了学费，同时需求又有弹性，

就能得出大学总会有更好的经济状况这样的结论。没错，在需求有弹性时降低学费会使收入增加，但是入学人数的增加也同时会引起总成本的上升。在这种情况下，大学就必须决定总收入的增加是不是超过了总成本的增加。

一个常见的错误：很多人认为，公司为了 "挣更多的钱 " ，需要做的全部事情就是提价。但是，一个公司如果把价格提高了 20% ，而需求量下降了超过 20% （需求法则！），总收入就会下降。

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The Myth of Vertical Demand

P

Q

D

P1

P2

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The Myth of Vertical Demand

Question Does a perfectly inelastic demand curve exist? Wil

l consumers buy the same quantity at all prices?

在整个价格范围内，需求曲线不是完全无弹性的，没有例外。

完全无弹性的需求曲线是一条垂直的线，表示对某种东西来说不存在替代品。

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The Myth of Vertical Demand 需求法则现在可以用弹性来表述：对整个价格范围而言，不存在完全无需求弹性的东西。

大多数购买者对于购买成本的变化至少会做出一点点反应，对于足够大的成本变化，所有购买者都会做出反应。

许多人常常谈论基本需求、最低要求、绝对必需品等等。但是需求曲线几乎从来不像他们设想的那样缺乏弹性。当然，这并不是说需求总是很有弹性的。

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All Scarce Goods Must Be Rationed Somehow Market prices and willingness to pay are our

primary criteria for rationing goods and services.

If a good is scarce, sacrifices must be made to obtain it.

Scarce goods must be rationed.

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All Scarce Goods Must Be Rationed Somehow Rationing can be done by willingness to pay

prices. Other ways to ration:

“rationing according to need” （按需分配） “Fist come, first served” （先到先得） Lottery （抽签） Equal shares for all （等分） “Might makes right” （强权就是公理） Merit （功绩，美德）

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All Scarce Goods Must Be Rationed Somehow 这些其他分配方式每种都有其优点。但是，这

些方式如果作为普遍方式来分配稀缺品，每种方式都有一些严重的缺点，在亚当 ·斯密所谓的 " 商业社会 " 的成员中分配尤其如此。 如果大多数商品都按照 "先来先得 " 的方式进行分配，人们就要花费极多的时间用来排队。

抽签本身是公平的，因为没有人对稀缺品有特殊的要求权，但是完全凭偶然性来分配却忽略了人们愿望和条件的多样性。

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All Scarce Goods Must Be Rationed Somehow

平均分配的困难在于，有时候某种东西很难被分割得很均匀，有时候每一份的价值太低，以至于对人没有意义。

“强权就是公理”的原则有明显的缺陷，不光是对弱者，对强者也是一样，人们被迫花费宝贵的资源来捍卫自己的那一份。

按照功绩分配不失为一个明智的方式，但是，前提条件是人们对什么是功绩以及用什么程序来决定某人拥有什么功绩要有一致的认识；而这些条件在很小的社会圈子（比如密切结合的家庭）之外几乎无法满足。

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All Scarce Goods Must Be Rationed Somehow 最重要的是，上面讨论的这些其他分配原则都

没有考虑供给的问题。很少有什么东西是从天而降，所能供给的数量很少是与分配方式无关的。

大多数商品都是人生产的，而人们都想让他们的努力得到回报。如果一种稀缺品的分配方式不能使生产这些商品的人们得到适当的回报，那么这种分配方式最终一定会崩溃。

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All Scarce Goods Must Be Rationed Somehow 我们把对供给的讨论推迟到下一章，这一章只

关注需求的概念。 即使只看需求这一面，把稀缺品分配给愿意出

最高价格的人这一方式也有若干被人忽视的优点。最基本的是，它能使人们根据各自的境况进行节约，从而扩大人们的自由和权力。

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All Scarce Goods Must Be Rationed Somehow 汽油的例子！ 油价上涨时，有很多种方式来节约汽油：多走路、坐公共汽车、骑自行车、几个人合开一辆车、搬到离单位近一点的地方住、在高速公路上开慢点、调节发动机、不出去兜风、更周密地计划、把几次旅行合并到一起、不去离家太远的地方度假，或者买一辆小点的、更省油的车。

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All Scarce Goods Must Be Rationed Somehow 每种节约方式的成本或代价对每个人来说都不

一样，有时甚至有很大的区别。 有些人住的地方周围有方便的公交条件，他们改乘公共汽车可能代价并不大，除非他们乘公共汽车会晕车。有些人可能会和同事比邻而居，这样他们几个人合开一辆车的成本就比较低。有些人正考虑买车，他们可以很方便地改买一辆小点的车，放弃买大车的计划，除非他们有一大家子人。

没有一个公式可以适用于所有人，没有一个最好的方式让人们节约。

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All Scarce Goods Must Be Rationed Somehow When the price of a good rises, users find it i

n their own interest to economize. They will choose ways to economize that ent

ail the smallest sacrifice. Individuals are in the best position to pick and

choose among the ways to economize. （既然人们远比别人更清楚自己的状况，他们就会在所有可能的方式当中选取最好的一种。 ）

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Is Money All That Matters? Money Costs, Other Costs, and Economic Calculation（算经济账） As the opportunity cost of an action

increases, the chooser will tend to undertake less of that action.

As the opportunity cost of an action decreases, the user will tend to undertake more of that action.

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Is Money All That Matters? Money Costs, Other Costs, and Economic Calculation People compare the expected additional ben

efit against the expected additional cost. In a commercial market economy, money is a

common denominator (i.e. a yardstick). Money allows individuals to calculate relative

costs and benefits. （用货币衡量的价格使算账成为可能！）

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Is Money All That Matters? Money Costs, Other Costs, and Economic Calculation 货币价格的变化是协调人们生产和消费的有用信号。这也正是经济学家如此重视价格变化的原因。

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Once Over Lightly

Trade-offs Needs Law of Demand Demand Curve Quantity Demanded Dependence

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Once Over Lightly

Inflation Time Price Elasticity of Demand Elasticity and Receipts Rationed Goods Money Costs