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TheMoneyIllusion货币幻觉

美国本特利大学经济学教授斯科特·萨姆纳(Scott Sumner)

 
 
 

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美国本特利大学经济学教授

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令人疑惑的供求关系  

2009-08-04 08:53:59|  分类: 默认分类 |  标签: |举报 |字号 订阅

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 [点击查看Scott Sumner的英文博客 [Scott Sumner中文博客]  

也许你不觉得供求关系令人疑惑,但这个问题确实困扰着我。问题开始于数年之前,当时我们的考试试卷上出现了这样一道题目。即“如果咖啡引发了健康问题,那么市场对于茶的需求会发生怎样的变化?”众所周知,咖啡与茶互为替代品。所以普通的思路是,由于引发健康问题,咖啡的价格将下降,而茶的需求将……如此等等。但是,我现在想更深入的讨论这一问题。

    时隔数年之后,我在曼昆经济学教材的第四章看到下面一段话:“如果冰冻酸奶的价格下降。那么供求规则将导致你购买更多的酸奶,而与此同时你将减少冰淇淋的需求。”这至少解决了咖啡与茶的问题。健康问题导致咖啡价格下降,而既然咖啡与茶互为替代品,咖啡价格的下降势必导致茶的需求减少。

    但是这是事实吗?曼昆的供求规则同时指出“酸奶价格下降将导致供给的下降”。而根据供求平衡原理,矛盾就此出现。

    另外,曼昆经济学教材的第四章一道练习题是:

    《纽约时报》一篇文章讲述了法国香槟酒业一次成功的营销活动。这篇文章指出:“许多行业管理人员欣喜于香槟酒的高价,但是同时又担心这种高价将导致需求下降,而随之而来的必然是价格下降。”这道习题的问题是:“这些管理人员在分析过程中出现了怎样的错误?”

    我认为,答案是营销活动导致需求增加,而同时造成量价同增的局面。但是这是错误的,因为曼昆已经指出,价格上升必然导致酸奶需求的下降,这也必然适用于香槟酒。所以这便引发了矛盾。

    这道题目来自于曼昆经济学教材的第三版,而到第四版时,这个存在矛盾的问题已经惊奇的消失了,而其他问题仍然在。我曾发邮件问曼昆为何最有趣的一道题目没有了,他表示,每一版都需要在使用前一版的教授们的反馈基础上进行改进。

    同时,酸奶的例子仍然出现在第四版中,这也许意味着,大多数教授都承认,酸奶价格的上涨必然导致需求的下降,但是这些教授也同时认为,那道香槟酒的题目的确令人疑惑。

    我开始意识到我是一个少数派。许多经济学家喜欢那种被我称为“价格变化推理”( reasoning from a price change)的思维方式,这种思维的一般方式如下:

第一,如果利率大幅下降,我们可以期待投资规模……

第二,如果石油价格下降,我们可以期待消费规模……

第三,如果美元贬值,我们可以期待进出口贸易……

    我们来讨论一下上述问题。今年的利率水平已经下降到十分低的水平,而投资规模也大幅下降,所以利率降低势必导致投资下降;石油价格已经大幅下降,而石油消费也同样在下降,所以油价下降势必导致石油消费的下降。也许此时你会发出疑问:“你并没有保持其他条件不变,其他条件都发生了改变。”

    但是这并不是问题的关键。当然,其他条件发生了改变,价格、利率及汇率怎么可能首先发生改变呢?但是这并不会影响我刚刚提到的上述举例,他们是供求规则的直接应用。画好供求曲线,然后将需求曲线向左平移,你可以清楚的看到价格及消费量的下降。

    那么,我们对于价格了解多少?我们所知的是,如果供应上升导致价格下降,那么消费必将上升,而如果需求下降导致价格下降,那么消费也将下降。换句话说,如果价格(或者利率及汇率)下降,我们可以猜测50%消费将上升,50%消费将下降。

    最近我看到有些人指出,能源价格的低水平是一件坏事,因为这将导致消费者消费行为趋于保守。这是十分有趣的,我也认为这是一件坏事,因为这反映了消费者正在保留能源,他们为什么保留能源?因为他们失业了。

    我看到也有人指出,美联储在2003年将基准利率下调至1%是一个巨大的错误,因为这导致房地产泡沫的形成。但是我认为,2003年利率的下降主要是因为投资的下降。

    我也发现,人们十分喜欢讨论关于汇率的问题。我并不太了解开放的宏观经济,但是我仍然十分想了解,人们讨论美元贬值问题时究竟想表明什么。为什么美元会首先贬值?也许有人指出,是由于国外投资者不愿再持有美元。所以,我们总可以为发生的事情找到根源,然后我们可以预测下面的结果。但是有一个问题是,为什么国外投资者不愿再持有美元?难道他们不愿继续与美国进行贸易?如果答案是肯定的,那么个中原因又是什么呢?他们希望增加投资?另外,国外投资者偏好的改变对美国贸易会造成怎样的影响?我们能知道的唯一事实就是,美国的贸易逆差正在消失。但是这对于美国经济又意味着什么?出口增加?进口减少?以下,是我通过 54年的经验所总结的一条规则:

    如果在几个经济周期内,人们总是说一件事是不可持续的,那么事实上这件事是可持续的。或者其持续的时间长过你的一生。也许有一天趋势会改变,但是既然你在自己的有生之年都无法看到,我们也没有必要进行讨论。

    那么我所指的那些被人们认为不可持续的事到底是哪些?亚洲国家向美国发债?医疗成本在GDP中所占比重不断攀升?顺便说一下,我们从“非医疗成本”的角度更容易看到后一个问题。如果非医疗成本每500年下降一半。那么总的非医疗成本仍然有可能在短期内上升。但是这种情况能够持续吗?难说。

教授经济学的经验

    有没有可能,当我们教授供求规则时,经济学专业的学生不再学习任何东西?我认为,关于供求规则,学生们应当掌握以下两点:

第一, 供给或者需求改变所带来的影响。

第二,价格及数量的变化将导致怎么的情形出现。

    我在一所集中优秀人才的学院教学。几乎这里所有的学生都知道供求不足所造成的影响。如果告诉他们一场冰冻袭击了佛罗里达的桔子林,他们都能够说出桔子价格的变化。如果告诉他们数百万中国人开始购买汽车,他们都将说出石油价格的变化。

    但是我同时发现,没有学生能够解释价格及数量数据的深层次含义。更重要的是,当他们离开我的课堂时依然一无所知。我经常问高阶段或者MBA学生下面这个问题:

    一份调查显示,当电影票价是6美元时,看电影的人数为100人,而当票价是9美元时,看电影的人数为300。这是否违背了供求规则?

    只有极少数学生能够回答这一问题,而如果你要求他们给出解释,能够给出满意答案的学生更是凤毛麟角。事实上,这是一个简单得不能再简单的问题。这是一个基本的供求关系问题。我所要问的不过是,但对电影的需求曲线发生变动时会产生怎样的情况。我已经不能找到更直白明了的问题。它到底有什么难的?难道真的几乎没人能够回答?来到我班上的学生,知道关于供求关系两个问题中的一个,而他们离开时同样如此。也许除去教授他们所谓“导致供给发生改变的五要素”或者“导致需求发生改变的五要素”,我们应该要求他们读读能够解释到底什么是经济学的文章。

    我认为,没有必要教授类似于佛罗里达桔子林遭遇冰冻的内容。至多我们只需要将这些内容放到教材的附录里供他们自学。学生们实在没有必要对诸如曼昆教材第四章这样的内容投入100%的精力。也许,我们可以让助教来为学生们上关于供求关系的课程,或者,我们可以完成放弃这方面的课程。我认为,机会成本、激励、边际分析才是值得我们关注的内容。

    我是一个乐于接受建议的人。但是,每当我阅读报刊时,甚至是诸如《纽约时报》、《华尔街日报》、《金融时报》这样的精英报刊时,我仍然觉得有些不妥之处。我认为,他们在努力的贴近经济生活,他们热衷于谈论价格、利率、汇率的变动,以及由此带来的影响。

(翻译纠错。读者发现任何翻译错误请发邮件给我们,谢谢:caijingblog#126.com 将#改为@)


英文原文(地址:http://blogsandwikis.bentley.edu/themoneyillusion/?p=1993 ):

Why is supply and demand so confusing?

Maybe you don’t find it confusing, but I do.  It all started a few years ago when we noticed that we had a “trick question” on our placement exam at Bentley.  The question asked what would happen to the demand for tea if there was a health scare regarding coffee.  Obviously coffee and tea are substitutes.  So if the health scare reduces the price of coffee then the demand for tea will . . . well, let’s think about it a bit more.

Then a few years later I noticed the following statement in chapter 4 of Mankiw’s intro text:

“Suppose that the price of frozen yogurt falls.  The law of demand says that you will buy more frozen yogurt.  At the same time you will probably buy less ice cream.”

Well at least that clears up the coffee/tea example.  A health scare will reduce the price of coffee.  Since coffee and tea are substitutes, if the price of coffee falls you will buy less tea.

Or will you?  Doesn’t Mankiw’s yogurt example also imply that “the law of supply says you will sell less yogurt” when the price falls.  And since the “law of logic” says that the number of units sold equal the number of units bought, we’ve got a problem here somewhere.

Things got more complicated in chapter 4’s “Problems and Applications.”  Question 12 asked:

An article in The New York Times described a successful marketing campaign by the French champagne industry.  The article noted that “many executives felt giddy about the stratospheric champagne prices.  But they also feared that sharp price increases would cause demand to decline, which would then cause prices to plunge.”  What mistake are the executives making in their analysis of the situation?”

I thought the answer was that the marketing campaign had caused the demand to increase, and that caused both price and quantity to increase.  But that can’t be right because Mankiw already told us that when the price goes up people will buy less yogurt—surely the same must be true of champagne?  So there seems to be a conflict.

That was the 3rd edition.  Then the 4th edition came out, and question 12 mysteriously disappeared, like a missing face from a Stalin-era group portrait.  All the other questions were still there.  I emailed Mankiw to ask him why my favorite question was removed, and he indicated that with each new edition changes were made based on feedback from professors who used the book.

The yogurt example is still there in the 4th edition.  Perhaps most professors agreed that a higher price of yogurt does reduce consumption, but thought that an accurate treatment of supply and demand such as question 12 would be “too confusing.”

That’s when I started notice that I was in the minority.  Many economists seem to like to do what I call “reasoning from a price change.”  It goes like this:

1.  If interest rates fall sharply we can expect investment to  . . .

2.  If the price of oil falls then consumers will . . .

3.  If the dollar depreciates then our imports/exports/trade balance will . . .

Let’s take these one at a time.  This year interest rates have fallen to low levels, and investment has plummeted.  So lower interest rates must reduce investment.  The price of oil has plunged, and oil consumption is also down.  So low oil prices would seem to reduce oil consumption.  Now at this point you might be getting fidgety.  “Wait, you’re not holding other things equal.  Something else changed.”

But that’s not really the problem.  Of course something else changed, how else could the price/interest rate/exchange rate have changed in the first place!  There is nothing “tricky” about the examples I just provided, they are straight-forward applications of supply and demand.  Draw a supply and demand curve.  Then shift demand to the left.  You will clearly see that at a lower price consumption will drop.

So what do we know about prices?  We know that if the price falls because supply increases, then consumption will increase, and if the price fell because demand fell, then consumption will decrease.  In other words we know that if the price (or interest rate or exchange rate) changes, we can predict with 50% confidence that quantity will increase, and 50% confidence that quantity will decrease.  So that’s progress, I guess.

Recently I read that the current low energy prices are actually a “bad thing,” because they will discourage consumers form conserving.  I found this interesting, as I think the current low prices are a bad thing because they reflect consumers conserving energy.  Why are they “conserving” energy?  Because they are out of work.

Often I read that the Fed made a huge mistake cutting interest rates to 1% in 2003, because it blew up a housing bubble.  I thought interest rates fell in 2003 because investment fell.

I notice that people really like to pontificate about exchange rates.  I really don’t know much about open economy macro, but I always wonder what people mean when they talk about the implications of a falling dollar.  Why would the dollar fall in the first place?  More sophisticated pundits will talk about a scenario where foreigners suddenly don’t want to hold American dollars anymore.  So now we finally have a root cause, and we can see what happens next.  But why do foreigners not want to hold dollars anymore?  Does it mean that they no longer want to run trade surpluses with the US?  And if so, why not?  Do they want to save less, or invest more?  And how will their preference for balanced trade affect us?  All we really know is that our trade deficit will disappear.  But what does that mean for the US economy?  More exports?  Or less imports?  Suppose the Fed is targeting NGDP when this collapse in confidence in the dollar occurs?  What then?  Here is the Sumner Rule, derived from 54 years of watching predictions fail to come true:

If over several business cycles people keep saying a trend is unsustainable, it is sustainable.  Or at least you will not live long enough to see it reversed.  It may be reversed some day, but since you will not live long enough to see it reversed, it’s not worth thinking about.

What sort of unsustainable trends am I thinking about?  How about Asians lending money to Aussies and Americans?  Or how about health care costs rising as a share of GDP?  By the way, on this point it might be easier to think in terms of “non-health care costs.”  Suppose the non-health share of GDP falls in half every 500 years.  Then total spending on non-health would still be able to rise as long as we had any kind of decent RGDP growth rate.  Can’t happen forever?  Don’t bet on it.

Lessons for teaching economics

Is it possible that economics students don’t learn anything when we teach them supply and demand?  It seems to me that there are basically two things we’d like our students to know about S&D (before we get into applications like price controls and taxes):

1.  The impact of a supply or demand shift.

2.  How to draw inferences from changes in prices and quantity.

I teach at an institution that is well above average, and here is what I have found.  Almost every single student comes into EC101 knowing the impact of supply and demand shocks.  Tell them a frost hits the Florida orange crop, and they can explain what happens to the price of oranges.  Tell them millions of Chinese start buying cars and they can tell you what happens to the price of oil.

I also find that almost no student comes into my class knowing how to interpret price and quantity data.  And what is worse, they leave the course equally ignorant.  I often ask the following question to upper level econ or MBA students who have already taken principles:

Question: A survey shows that on average 100 people go to the movies when the price is $6 and 300 people go when the price is $9.  Does this violate the laws of supply and demand?

Very, very few can answer this question, especially if you ask for an explanation.  Even worse, I think there is a perception that there is something ‘tricky’ about this question, something unfair.  In fact, it is as easy a question as you could imagine.  It’s basic S&D.  It’s merely asking students what happens when the demand for movies shifts.  I cannot imagine a less tricky question, or a more straightforward application of the laws of supply and demand.  In the evening hours the demand for movies shifts right.  Price rises.  Quantity supplied responds.  What’s so hard about that?  And yet almost no student can get it right.  Our students enter EC101 knowing one of the two things they need to know about S&D, and they leave knowing one of the two things they need to know about S&D.  Maybe instead of having them memorize mind-numbing lists of “5 factors that shift supply,” and “5 factors that shift demand,” we should just tell them to read something that will explain what economics is all about, something that portrays economists as detectives trying to solve the identification problem, something like Freakonomics.

If there are any other economics instructors out there I’d like to know what you think.  I really don’t think we need to teach students what happens when frost hits the Florida orange crop.  Perhaps we should just put supply and demand into an appendix and tell them to study it if they need to.  Instead devote 100% of chapter 4 to the identification problem.  Leave all the technical stuff for students majoring in economics to take in their intermediate level courses.  Or maybe the identification problem is too hard, and we should simply forget about teaching supply and demand.  Devote the whole course to opportunity costs, incentives, marginal analysis, etc.

I’m open to suggestions.  But when I read the newspaper, even the elite newspapers like the NYT, FT, WSJ, etc, I feel like something is wrong.  What they are doing is about as closely related to economics as astrology is to astronomy.  Talking about the implications of price/interest rate/exchange rate changes is about as useful as talking about the implications of Saturn being in Aquarius.

PS.  In case Greg Mankiw is reading:  The fact that I use your book shows that I think it is the best one out there.

PPS.  In case Tyler Cowen is reading:  Ignore the previous PS, I haven’t had a chance to check out your (and Alex’s) book yet.

 [点击查看Scott Sumner的英文博客 [Scott Sumner中文博客]  

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