为什么宏观经济学这么难讲？

Why is macroeconomics so hard to teach?

2021年2月1日

这是2018年8月11日《经济学家》杂志上的一篇专栏文章。因为这杂志好像也是时不时地被wall，所以将原文译出搬到这里，供大家随便看看。毕竟只是专栏文章，写的相对不够深入。不过饶是如此，我也有几个地方吃的不准，不敢保证译出了原作者的真实意思，有兴趣的自己去翻原文吧。

https://www.economist.com/finance-and-economics/2018/08/09/why-is-macroeconomics-so-hard-to-teach

Nick Rowe先生长期在（位于渥太华的 ）卡尔顿大学讲授宏观经济学。上个月他做了个噩梦，梦到在距秋季学期开学的第一节课还有5分钟时，他找不到教室了。惊醒过来后才欣慰地想起来，他刚刚已经退休了。学习宏观经济学是许多学生焦虑的根源，事实上讲授这门课也会让他们的任课老师感到不安。众所周知，这门课程涉及的主题都是很难解释清楚的。Nick Rowe先生在卡尔顿的37年里，他自己也承认，作为一名研究人员，他一直处于“相当低的地位”。但他却成了传达宏观经济直觉的雷鸟：在过去的十年里，他为经济学博客“值得一提的加拿大推广计划”（Worthwhile Canadian Initiative）撰写了大量文章，开创了其知识分子职业生涯的第二阶段。他通过巧妙的类比或以数值化的比喻为许多争论提供了有益的视角，常常会令人听君一席话，胜读十年书之感。他的许多类比和比喻都会以某种水果为例。

宏观经济学的教师们可能会发现，在面对宏观经济学的课堂时，永远是准备不足的。想要做宏观领域的学术研究，他们必须解答该领域的大牛们提出的深奥问题。但是要想成为一名优秀的初级宏观经济学教师，他们必须对那些懵懵懂懂的学生给出清晰的答案。这需要对讲课内容有良好的直觉，仅仅通过摆弄方程式是不够的。

事实上，Nick Rowe先生自认为，自己做为一名教师之所以还算得上成功，至少是部分地由于自己作为数学家的短板。他引述过琼·罗宾逊（Joan Robinson）的一段话：“我从来没有学过数学，所以我不得不思考。”因为答案并不会从方程式中跳出来，他不得不更专注于代数式背后的经济行为。

宏观经济学之所以难以讲授，部分原因在于这个领域的理论家们（古典主义、凯恩斯主义、货币主义、新古典主义和新凯恩斯主义等等）彼此的看法是大相迳庭。造成这门课程讲授困难的另一个原因又恰恰是各类教科书的大同小异。为了吸引尽可能多的读者，大多数教科书都提供了相似的材料：各种各样的模型，而这些模型之间常常是彼此并不相容，有些模型甚至都不能自洽。结果就是许多教师必须要讲授那些连他们自己都不相信的东西。

宏观经济学的老师们有时也会忘记，宏观经济学充满了语义陷阱（假性同源词faux amis）：有些术语与日常用语中的含义有所不同。“储蓄”就是一个例子。在日常生活中，储蓄的含义是与支出相对应的。在宏观经济学中，储蓄则与消费是相对应（或者更准确地说，从生产中获得的收入没有用于购买新的消费品）。在宏观经济学中，一个人花了一大笔钱买房子，就算他要掏空自己的银行账户才能买这房子，这也算是储蓄。这个词可能非常混乱，Nick Rowe先生认为应该把它从宏观经济学中剔除掉。

Nick Rowe先生认为，宏观经济学这门学科多少有点“怪异”，这使得它更难讲授。对他来说，这门学科的根本问题是让·萨伊（Jean-Baptiste Say）在200年前提出的：供给创造自己的需求吗？答案往往是否定的，这很奇怪。如果人们不能用收益获得同样有价值的商品（从而增加需求），为什么还要费劲生产和销售东西（从而增加供给）？由于学生们对经济衰退司空见惯，所以认为经济衰退是理所当然的，他们可能没有意识到经济衰退其实并非常态。教师们可能会意识到学生们的这种困惑，但他们有时会纠结或者干脆忽略对其加以解释。Nick Rowe先生本人是直到进入研究生院才真正地遇到了萨伊定律。

作为一个货币主义者，Nick Rowe先生认为经济衰退的原因在这于对货币这个交换媒介的过度需求。为了说明这一点，他建立了一个“极简主义”的宏观经济模型，这是他能构建的最小的模型。模型目的是说明发生经济衰退的必要条件是什么，以及它遗漏了什么，不一定需要什么。不出所料，它的模型又用到了水果来举例。

在这个模型中，有一半的人拥有苹果，另一半人拥有香蕉。这两组人也都有些芒果，但没有那么多。卖苹果的人想要更多的香蕉；卖香蕉的人想要更多的苹果。但他们最想要的是更多的芒果。

在这个世界上的人们显然可以从用苹果换香蕉这样的交易中获益。在物物交换的经济中，情况的确是如此。但是如果把这些水果中的一种，芒果，作为交换的媒介呢？如果苹果和香蕉都可以用来交换芒果，二者之间却不能直接相互交换呢？这与现实世界非常相似，在现实世界中，商品交易通常是通过货币进行的，而无法直接进行物物交换。

在这种情况下，转手的水果将减少，贸易的潜在收益将丧失。人们不愿意用他们囤积的芒果买很多东西。结果，他们自己也无法把大部分的水果卖给其他人以换取芒果，因为其他人也是都想囤积一些芒果。按照Nick Rowe先生的说法，这就是经济衰退的样子。对交换媒介的过度需求抑制了贸易。工人们不能用他们的劳动换得货币，部分原因是他们（和其他人一样都）不愿意用他们的货币换来别人劳动的成果。

货币主义者认为，由于各种原因，交换媒介是独特的。当人们想要获得更多的任何其他商品或资产，他们必须通过购买行为来实现。然而，如果他们想要更多的货币，他们可以简单地通过克制自己购买其他东西的方式来实现，这正是经济衰退时的特征之一，支出下降。同样，如果任何其他资产或商品需求旺盛，其价格将上涨，除非其需求被抑制住。但因为每样东西都是用货币来定价的，所以它没有自己的价格。只有当其他所有东西的价格下跌时，它才能升值，通货紧缩压力也是衰退的特征。

讲好宏观经济学的秘密

没有对宏观经济现象的强大的直觉，你不可能教好宏观经济学。但要想获得这种直觉，最好的方法就是讲授这门课程。“我每次都学到了一些东西，”Nick Rowe先生说。在“评师网”（Rate my Professors）这个网站上，一个学生向他致以最崇高的敬意：他使得上午8点半的课程成为了最值得一听的课。在他漫长的教学生涯结束时，他的学生们是如何表达他们的感激之情的呢？当然，是给他一个苹果和一根香蕉。

原文如下：

LAST month NickRowe had a bad dream. It was five minutes before the first class of the autumn term at Carleton University in Ottawa, where he has long taught macroeconomics. But he could not find the classroom. Then he woke up and remembered with relief that he had just retired.

Learning macro is a source of anxiety for many students. Teaching it can give their professors the jitters, too. The subject is notoriously difficult to explain well. During his 37 years at Carleton Mr Rowe remained, by his own admission, “fairly low down the totem pole” as a researcher. But he became a thunderbird at conveying macroeconomic intuition. In the past decade this served him well in his second intellectual career, contributing to Worthwhile Canadian Initiative, an economicsblog. Many a controversy has benefited from one ofhis ingenious analogies or numerical parables, usually involving some kind offruit.

Professors may find themselves ill-prepared for the macro classroom. To become academics they had to answer erudite questions posed by more senior members of the discipline. To become good teachers of introductory macro, they have to give clearanswers to muddled students. That requires an intuitive feel for the subject. It is not enough to crank through the equations.

Indeed, Mr Rowe attributes part of his success as a teacher to his shortcomings as a mathematician. He quotes Joan Robinson, another clear expositor of macroeconomics: “I never learned maths, so I had to think.” Because the answers did not leap out at him from the equations, he had to dwell on the economic behaviour underneath the algebra.

Macroeconomics is difficult to teach partly because its theorists (classical, Keynesian, monetarist, New Classical and New Keynesian, among others) disagree about so much. It is difficult also because the textbooks disagree about so little. To reach the widestpossible audience, most coversimilarmaterial: a miscellany of models that are not always consistent with each other or even with themselves. The result is that many professors must teach things they do not believe.

Professors can also sometimes forget that macroeconomics is full of faux amis: words that mean something different in everyday speech. “Saving” is an example. In ordinary life, it means the opposite of spending. In macroeconomics it means the opposite of consumption (or, more precisely, not buying new consumer goods with income earned from production). In macro, someone who spends a fortune on a house is saving even if they have emptied their bank account to do so. The term can be so confusing that Mr Rowe thinks it should be banished from the discipline.

More difficulties, Mr Rowe suggests, follow from the fact that macroeconomics is a bit “weird”. For him, the discipline’s fundamental question is the one broached by Jean-Baptiste Say 200 years ago: does supply create its own demand? The answer, which is often no, is odd. Why do people go to the trouble of producing and marketing stuff (thereby adding to supply) if not to obtain equally valuable goods with the proceeds (thereby adding to demand)? Because students take recessions for granted, they may not realise how peculiar they are. Professors may recognise the strangeness. But they sometimes struggle or neglect to explain it. Mr Rowe did not encounter Say’s law explicitly until well into graduate school.

As a monetarist, he thinks the explanation for recessions lies in an excess demand for money, the medium of exchange. To illustrate the point he has built a “minimalist” macroeconomic model, the smallest he can get away with. Its aim is to show what is required for a recession and, by what it leaves out, what is not necessarily required. Inevitably, it involves fruit.

In this model half the people have apples, the other half bananas. The two groups also have mangoes, but not as many. The apple-sellers would like more bananas; the banana-sellers more apples. But what they all want most is more mangoes.

People in this world can clearly gain from trading apples for bananas. And in a barter economy that is exactly what happens. But what if one of the fruits—mangoes—serves as the medium of exchange? What if apples and bananas can be traded for mangoes but not directly with each other? This parallels the real world where goods are typically traded for money but not each other.

In this scenario less fruit will change hands and potential gains from trade will be lost. People are unwilling to buy much with their mangoes, which they hoard. As a result they are themselves unable to sell much of their fruit for the mangoes that everyone else is similarly hoarding. This, according to Mr Rowe, is what a recession looks like. An excess demand for the medium of exchange depresses trade. Workers are unable to sell their labour for money, partly because they (and everyone else) are unwilling to part with their money for the fruits of anyone else’s labour.

Monetarists think the medium of exchange is distinctive for a variety of reasons. With any other good or asset, when people want more they must buy it. If they want more money, however, they can simply refrain from buying other things, a drop in spending characteristic of a recession. Similarly, if any other asset or good is in hot demand, its price will rise until the demand is quenched. But because everything is priced in money, it has no price of its own. It can rise in value only if the price of everything else falls, a deflationary pressure also characteristic of recessions.

The hidden fundamentals of macro

You cannot teach macro well without a strong intuitive feel for the subject. But the best way to gain a feel for the subject is to teach it. “I learn something every time,” Mr Rowe says. On Rate my Professors, a website, one student paid him the ultimate tribute: he made an 8.30am class worth attending. And how, at the end of his long teaching career, did his students show their appreciation? Naturally, by giving him an apple and a banana.