曼昆教授是一位高产的学者和一位学术与政治争论的经常参与者。他的著作发表在《美国经济评论》（American Economic Review）、《政治经济学杂志》（Journal of Political Economy）和《经济学季刊》（Quarterly Journal of Economics）等学术杂志，以及《纽约时报》（The New YorkTimes）、《金融时报》（The Financial Times）、《华尔街日报》（The Wall Street Journal）和《财富》（Fortune）等热门报刊上。他也是最畅销的中级经济学教科书《宏观经济学》（WorthPublishers出版）的作者。除了教学、研究和写作之外，曼昆教授还是美国国家经济研究局的合作研究人员、波士顿联邦储备银行和国会预算办公室的顾问。以及ETS考试研发委员会下设的经济学高级水平考试委员会成员。从2003年到2005年。他担任总统经济顾问委员会主席。
To find a substitute for laboratory experiments, economists pay close attention to the natural experiments offered by history. When a war in the MiddleEast interrupts the flow of crude oil, for instance, oil prices skyrocket around theworld. For consumers of oil and oil products, such an event depresses livingstandards. For economic policymakers, it poses a difficult choice about how bestto respond. But for economic scientists, the event provides an opportunity tostudy the effects of a key natural resource on the world's economies, and thisopportunity persists long after the wartime increase in oil prices is over.Throughout this book, therefore, we consider many historical episodes. Theseepisodes are valuable to study because they give us insight into the economy ofthe past and, more important, because they allow us to illustrate and evaluateeconomic theories of the present.
The Role of Assumptions
If you ask a physicist how long it would take for a marble to fall from the top ofa ten-story building, she will answer the question by assuming that the marblefalls in a vacuum. Of course, this assumption is false. In fact, the building is surrounded by air, which exerts friction on the falling marble and slows it down.Yet the physicist will correctly point out that friction on the marble is so smallthat its effect is negligible. Assuming the marble falls in a vacuum greatly simplifies the problem without substantially affecting the answer.
Economists make assumptions for the same reason: Assumptions can simplifythe complex world and make it easier to understand. To study the effects ofinternational trade, for example, we may assume that the world consists of onlytwo countries and that each country produces only two goods. Of course, thereal world consists of dozens of countries, each of which produces thousands ofdifferent types of goods. But by assuming two countries and two goods, we canfocus our thinking on the essence of the problem. Once we understand international trade in an imaginary world with two countries and two goods, we are ina better position to understand international trade in the more complex world inwhich we live.
The art in scientific thinking-whether in physics, biology, or economics-isdeciding which assumptions to make. Suppose, for instance, that we were drop-ping a beachball rather than a marble from the top of the building. Our physicistwould realize that the assumption of no friction is far less accurate in this case:Friction exerts a greater force on a beachball than on a marble because a beach-ball is much larger. The assumption that gravity works in a vacuum is reasonable for studying a falling marble but not for studying a falling beachball.
Similarly, economists use different assumptions to answer different questions.Suppose that we want to study what happens to the economy when the government changes the number of dollars in circulation. An important piece of thisanalysis, it turns out, is how prices respond. Many prices in the economy changeinfrequently; the newsstand prices of magazines, for instance, change only everyfew years. Knowing this fact may lead us to make different assumptions whenstudying the effects of the policy change over different time horizons. For studying the short-run effects of the policy, we may assume that prices do not changemuch. We may even make the extreme and artificial assumption that all pricesare completely fixed. For studying the long-run effects of the policy, however,we may assume that all prices are completely flexible. Just as a physicist usesdifferent assumptions when studying falling marbles and falling beachballs,economists use different assumptions when studying the short-run and long-runeffects of a change in the quantity of money.