书本前言

Paul Samuelson once stated that “macroeconomics, even with all of our computers and with all of our information is not an exact science and is incapable of being an exact science”. Perhaps this quote captures the view that the feld of macroeconomics, the study of aggregate behaviour of the economy, is full of loose ends and inconsistent statements that make it difcult for economists to agree on anything.

保罗·萨缪尔森曾说过，“宏观经济学，即使有了我们所有的计算机和所有的信息，也不是一门精确的科学，也不可能成为一门精确的科学”。也许这句话概括了这样一种观点，即宏观经济学领域，即对经济总体行为的研究，充满了漏洞和不一致的陈述，使得经济学家很难在任何事情上达成一致。

While there is truth to the fact that there are plenty of disagreements among macroeconomists, we believe such a negative view is unwarranted. Since the birth of macroeconomics as a discipline in the 1930s, in spite of all the uncertainties, inconsistencies, and crises, macroeconomic performance around the world has been strong. More recently, dramatic shocks, such as the Great Financial Crisis or the Covid pandemic, have been managed – not without cost, but with efective damage control. Tere is much to celebrate in the feld of macroeconomics.

尽管宏观经济学家之间存在诸多分歧，但我们认为这种负面观点是没有根据的。自 20 世纪 30 年代宏观经济学作为一门学科诞生以来，尽管存在各种不确定性、不一致性和危机，但世界各地的宏观经济表现一直强劲。最近，诸如全球金融危机或 Covid 疫情等剧烈冲击也得到了控制——虽然并非没有代价，但进行了有效的损害控制。宏观经济学领域有很多值得庆祝的地方。

Macroeconomics was born under the pain of both U.S. and UK’s protracted recession of the 1930s. Until then, economics had dealt with markets, efciency, trade, and incentives, but it was never thought that there was place for a large and systematic breakdown of markets. High and persistent unemployment in the U.S. required a diferent approach.

宏观经济学诞生于 20 世纪 30 年代美国和英国长期衰退的痛苦之中。在此之前，经济学研究的是市场、效率、贸易和激励，但从未有人认为市场会出现大规模和系统性的崩溃。美国持续的高失业率需要一种不同的方法。

The main distinctive feature to be explained was the large disequilibrium in the labour market. How could it be that a massive number of people wanted to work, but could not fnd a job? Tis led to the idea of the possibility of aggregate demand shortfalls – and thus of the potential role for government to prop it up, and, in doing so, restore economic normalcy. “Have people dig a hole and fll them up if necessary” is the of-quoted phrase by Keynes. In modern economic jargon, increase aggregate demand to move the equilibrium of the economy to a higher level of output.

需要解释的主要特点是劳动力市场的大规模失衡。 为什么会有大量的人想工作，却找不到工作？ 这就引出了总需求不足的可能性——以及政府支持总需求，从而恢复经济常态的潜在作用。“让人挖个坑再填上，如有必要”是凯恩斯经常被引用的一句话。 用现代经济术语来说，就是增加总需求，将经济的均衡点转移到更高的产出水平。

Thus, an active approach to fscal and monetary policy developed, entrusting policy makers with the role of moderating the business cycle. Te relationship was enshrined in the so-called Phillips curve, a relationship that suggested a stable tradeof between output and infation. If so, governments simply had to choose their preferred spot on that tradeof.

因此，一种积极的财政和货币政策方法得以发展，赋予政策制定者缓和商业周期的角色。这种关系体现在所谓的菲利普斯曲线中，这种曲线表明产出和通货膨胀之间存在稳定的权衡。如果是这样，政府只需选择他们在该权衡中的首选位置。

Then things changed. Higher infation in the 60s and 70s, challenged the view of a stable tradeof between output and infation. In fact, infation increased with no gain in output, the age of stagfation had arrived. What had changed?

然后事情发生了变化。60 年代和 70 年代更高的通货膨胀挑战了产出和通货膨胀之间稳定权衡的观点。事实上，通货膨胀上升，但产出没有增加，滞胀时代已经到来。什么发生了变化？

Answer had to do with the role of expectations in macroeconomics[1]

答案与预期在宏观经济中的作用有关

Te stable relationship between output and infation required static expectations. People did not expect infation, then the government found it was in its interest to generate a bit of infation – but that meant people were always wrong! As they started anticipating the infation, then its efect on employment faded away, and the efectiveness of macro policy had gone stale.

产出和通胀之间稳定的关系需要静态预期。人们不预期通胀，然后政府发现制造一点通胀符合其利益——但这意味着人们总是错的！当他们开始预期通胀时，它对就业的影响就消失了，宏观政策的有效性也变得陈旧。

Te rational expectations revolution in macroeconomics, initiated in the 1970s, imposed the constraint that a good macro model should allow agents in the model to understand it and act accordingly. Tis was not only a theoretical purism. It was needed to explain what was actually happening in the real world. Te methodological change took hold very quickly and was embraced by the profession. As a working assumption, it is a ubiquitous feature of macroeconomics up to today.

20 世纪 70 年代发起的宏观经济学中的理性预期革命，提出了一个约束条件，即一个好的宏观模型应该允许模型中的行为者理解它并采取相应的行动。这不仅仅是一种理论上的纯粹主义。它需要解释现实世界中实际发生的事情。这种方法论的改变很快被接受，并被该行业所采纳。作为一个工作假设，它至今仍然是宏观经济学中普遍存在的一个特征。

Ten an additional challenge to the world of active macroeconomic policy came about. In the early 1980s, some macroeconomists started the “real business cycles” approach: they studied the neoclassical growth model – that is, a model of optimal capital accumulation – but added to it occasional productivity shocks. Te result was a simulated economy that, they argued, resembled on many dimensions the movements of the business cycle. Tis was a dramatic fnding because it suggested that business cycles could actually be the result of optimal responses by rational economic agents, thereby eschewing the need for a stabilising policy response. What is more, active fscal or monetary policy were not merely inefective, as initially argued by the rational expectations view: they could actually be harmful.

对积极的宏观经济政策而言，又出现了一个额外的挑战。在 20 世纪 80 年代初，一些宏观经济学家开始了“真实商业周期”方法：他们研究了新古典增长模型——即最优资本积累模型——但偶尔会加入生产力冲击。结果是一个模拟经济，他们认为，在许多方面都类似于商业周期的运动。这是一个戏剧性的发现，因为它表明商业周期实际上可能是理性经济主体最优反应的结果，从而避免了对稳定政策反应的需求。更重要的是，正如理性预期观点最初认为的那样，积极的财政或货币政策不仅无效，而且实际上可能是有害的。

This was the state of the discussion when a group of economists tackled the task of building a framework that recovered some of the features of the old Keynesian activism, but in a model with fully rational agents. Tey modelled price formation and introduced market structures that departed from a perfectly competitive allocation. Tey adhered strictly to the assumptions of rational expectations and optimisation, which had the added advantage of allowing for explicit welfare analyses. Tus, the New Keynesian approach was built. It also allowed for shocks, of course, and evolved into what is now known as dynamic stochastic general equilibrium (DSGE) models.

当时讨论的焦点正是在此。一群经济学家致力于构建一个理论框架，以期在完全理性人的模型中，重新拾起旧凯恩斯主义积极干预政策的若干特征。他们对价格形成机制进行建模，并引入了偏离完全竞争配置的市场结构。他们严格遵循理性预期和最优化的假设，这还带来了一个额外的好处，即能够进行明确的福利分析。由此，新凯恩斯主义方法应运而生。当然，该方法也纳入了冲击的考量，并最终发展成为如今广为人知的动态随机一般均衡（DSGE）模型。

Macroeconomic policymaking evolved along those lines. Nowadays, DSGE models are used by any respectable central bank. Furthermore, because this type of model provides fexibility in the degree of price rigidities and market imperfections, it comprises a comprehensive framework nesting the diferent views about how individual markets operate, going all the way from the real business cycle approach to specifcations with ample rigidities.

宏观经济政策的制定沿着这些方向演变。如今，任何一家体面的中央银行都在使用 DSGE 模型。此外，由于这种类型的模型在价格刚性和市场不完善程度上提供了灵活性，因此它构成了一个全面的框架，涵盖了关于个体市场如何运作的不同观点，从实际商业周期方法一直到具有充足刚性的规范。

But the bottom line is that macroeconomics speaks with a common language. While diferences in world views and policy preferences remain, having a common framework is a great achievement. It allows discussions to be framed around the parameters of a model (and whether they match the empirical evidence) – and such discussions can be more productive than those that swirl around the philosophical underpinnings of one’s policy orientations.

但最重要的是，宏观经济学使用一种通用语言。虽然世界观和政策偏好仍然存在差异，但拥有一个共同的框架是一项伟大的成就。它允许围绕模型的参数（以及它们是否与经验证据相符）来构建讨论——而且这种讨论可能比那些围绕一个人的政策导向的哲学基础进行的讨论更有效率。

This book, to a large extent, follows this script, covering the diferent views – and very importantly, the tools needed to speak the language of modern macroeconomic policymaking – in what we believe is an accessible manner. Tat language is that of dynamic policy problems.

本书在很大程度上秉承这一宗旨，旨在阐述各种观点，尤其侧重于理解并掌握现代宏观经济政策制定的语言工具，并力求以平易近人的方式进行解读。这种语言的核心在于动态政策问题。

We start with the Neoclassical Growth Model – a framework to think about capital accumulation through the lens of optimal consumption choices – which constitutes the basic grammar of that language of modern macroeconomics. It also allows us to spend the frst half of the book studying economic growth – arguably the most important issue in macroeconomics, and one that, in recent decades, has taken up as much attention as the topic of business cycles. Te study of growth will take us through the discussion of factor accumulation, productivity growth, the optimality of both the capital stock and the growth rate, and empirical work in trying to understand the proximate and fundamental causes of growth. In that process, we also develop a second canonical model in modern macroeconomics: the overlapping generations model. Tis lets us revisit some of the issues around capital accumulation and long-run growth, as well as study key policy issues, such as the design of pension systems.

我们从新古典增长模型开始——这是一个通过最优消费选择的视角来思考资本积累的框架——它构成了现代宏观经济学语言的基本语法。它还允许我们用本书的前半部分来研究经济增长——这可以说是宏观经济学中最重要的议题，并且在近几十年里，它所受到的关注与商业周期这一主题一样多。对增长的研究将引导我们讨论要素积累、生产率增长、资本存量和增长率的最优性，以及试图理解增长的直接和根本原因的实证研究。在这个过程中，我们还将发展现代宏观经济学中的第二个规范模型：世代交叠模型。这让我们重新审视围绕资本积累和长期增长的一些问题，并研究关键的政策问题，例如养老金制度的设计。

We then move to discuss issues of consumption and investment. Tese are the key macroeconomic aggregates, of course, and their study allows us to explore the power of the dynamic tools we developed in the frst part of the book. Tey also let us introduce the role of uncertainty and expectations, as well as the connections between macroeconomics and fnance.

然后我们转而讨论消费和投资问题。当然，这些是关键的宏观经济总量，通过对它们的研究，我们可以探索我们在本书第一部分中开发的动态工具的力量。它们还让我们介绍了不确定性和预期，以及宏观经济学和金融之间的联系。

Then, in the second half of the book, we turn to the study of business cycle fuctuations, and what policy can and should do about it. We start with the real business cycle approach, as it is based on the neoclassical growth model. Ten we turn to the Keynesian approach, starting from the basic IS-LM model, familiar to anyone with an undergraduate exposure to macroeconomics, but then showing how its modern version emerged: frst, with the challenge of incorporating rational expectations, and then with the fundamentals of the New Keynesian approach. Only then, we present the canonical New Keynesian framework.

然后在本书的后半部分，我们将转向对商业周期波动的研究，以及政策可以而且应该如何应对。我们从实际商业周期方法开始，因为它基于新古典增长模型。然后我们转向凯恩斯主义方法，从基本的 IS-LM 模型开始，任何具有宏观经济学本科背景的人都熟悉它，然后展示它的现代版本是如何出现的：首先，通过纳入理性预期的挑战，然后是新凯恩斯主义方法的基础。只有这样，我们才会介绍规范的新凯恩斯主义框架。

Once we’ve covered all this material, we discuss the scope and efectiveness of fiscal policy. We also go over what optimal fscal policy would look like, as well as some of the reasons for why in practice it departs from those prescriptions. We then move to discuss monetary policy: the relationship between money and prices, the debate on rules vs discretion, and the consensus that arose prior to the 2008 fnancial crisis and the Great Recession. We then cover the post-crisis development of quantitative easing, as well as the constraints imposed by the zero lower bound on nominal interest rates. We fnish of by discussing some current topics that have been infuencing the thinking of policymakers on the fscal and monetary dimensions: secular stagnation, the fscal theory of the price level, and the role of asset-price bubbles and how policy should deal with them.

一旦我们学习完所有这些内容，我们将探讨财政政策的范围和有效性。我们还将分析最优财政政策的形式，并研究在实践中，财政政策为何会偏离这些既定方案的一些原因。随后，我们转而讨论货币政策：货币与价格之间的关系、规则与相机抉择之争，以及2008年金融危机和次贷危机之前形成的共识。此后，我们还将探讨危机后量化宽松的发展，以及名义利率零下限带来的约束。最后，我们将讨论当前影响财政和货币政策决策者思路的一些议题：长期停滞、物价水平的财政理论，以及资产价格泡沫的作用以及政策应如何应对这些泡沫。

As you can see from this whirlwind tour, the book covers a lot of material. Yet, it has a clear methodological structure. We develop the basic tools in the frst part of the book, making clear exactly what we need at each step. All you need is a basic knowledge of calculus, diferential equations, and some linear algebra – and you can consult the mathematical appendix for the basics on the tools we introduce and use in the book. Troughout, we make sure to introduce the tools not for their own sake, but in the context of studying policy-relevant issues and helping develop a framework for thinking about dynamic policy problems. We then study a range of policy issues, using those tools to bring you to the forefront of macroeconomic policy discussions. At the very end, you will also fnd two appendices for those interested in tackling the challenge of running and simulating their own macroeconomic models.

正如你从这次旋风式的浏览中所看到的，本书涵盖了大量内容。然而，它有一个清晰的方法论结构。我们在本书的第一部分开发了基本工具，清楚地说明了我们在每个步骤中需要什么。你只需要具备微积分、微分方程和一些线性代数的基本知识——你可以查阅数学附录，了解我们在书中介绍和使用的工具的基本知识。自始至终，我们确保介绍这些工具不是为了它们本身，而是在研究与政策相关的问题和帮助开发一个思考动态政策问题的框架的背景下。然后，我们研究一系列政策问题，使用这些工具将你带到宏观经济政策讨论的前沿。最后，你还会发现两个附录，供那些有兴趣应对运行和模拟他们自己的宏观经济模型挑战的人使用。

All in all, Samuelson was right that macroeconomics cannot be an exact science. Still, there is a heck of a lot to learn, enjoy and discover – and this, we hope, will help you become an informed participant in exciting macroeconomic policy debates. Enjoy!

总而言之，萨缪尔森认为宏观经济学不可能成为一门精确的科学，他是对的。不过，仍然有很多东西值得学习、享受和发现——我们希望这本书能帮助你成为激动人心的宏观经济政策辩论中一位知情的参与者。祝你愉快！

Note 注意

[1] Surprisingly, the answer came from the most unexpected quarter: the study of agricultural markets. As early as 1960 John Muth was studying the cobweb model, a standard model in agricultural economics. In this model the farmers look at the harvest price to decide how much they plant, but then this provides a supply the following year which is inconsistent with this price. For example a bad harvest implies a high price, a high price implies lots of planting, a big harvest next year and thus a low price! Te low price motivates less planting, but then the small harvest leads to a high price the following year! In this model, farmers were systematically wrong, and kept being wrong all the time. Tis is nonsense, argued Muth. Not only should they learn, they know the market and they should plant the equilibrium price, namely the price that induces the amount of planting that implies that next year that will be the price. Tere are no cycles, no mistakes, the market equilibrium holds from day one! Transferred to macroeconomic policy, something similar was happening.

令人惊讶的是，答案来自最意想不到的方面：农业市场研究。早在 1960 年，John Muth 就在研究蛛网模型，这是农业经济学中的一个标准模型。在这个模型中，农民根据收获价格来决定种植多少，但这会在第二年提供与该价格不一致的供应。例如，歉收意味着高价，高价意味着大量种植，明年大丰收，因此价格低廉！低价促使减少种植，但随后的小丰收导致第二年价格高涨！在这个模型中，农民系统性地犯错，并且一直犯错。Muth 认为这是胡说八道。他们不仅应该学习，而且应该了解市场，并且应该种植均衡价格，即诱导种植量的价格，这意味着明年这将是该价格。没有周期，没有错误，市场均衡从第一天开始就成立！转移到宏观经济政策，类似的事情正在发生。