宏觀經濟理論爲何需要重建

Economics is, like medicine (and unlike, say, cosmology), a practical discipline. Its goal is to make the world a better place. This is particularly true of macroeconomics, which was invented by John Maynard Keynes in response to the Great Depression. The tests of this discipline are whether its adepts understand what might go wrong in the economy and how to put it right. When the financial crisis that hit in 2007 caught the profession almost completely unawares, it failed the first of these tests. It did better on the second. Nevertheless, it needs rebuilding.

經濟學是一門實用學科,與醫學一樣，而與（比如說）宇宙學不一樣。經濟學的目的是讓這個世界變得更美好。宏觀經濟學尤其如此，它由約翰?梅納德?凱恩斯(John Maynard Keynes)在對大蕭條(Great Depression)做出迴應時所創。對這門學科的檢驗在於，宏觀經濟學專家是否瞭解經濟中哪裏可能出問題以及如何糾正。2007年爆發的金融危機幾乎讓該學科完全始料不及，第一次檢驗不及格。第二次的成績好一些。但它需要重建。

In a blog for the Financial Times in 2009, Willem Buiter, now at Citi, argued that: “Most mainstream macroeconomic theoretical innovations since the 1970s?.?.?.?have turned out to be self-referential, inward-looking distractions at best.” An exceptionally thorough analysis, published in the Oxford Review of Economic Policy, under the title “Rebuilding Macroeconomic Theory”, leads this reader to much the same position. The canonical approach did indeed prove gravely defective. Moreover, top class professional economists differ profoundly on what to do about it. Socrates might say that awareness of one’s ignorance is far better than the illusion of knowledge. If so, macroeconomics is in good shape.

在2009年爲英國《金融時報》撰寫的博客中，現供職於花旗(Citi)的威廉姆?比特(Willem Buiter)指出：“自上世紀70年代以來的多數主流宏觀經濟理論創新……被證明充其量是自我參照、封閉的消遣。”發表在《牛津經濟政策評論》(Oxford Review of Economic Policy)的一篇非常深入的分析文章題目爲“重建宏觀經濟理論”，它引導讀者得出大致相同的觀點。墨守成規的研究方法確實被證明存在嚴重缺陷。另外，頂尖職業經濟學家對於如何解決這個問題存在深刻分歧。蘇格拉底(Socrates)可能會說，意識到自己的愚昧無知要比處於知識幻覺好得多。如果是這樣的話，宏觀經濟學目前狀況良好。

As David Vines and Samuel Wills explain in their excellent overview, the core macroeconomic model rested on two critical assumptions: the efficient markets hypothesis and rational expectations. Neither looks convincing today. It is questionable whether it is even possible to have “rational expectations” of a profoundly uncertain future. Such uncertainty helps explain the existence of institutions — money, debt and banks — whose effects are so significant and yet were largely ignored in standard models. It is better to be roughly right than precisely wrong. Thus, Hyman Minsky’s view of the dangers of speculative tendencies in finance was roughly right, while many of the brightest macroeconomists proved precisely wrong.

正如大衛?維內斯(David Vines)和薩姆?威利斯(Sam Willis)在二人精彩的概述中解釋的那樣，核心宏觀經濟模型取決於兩個重要假設：有效市場假說和理性預期。這兩點現在看上去都沒有說服力。對高度不確定的未來是否有可能有“理性預期”甚至都存在疑問。這種不確定性有助於解釋貨幣、債務和銀行等體系的存在，這些體系的影響非常重要，但在標準模型中基本被忽視。大致正確好於完全錯誤。海曼?明斯基(Hyman Minsky)有關金融領域投機傾向危險性的觀點大致正確，而很多最聰明的宏觀經濟學家被證明是完全錯誤的。

It is not good enough to argue that the canonical model works in normal times. We need also to understand the risks of crises and what to do about them. This is partly because crises are, as the Nobel-laureate Joseph Stiglitz notes, the most costly events. A macroeconomics that does not include the possibility of crises misses the essential, just as would a medicine that assumes away the possibility of heart attacks. Moreover, crises are endogenous: that is to say, they come from within the economy. They are a result of the interaction between tendencies towards excessive optimism and the fragility of any system of highly leveraged financial intermediaries.

辯稱標準模型在常規時期有用是不夠的。我們還需要了解危機的風險以及如何應對。這在一定程度上是因爲危機是代價最高的事件，正如諾貝爾獎得主約瑟夫?斯蒂格利茨(Joseph Stiglitz)指出的那樣。不包括危機可能性的宏觀經濟學沒有抓住重點，就好像一種藥物不假設出現心臟病的可能性。另外，危機是有內源性的：這就是說，它們來自經濟內部。它們是過分樂觀的傾向以及高槓杆金融中介體系的脆弱之間相互作用的結果。

My colleague Martin Sandbu notes, in particular, the possibility of “multiple equilibria” — the idea that economies might end up in self-reinforcing bad states of the world. This possibility makes it vital to respond to crises forcefully. Doctors’ first response to a heart attack is, after all, not to tell the patient to go on a diet. That happens only after they have dealt with the attack itself.

我的同事馬丁?桑德布(Martin Sandbu)特別指出“多重均衡”的可能性，這種觀點認爲，經濟可能最終會讓全球的糟糕狀態自我強化。這種可能性讓對危機做出有力應對變得至關重要。畢竟，醫生面對心臟病的第一反應不是告訴患者要節食。這種事情只有在他們處理過心臟病後纔會發生。

So a big question is not only whether we know how to respond to a crisis, but whether we did so. In his contribution, the Nobel laureate Paul Krugman argues, to my mind persuasively, that the basic Keynesian remedies — a strong fiscal and monetary response — remain right. Also vital is swift revitalisation of the banking system. The contrast between the swifter US recovery and the dreadful delays in the eurozone gives striking support for this view. Essentially, the latter lost five years before the recovery began.

因此，重要問題不僅是我們是否明白如何應對一場危機，還有我們是否這麼做了。諾貝爾獎得主保羅?克魯格曼（Paul Krugman)指出（我認爲很有說服力），凱恩斯的基本補救方法——強有力的財政和貨幣應對措施——仍然正確。同樣重要的還有銀行體系快速恢復活力。美國經濟更快復甦與歐元區經濟復甦的可怕延遲之間的對比，爲這種觀點提供了明顯支持。基本上，歐元區失去了5年纔開始復甦。

A comparison between what happened in the 1930s and this post-crash period shows we have indeed learnt some important things. Compared with the Great Depression, the immediate declines in output and rises in unemployment were far smaller. Moreover, prices have also been far more stable this time. These are true successes. Nevertheless, after a decade, the level of output per head, relative to pre-crisis levels, is less impressive: Germany and the UK did even better last time. Moreover, the worst hit eurozone countries have suffered badly, by any standards. This recovery really has not been a triumph.

比較一下上世紀30年代和此次危機之後的時期，我們會發現，我們實際上學到了一些重要的東西。與“大蕭條”相比，產出的立即下滑以及失業率上升幅度要小得多。另外，這次物價也一直穩定得多。這些是真正的成功。然而，在危機發生了10年後，相對於危機前的人均產出水平則不如人意：德國和英國甚至上一次表現得更好。另外，以任何標準來衡量，遭受最嚴重打擊的歐元區國家都受損嚴重。這次復甦實際上算不上成功。

This suggests that fixing a huge crisis after the event is terribly hard. The obvious need then is to make economies more resilient. Even if we do not fully understand the economic dynamics, the broad lessons for the reform of our economies seem clear. Economies would be more resilient if they were less highly leveraged and, in particular, if they depended less on holdings of money backed by risky assets owned by the highly leveraged financial intermediaries known as banks. Obvious solutions include eliminating the incentives towards leverage in our tax systems, encouraging greater use by the economy of equity finance and debt that can be readily converted into equity, raising the reserve and capital requirements of banks and moving swiftly towards the issuance of digital central bank cash.

這表明，事後修復一場嚴重危機非常困難。因此，明顯有必要讓經濟變得更有韌性。儘管我們不完全瞭解經濟運行動態，但該如何改革經濟的普遍教訓似乎是顯而易見的。如果槓桿不那麼高，尤其是如果減少對高槓杆金融中介機構（即銀行）所有的風險資產支持的資金的依賴，經濟的韌性將會增強。顯而易見的解決方案包括消除我們稅收體系中鼓勵使用槓桿的激勵措施、鼓勵擴大經濟對股權融資以及可輕易轉換爲股權的債務的使用、提高銀行的準備金和資本金要求、以及快速轉向發行央行數字貨幣。

The analysis of fundamental macroeconomic theory suggests substantial ignorance of how our economies work. This is not that surprising. We may never understand how such complex systems — animated, as they are, by human desires and misunderstandings — actually function. This does not mean that attempting to improve understanding is a foolish exercise. On the contrary, it is important. But it is arguably more vital in practice to focus on two other tasks. The first is how to make the body economic more resistant to the consequences of manias and panics. The second is how to restore it to health as quickly as possible. On both counts, we need to think more and do more. These are the practical challenges before us.

對基本宏觀經濟理論的分析顯示，我們經濟的運行方式被極大地忽視了。這並不那麼令人意外。我們可能永遠不理解這些複雜的體系（由人類慾望和誤解驅動）實際上如何運轉。這並不意味着努力提高認識是愚蠢之舉。相反，這很重要。但實際上，關注另外兩個任務或許更爲重要。第一個是如何讓經濟更能抵禦狂熱和恐慌的影響。第二個是如何儘可能快地令其恢復健康。從這兩方面來看，我們需要思考更多並採取更多行動。這些是我們面臨的實際挑戰。