谁该对这场大萧条以来最严重的衰退负责，是风险管理工作糟糕透顶的金融家，还是未能阻止他们的监管者？指责游戏仍在继续。但经济学界承担的不只是一点点罪责。它提供的模型，让监管机构安心地相信市场可以自我监管；相信模型是有效的，而且会自我修正。市场价格完全揭示了所有相关信息的有效市场假说曾风靡一时。如今，不仅我们的经济陷入一团糟，就连在危机发生前几年占据主导地位的经济范式也是如此——或至少应该是。
非经济学专业的人士很难理解，主流宏观经济模型有多么奇特。许多人以为，需求必定会等于供给——这意味着不可能出现失业（目前有许多人在享受额外的闲暇；“他们为何不快乐”是一个心理学问题，而不是经济学问题）。许多人使用“代表性个人模型”——即假定所有个人都是一样的，这意味着不可能存在有意义的金融市场（谁把借钱给谁？）。作为现代经济学基石的信息不对称假说也无处立足：它们只有在个体患上严重的精神分裂症时才会成立。这种假设与人们所青睐的完全理性假说是矛盾的。
糟糕的模型会导致糟糕的政策：例如，各央行曾一心关注通胀导致的轻微经济效率低下，而没有理会金融市场功能失调和资产价格泡沫导致的严重得多得多的效率低下。毕竟，他们的模型说，金融市场总是有效的。引人注目的是，标准的宏观经济模型甚至没有对银行进行足够的分析。难怪美联储(Fed)前主席艾伦•格林斯潘(Alan Greenspan)在他著名的道歉声明中，会对银行没能在风险管理方面做得更好感到意外。真正的意外是他竟然会感到意外：只要扫一眼银行及其经理人面对的不合理激励，就能预测出他们会采取过度冒险的短视行为。
应当根据预测能力给标准模型评级——尤其是它们在紧要形势下的预测能力。提高正常时期预测的准确性——知道明年的经济增长率是2.4%还是2.5%，远没有了解重大衰退的风险那么重要。在这方面，宏观经济模型败得很惨。到目前为止，基于这些模型的预测完全削弱了政策制定者的可信度。它们未能预见到危机的来临，称泡沫破裂后的影响是有限的，而且远没有认识到其后果会有现实发生的那么持久和严重。
幸运的是，尽管多数主流经济学家将目光聚焦在这些有缺陷的模型，但仍有许多研究人员致力于开发替代方法。经济学理论已经表明，标准模型的许多核心结论并不严谨——也就是说，假设条件的轻微改变，就会导致结论发生重大变化。即使是轻微的信息不对称或风险市场中的不完美，也意味着市场不是有效的。亚当•斯密(Adam Smith)的“看不见的手”等著名结论是站不住脚的——看不见的手之所以看不见，是因为这只手并不存在。现在很少有人会认为，银行经理人在追求个人利益时，也促进了全球经济的福祉。
货币政策通过信用的可获得性（以及获得信用的条款，尤其是针对中小企业的条款）来影响经济。要理解这一点，需要我们分析银行及银行与影子银行业的相互影响。国债利率与贷款利率之间的利差可以大幅改变。除了少数例外，大多数央行很少注意系统性风险和信用相互关联造成的风险。此次危机爆发前几年，一些研究人员聚焦于这些问题，包括连续破产将在本次危机中以如此重要的方式出现的可能性。这个例子表明，谨慎建立经济个体（家庭、企业和银行）之间复杂关系的模型非常重要——这种关系在“假定所有人都一样”的模型里是无法研究的。就连至高无上的“理性”假设也受到了攻击：人们需要研究宏观经济行为的理性和后果的系统偏差。
改变范式并不容易。太多的人投入了太多时间和精力到错误的模型之中。就像托勒密(Ptolemaic)试图维护地球是宇宙中心的观点一样，总会有人勇敢地尝试让标准范式变得更加复杂和完善。由此得出的模型会有所改进，基于这些模型的政策也会做得更好，但它们仍有可能失败。没有比改变范式更好的了。
但我相信，我们是可以掌握新的范式的：智力构件已经存在，而新经济思维研究所(INET)正在搭建框架，以便将形形色色努力创建新范式的学者聚合在一起。当然，陷入危险的不仅是经济学界或依赖经济学理念的政策制定者的可信度：还有我们经济的稳定与繁荣。
本文作者为哥伦比亚大学(Columbia University)校级教授，于2001年获得诺贝尔经济学奖。他曾担任比尔•克林顿(Bill Clinton)的经济顾问委员会(council of economic advisers)主席和世界银行(World Bank)首席经济学家，目前是新经济思维研究所咨询委员会成员。
译者/君悦
http://www.ftchinese.com/story/001034219
The blame game continues over who is responsible for the worst recession since the Great Depression – the financiers who did such a bad job of managing risk or the regulators who failed to stop them. But the economics profession bears more than a little culpability. It provided the models that gave comfort to regulators that markets could be self-regulated; that they were efficient and self-correcting. The efficient markets hypothesis – the notion that market prices fully revealed all the relevant information – ruled the day. Today, not only is our economy in a shambles but so too is the economic paradigm that predominated in the years before the crisis – or at least it should be.
It is hard for non-economists to understand how peculiar the predominant macroeconomic models were. Many assumed demand had to equal supply – and that meant there could be no unemployment. (Right now a lot of people are just enjoying an extra dose of leisure; why they are unhappy is a matter for psychiatry, not economics.) Many used “representative agent models” – all individuals were assumed to be identical, and this meant there could be no meaningful financial markets (who would be lending money to whom?). Information asymmetries, the cornerstone of modern economics, also had no place: they could arise only if individuals suffered from acute schizophrenia, an assumption incompatible with another of the favoured assumptions, full rationality.
Bad models lead to bad policy: central banks, for instance, focused on the small economic inefficiencies arising from inflation, to the exclusion of the far, far greater inefficiencies arising from dysfunctional financial markets and asset price bubbles. After all, their models said that financial markets were always efficient. Remark-ably, standard macroeconomic models did not even incorporate adequate analyses of banks. No wonder former Fed Reserve chairman Alan Greenspan, in his famous mea culpa, could express his surprise that banks did not do a better job at risk management. The real surprise was his surprise: even a cursory look at the perverse incentives confronting banks and their managers would have predicted short-sighted behaviour with excessive risk-taking.
The standard models should be graded on their predictive ability – and especially their ability to predict in circumstances that matter. Increasing the accuracy of forecast in normal times (knowing whether the economy next year will grow at 2.4 per cent or 2.5 per cent) is far less important than knowing the risk of a major recession. In this the models failed miserably, and the predictions based on them have, by now, totally undermined the credibility of policymakers. They failed to see the crisis coming, said its effects were contained after the bubble broke, and thought the consequences would be far more short-lived and less severe than they have been.
Fortunately, while much of the mainstream focused on these flawed models, numerous researchers were engaged in developing alternative approaches. Economic theory had already shown that many of the central conclusions of the standard model were not robust – that is, small changes in assumptions led to large changes in conclusions. Even small information asymmetries, or imperfections in risk markets, meant that markets were not efficient. Celebrated results, such as Adam Smith’s invisible hand, did not hold; the invisible hand was invisible because it was not there. Few today would argue that bank managers, in their pursuit of their self-interest, had promoted the well-being of the global economy.
Monetary policy affects the economy through the availability of credit – and the terms on which it is made available, especially to small- and medium-sized enterprises. Understanding this requires us to analyse banks and their interaction with the shadow banking sector. The spread between the Treasury bill rate and lending rates can change markedly. With a few exceptions, most central banks paid little attention to systemic risk and the risks posed by credit interlinkages. Years before the crisis, a few researchers focused on these issues, including the possibility of the bankruptcy cascades that were to play out in such an important way in the crisis. This is an example of the importance of modelling carefully complex interactions among economic agents (households, companies, banks) – interactions that cannot be studied in models in which everyone is assumed to be the same. Even the sacrosanct assumption of rationality has been attacked: there are systemic deviations from rationality and consequences for macroeconomic behaviour that need to be explored.
Changing paradigms is not easy. Too many have invested too much in the wrong models. Like the Ptolemaic attempts to preserve earth-centric views of the universe, there will be heroic efforts to add complexities and refinements to the standard paradigm. The resulting models will be an improvement and policies based on them may do better, but they too are likely to fail. Nothing less than a paradigm shift will do.
But a new paradigm, I believe, is within our grasp: the intellectual building blocks are there and the Institute for New Economic Thinking is providing a framework for bringing the diverse group of scholars striving to create this new paradigm together. What is at stake, of course, is more than just the credibility of the economics profession or that of the policymakers who rely on their ideas: it is the stability and prosperity of our economies.
The writer, recipient of the 2001 Nobel Memorial Prize in economics, is University Professor at Columbia University. He served as chairman of President Bill Clinton’s Council of Economic Advisers and as chief economist of the World Bank. He is on the Advisory Board of INET
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