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公众可能比专家更能够理解造成危机的原因。
很少有宏观经济学家能提前预知经济危机，他们似乎总是事后诸葛亮般，修改已有的经济理论模型。
本文主要介绍了一种新的引人注目的宏观经济学理论：「外推预期理论」（extrapolative expectations）。
该理论主要认为，当房价、股票等资产价格（asset prices），接连不断上涨时，投资者开始相信这种趋势代表了一种新常态。
他们蜂涌入这些资产，进一步抬高了价格，似乎证实了这种趋势永不会停止的观点。 但是当外推者（extrapolators）的资金耗尽了，现实露出真面目，随之而来的就是崩溃。
在经济繁荣时期的某个时刻，非理性繁荣（irrational exuberance）占据优势，推高了股价、房价，或者将两者的价格都炒上了天。当它们不可避免地下跌时，银行破产，连带着其余的经济一起崩盘。
这种理论似乎极好的解释了当下的各种金融乱像（比如收割本金的匹凸匹，收智商税的空气币等等）。
Macroeconomics tends to advance — or, at least, to change — one crisis at a time.
每面临一次经济危机，宏观经济学倾向于更新（或至少是改进）一下经济模型。
The Great Depression discredited the idea that economies were basically self-correcting, and the following decades saw the development of Keynesian theory and the use of fiscal stimulus.
上个世纪30年代的大萧条（Great Depression）否定了经济基本上能够自我调整的观点，在随后的几十年里，见证了凯恩斯理论（Keynesian theory）的发展以及财政刺激（fiscal stimulus）的使用。
The stagflation of the 1970s led to the development of real business cycle models, which saw recessions as the efficient working of the economy, and central bank meddling as likely only to cause inflation.
上世纪70年代的滞胀（stagflation）导致了实际经济周期模型（real business cycle models）的发展，该模型视衰退（recessions）为经济的有效运作方式，而央行的干预反而有可能导致通货膨胀（inflation）。
The painful recessions of the early 1980s saw a shift to so-called New Keynesian models, in which monetary policy is the central stabilizing force in the economy.
上世纪80年代初的痛苦的大衰退（Great Recession），转向了所谓的新凯恩斯模型（New Keynesian models），即货币政策（monetary policy）才是经济中的核心稳定力量。
The housing bubble that peaked in 2006, the financial crisis of 2008, and the Great Recession that followed constitute another crisis. So far, however, it has produced mostly evolution, rather than revolution, in economists’ conception of the business cycle.
2006年达到巅峰的房地产泡沫（housing bubble），2008年的金融危机（financial crisis），以及随之而来的大衰退（Great Recession）构成了另一场危机。然而，迄今为止，经济学家对于经济周期的理解，仅仅是有了各种变化，而非根本性的变革。
The bubble and the following crisis convinced macroeconomists that recessions often emanate from the financial sector — an idea that had often been resisted or overlooked before.
房产泡沫及随后的危机使宏观经济学家相信，经济衰退往往源自于金融领域（financial sector），这种观点以前经常遭到抵制或忽视。
There was immediately a flurry of activity, as economists hastened to shoehorn finance into their standard models. Some now believe that the addition of finance will allow New Keynesian models to forecast crises before they happen; others are, understandably, skeptical.
随着经济学家匆忙将金融纳入其标准模型，一系列措施随之而来。一些人现在认为，金融的纳入将使新凯恩斯模型能够在经济危机发生前就提前预测到；而其他人，当然对此表示怀疑。
Another important insight from the Great Recession was that traditional monetary policy isn’t always enough to stabilize the economy — when interest rates hit zero, other measures are needed. These could include quantitative easing, forward guidance or fiscal stimulus.
大衰退引发的另一个重要观点是，传统的货币政策（monetary policy）不足以稳定经济--当利率降到零时，还需要其他措施。这些措施包括量化宽松（quantitative easing）、前瞻性指导（forward guidance）或财政刺激（fiscal stimulus）。
As New Keynesian pioneer Jordi Gali noted in a recent summary, there has been much work figuring out how New Keynesian models can deal with zero interest rates. There has also been much work on making the models more realistic by taking into account of the big differences among consumers and companies.
正如新凯恩斯主义先锋乔迪·加利（Jordi Gali）在其最近的摘要中指出，目前已有许多工作致力弄清楚新凯恩斯模型如何处理零利率问题。通过考虑消费者和公司之间的巨大差异，在这方面采取措施，使得该模型更加现实可行。
These are important innovations, and they address glaring deficiencies in the pre-2008 models. But they don’t feel like a big break with the status quo. Most importantly, the basic notion of recessions as driven by rational actors’ responses to unpredictable, sudden events — or shocks, as economists call them — remains in place.
这些都是重要的创新，解决了2008年之前模型中的明显缺陷。但与现状相比，这并非很大的突破。最重要的是，经济学家依旧认为，衰退是由于理性行为者（rational actors）对不可预测的突发事件（或经济学家所谓的冲击（shocks））的反应所驱动的。
That would come as a jarring surprise to many outside academia. To lots of people, it seems obvious that the 2008 crisis was long in the making — the product of years of financial and regulatory folly.
但对于学术界之外的大多数来说，这将是令人震惊的意外。许多人认为，2008年的危机显然是经过长期酝酿的，是多年金融和监管愚蠢行为的产物。
In general, the notion that economic booms cause busts, instead of being random unrelated events — an idea advanced by the maverick economist Hyman Minsky — seems to have much more currency beyond the ivory tower than within it.
总的来说，这个看法--即经济繁荣导致了经济萧条，而非随机无关事件（这是由特立独行的经济学家海曼·明斯基（Hyman Minsky）提出的观点），似乎在学术界之外获得了更多的认同。
But at least a few economists are working on something more revolutionary — a new interpretation of recessions, booms and financial markets that more closely matches the popular idea that business cycles are both predictable and driven by irrationality.
但至少有一些经济学家正在研究一些更具变革性的理论--对衰退、繁荣以及金融市场的一种新解读，更接近于流行观点--即经济周期既是可预测（predictable）的又受非理性行为（irrationality）驱动。
The basics of this new idea are laid out in a presentation by Nicola Gennaioli and Andrei Shleifer — two behavioral finance specialists venturing into the realm of macroeconomics.
Nicola Gennaioli 和 Andrei Shleifer 这两位行为金融学家，在一篇文章中阐述了这一新理念的基础。（注： http://papers.nber.org/conf_papers/f114470/f114470.slides.pdf）
Gennaioli and Shleifer take their cue from a number of recent papers hinting that recessions are actually possible to predict years in advance, if one simply pays attention to the right variables.
Gennaioli 和 Shleifer 从最近的一些论文中得到启发，这些论文认为，若是仔细观察正确的变量，事实上可以提前几年预测经济衰退。
One of these is a 2013 paper by Robin Greenwood and Samuel Hanson, showing that when junk bond issuance increases and credit spreads narrow, a credit bust often tends to follow two or three years later.
其中一篇是 Robin Greenwood 和 Samuel Hanson 在2013年发表的论文，显示当企业垃圾债券（junk bond）发行增加，且信用利差（credit spreads）缩小时，信贷泡沫破裂（credit bust）往往会在两三年之后出现。
Another is a 2016 paper by Matthew Baron and Wei Xiong, showing a similar result for bank lending instead of corporate bonds.
另一篇是 Matthew Baron 和 Wei Xiong 在2016年发表的论文，显示了银行信贷（bank lending）泡沫破灭的结局，与上述企业债券（corporate bonds）的崩溃像类似。
A third recent paper, by David López-Salido, Jeremy C. Stein, and Egon Zakrajšek, adds term spreads to Greenwood and Hanson’s list of forecasters, and find that together these indicators give a decent amount of warning about recessions two or three years down the road.
第三篇来自于 David López-Salido、Jeremy C. Stein 和 Egon Zakrajšek 最近发表的论文，为 Greenwood 和 Hanson 的预测指标增加了期限息差（term spreads），并发现这些指标合在一起，能够对提前两三年对经济衰退发出充分的预警。
Other papers find a correlation between rapid credit growth and heightened recession risk.
其他论文发现信贷快速增长和衰退风险加剧之间存在相关性。
All of these papers have one thing in common — they use debt to predict recessions years in advance. That fits with the emerging post-crisis wisdom that problems in credit markets are the source of both financial crashes and the ensuing economic slowdowns.
所有这些论文都有一个共同点，那就是利用债务（这个指标）来提前几年预测经济衰退。这符合后危机时代的现行观念，即信贷市场（credit markets）的问题才是金融崩溃（financial crashes）与随之而来的经济衰退（economic slowdowns）的根源。
Gennaioli and Shleifer explain these patterns by turning to their own preferred theory of human irrationality — the theory of extrapolative expectations.
Gennaioli 和 Shleifer 通过自己偏爱的人类非理性理论来阐述这些模型--即外推预期理论（extrapolative expectations）。（注：Barberis, Nicholas, Robin Greenwood, Lawrence Jin, and Andrei Shleifer. 2018. “Extrapolation and Bubbles.” Journal of Financial Economics 129 (2): 203-227.）
Basically, this theory holds that when asset prices rise — home values, stocks and so on — without a break, investors start to believe that this trend represents a new normal.
该理论主要认为，当房价、股票等资产价格（asset prices），接连不断上涨时，投资者开始相信这种趋势代表了一种新常态。
They pile into the asset, pumping up the price even more, and seeming to confirm the idea that the trend will never end.
他们蜂涌入这些资产，进一步抬高了价格，似乎证实了这种趋势永不会停止的观点。
But when the extrapolators’ money runs out, reality sets in and a crash ensues.
但是当外推者（extrapolators）的资金耗尽了，现实露出真面目，随之而来的就是崩溃。
Gennaioli, Shleifer, and their coauthors have been only one of several teams of researchers to investigate this idea and its implications in recent years.
近几年来只有几个团队研究这个观点及其影响，Gennaioli、Shleifer 及其合著者乃是其中之一。
When extrapolative expectations are combined with an inherently fragile financial system, a predictable cycle of booms and busts is the result.
当外推预期与先天脆弱的金融体系相结合时，结果就是可预见的繁荣和萧条的循环周期。
At some point during good economic times, irrational exuberance takes hold, pushing stock prices, house values, or both into the stratosphere. When they inevitably come down, banks collapse, taking the rest of the economy with them.
在经济繁荣时期的某个时刻，非理性繁荣（irrational exuberance）占据优势，推高了股价、房价，或者将两者的价格都炒上了天。当它们不可避免地下跌时，银行破产，连带着其余的经济一起崩盘。
This story, if it became the standard model of the business cycle, would represent a true revolution in macroeconomics. It discards two pillars of recent macroeconomic thought — rational expectations, and shock-driven unpredictable recessions.
如果这个理论成为经济周期的标准模型，它将代表宏观经济学的一场真正的革命。它抛弃了近期宏观经济思想的两大支柱--理性预期（rational expectations）和冲击驱动的不可预测的衰退（shock-driven unpredictable recessions）。
It would represent a triumph for Minsky’s ideas, and for those outside the academy who have long urged macroeconomists to pay more attention to debt markets and human psychology.
这将是明斯基（Minsky）思想的一次胜利，对于那些长期以来一直敦促宏观经济学家更多的关注债务市场和人类心理学的人来说，亦是一场胜利。
And if the code of booms and busts can finally be cracked, there may be ways for central banks, regulators or other policy makers to head off crises before they begin, instead of cleaning up afterward.
如果繁荣和萧条的准则最终可以被破解，央行、监管机构或其他政策制定者就能够提前阻止危机的爆发，而不是在事后清理。
So far, Gennaioli and Shleifer’s story isn’t close to achieving dominance in macro. But of all the ideas being put forth in the field, this seems like the most interesting to watch.
到目前为止，Gennaioli 和 Shleifer 的理论还远未在宏观经济学占主导地位。但是在该领域提出的所有想法中，这似乎是最值得关注的。
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原文：bloomberg.com/view/articles/2018-07-29/what-economists-still-don-t-get-about-2008-crisis?utm_source=pocket&utm_medium=email&utm_campaign=pockethits
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