永远的泡沫

（核心提示）

投机泡沫的最准确定义：关于价格上涨的新闻刺激了投资者热情，投资者热情通过心理感染传播，并在此过程中放大可以证明价格上涨合理性的故事。”这吸引了“越来越庞大的投资者群体，他们尽管怀疑投资的真实价值，但也被吸引入局，部分是因为他们嫉妒他人的成功，部分是因为赌徒的兴奋。”

罗伯特·席勒是2013年诺贝尔经济学奖得主。作为新凯恩斯学派成员之一，席勒认为，市场并非总是有效的，投资者的情绪甚至外在气候等因素都能对投资行为产生影响。

席勒同时也是一位畅销书作者，《动物精神》和《非理性繁荣》使他的理论深入人心。

席勒与中国关系密切，曾多次到访中国，并多次提及对中国房地产的看法，称中国房地产泡沫严重。本栏目选登席勒的文章《永远的泡沫》，供对看懂中国房市泡沫有着强烈的现实意义。

在2006年史上最大全球房地产泡沫破灭、2007年世界股市大泡沫结束之后，你大概以为我们已经进入了后泡沫时代。但关于泡沫的讨论仍没有淡出视野——许多国家的房地产市场仍有泡沫或出现了新的泡沫；新的全球股市泡沫；美国和其他国家长期债券市场泡沫；石油价格泡沫；黄金泡沫，等等。

尽管如此，当我上个月访问哥伦比亚时，并没有预期会看到泡沫。但是，人们再一次和我谈到了进行中的房地产泡沫，我的司机指向海滨度假小镇卡塔赫纳（Cartagena），带着惊愕的口气说，有的房子最近卖到了数百万美元。

哥伦比亚央行有一个追踪三大城市——波哥大、麦德林和卡利的住房价格指数。自2004年以来，该指数实际值（经通胀调整）上涨了69%，其中大部分涨幅发生在2007年之后。房价上升之快令人想起了美国，标普/凯斯-希勒（S&P/Case-Shiller）十城市住房价格指数从1997年的底部到2006年的顶部总共上涨了131%。

问题由此而生：什么才是壮观的泡沫？《牛津英语词典》对泡沫的定义是“任何易碎、虚幻、空洞、无价值的东西；骗人的把戏。自17世纪以来常用于欺骗性的商业或金融计划。”问题在于，“把戏”和“项目”之类的词表明泡沫是有意为之之物，而不是不听从任何人指挥的普遍的社会现象。

也许“泡沫”一词有滥用之嫌。

尤金·法马（Eugene Fama）显然这样认为。作为“有效市场假说”最重要的支持者，法马否认泡沫的存在。在2010年接受《纽约客》采访时，法马对约翰·卡西迪（John Cassidy）说：“我甚至不知道泡沫是什么意思。这些词倒是很流行。我不认为它们有什么意义。”

在我的书《非理性繁荣》（Irrational Exuberance）第二版中，我尝试给泡沫下一个更好的定义。“投机泡沫”我写道，是“这样一种情况：

关于价格上涨的新闻刺激了投资者热情，投资者热情通过心理感染传播，并在此过程中放大可以证明价格上涨合理性的故事。”这吸引了“越来越庞大的投资者群体，他们尽管怀疑投资的真实价值，但也被吸引入局，部分是因为他们嫉妒他人的成功，部分是因为赌徒的兴奋。”

这似乎是泡沫这个词最核心的意思，也是最常用的用法。这个定义隐含的含义是“聪明钱”为何极难通过赌反向下注从泡沫中赚钱：心理传染催生了论证价格上涨合理性的观念，因此参与泡沫可谓“几乎”（almost）理性。但其实是不理性的。

每个国家的故事各不相同，因为各国各有各的新闻，与其他国家并不总是相同。比如，目前哥伦比亚的故事是，在总统桑托斯广受好评的治理下，哥伦比亚政府将通胀和利率压到了发达国家水平，此外，消除FARC叛军的威胁不啻向哥伦比亚经济注入了新活力。这个好故事足以推动住房泡沫。

因为泡沫从本质上说是一种社会心理学现象，因此天生难以控制。自金融危机以来的监管行动或许能抑制未来泡沫。但公众对泡沫的恐慌也可能加强心理传染，助长更加自我实现的预言。

泡沫一词的一个问题是它创造了一幅扩张的肥皂泡心理情景，这个肥皂泡注定会突然地、不可逆转地破灭。但投机泡沫并没有那么容易结束；事实上，它们可能先因为故事的改变而收缩一定幅度，然后再度膨胀。

更准确的做法是将这些发作称为投机传染病。从流感中我们知道，新的传染病可以在旧传染病正在消失时突然爆发，如果新的病毒种类出现的话，或者如果某些环境因素提高了传染率的话。类似地，一个新的投机泡沫可以随时出现，如果关于经济的新故事出现，且故事性足够强以至于投资者被新思维传染的话。

20世纪20年代美国牛市便是如此。这次牛市在1929年见顶。我们认为泡沫是一段价格狂飙突进的时期，接着是突如其来的转折点和决定性的大跌，这扭曲了历史。事实上，“黑色星期二”后，美国股市实际价格出现过一次大涨，1930年，股市收复了1929年跌幅的一半。接着到来的是第二次崩溃、1932—1937年的再次繁荣，以及第三次崩溃。

投机泡沫不会像短篇故事、小说或戏剧那样结束。不存在将所有线索同时推向令人难忘的结局的剧终场景。在真实世界中，我们永远不知道故事什么时候结束。

 

英文原题：Bubbles Forever

You might think that we have been living in a post-bubble world since the collapse in 2006 of the biggest-ever worldwide real-estate bubble and the end of a major worldwide stock-market bubble the following year. But talk of bubbles keeps reappearing – new or continuing housing bubbles in many countries, a new global stock-market bubble, a long-term bond-market bubble in the United States and other countries, an oil-price bubble, a gold bubble, and so on.

Nevertheless, I was not expecting a bubble story when I visited Colombia last month. But, once again, people there told me about an ongoing real-estate bubble, and my driver showed me around the seaside resort town of Cartagena, pointing out, with a tone of amazement, several homes that had recently sold for millions of dollars.

The Banco de la República, Colombia’s central bank, maintains a home price index for three main cities – Bogotá, Medellín, and Cali. The index has risen 69% in real (inflation-adjusted) terms since 2004, with most of the increase coming after 2007. That rate of price growth recalls the US experience, with the S&P/Case-Shiller Ten-City Home Price Index for the US rising 131% in real terms from its bottom in 1997 to its peak in 2006.

This raises the question: just what is a speculative bubble? The Oxford English Dictionary defines a bubble as “anything fragile, unsubstantial, empty, or worthless; a deceptive show. From 17th c. onwards often applied to delusive commercial or financial schemes.” The problem is that words like “show” and “scheme” suggest a deliberate creation, rather than a widespread social phenomenon that is not directed by any impresario.

Maybe the word bubble is used too carelessly.

Eugene Fama certainly thinks so. Fama, the most important proponent of the “efficient markets hypothesis,” denies that bubbles exist. As he put it in a 2010 interview with John Cassidy for The New Yorker, “I don’t even know what a bubble means. These words have become popular. I don’t think they have any meaning.”

In the second edition of my book Irrational Exuberance, I tried to give a better definition of a bubble. A “speculative bubble,” I wrote then, is “a situation in which news of price increases spurs investor enthusiasm, which spreads by psychological contagion from person to person, in the process amplifying stories that might justify the price increase.” This attracts “a larger and larger class of investors, who, despite doubts about the real value of the investment, are drawn to it partly through envy of others’ successes and partly through a gambler’s excitement.”

That seems to be the core of the meaning of the word as it is most consistently used. Implicit in this definition is a suggestion about why it is so difficult for “smart money” to profit by betting against bubbles: the psychological contagion promotes a mindset that justifies the price increases, so that participation in the bubble might be called almost rational. But it is not rational.

The story in every country is different, reflecting its own news, which does not always jibe with news in other countries. For example, the current story in Colombia appears to be that the country’s government, now under the well-regarded management of President Juan Manuel Santos, has brought down inflation and interest rates to developed-country levels, while all but eliminating the threat posed by the FARC rebels, thereby injecting new vitality into the Colombian economy. That is a good enough story to drive a housing bubble.

Because bubbles are essentially social-psychological phenomena, they are, by their very nature, difficult to control. Regulatory action since the financial crisis might diminish bubbles in the future. But public fear of bubbles may also enhance psychological contagion, fueling even more self-fulfilling prophecies.

One problem with the word bubble is that it creates a mental picture of an expanding soap bubble, which is destined to pop suddenly and irrevocably. But speculative bubbles are not so easily ended; indeed, they may deflate somewhat, as the story changes, and then reflate.

It would seem more accurate to refer to these episodes as speculative epidemics. We know from influenza that a new epidemic can suddenly appear just as an older one is fading, if a new form of the virus appears, or if some environmental factor increases the contagion rate. Similarly, a new speculative bubble can appear anywhere if a new story about the economy appears, and if it has enough narrative strength to spark a new contagion of investor thinking.

This is what happened in the bull market of the 1920’s in the US, with the peak in 1929. We have distorted that history by thinking of bubbles as a period of dramatic price growth, followed by a sudden turning point and a major and definitive crash. In fact, a major boom in real stock prices in the US after “Black Tuesday” brought them halfway back to 1929 levels by 1930. This was followed by a second crash, another boom from 1932 to 1937, and a third crash.

Speculative bubbles do not end like a short story, novel, or play. There is no final denouement that brings all the strands of a narrative into an impressive final conclusion. In the real world, we never know when the story is over.

(Robert J. Shiller is Professor of Economics at Yale University, the co-creator of the Case-Shiller Index of US house prices, and the winner of the 2013 Nobel Prize in Economics.)