市场抛盘是怎样发生的？

    同样，我也试图深入挖掘多年来一直困扰市场观察人士和财经记者（包括我自己）的一个现象：是什么导致投资者从乐观转向焦虑，再转向恐慌，然后又重新乐观起来？

    经济学家们给出的答案大多无法令人满意，或被证明是错误的。有一段时间，很多人接受这样的观点：市场本质上是随机的，仅此而已。

    但随后发生了几乎吞噬整个美国经济的金融危机。寻找金融传染的起因以及如何控制传染，成为热门话题。

    好消息是，我们对什么造成市场恐慌进行了新的研究，包括几个月前刚刚发布的一项重要研究。坏消息是，这些研究可能不会平息争论。关于投资者和他们的非理性反应，我们知道一些，但也有没搞明白的地方。

    询问大多数专业投资者和市场策略师“为什么会发生市场恐慌”，你多数情况会得到同样的回答：股价涨得太高了。

    “用市场术语讲，就是疲竭，”热门新闻简讯《高科技策略师》（The High-Tech Strategist）资深编辑弗雷德•希基说。

    意识到股票被显著高估，看起来导致了2000年科技泡沫的破裂。2000年3月，《巴伦周刊》（Barron’s）发表了一篇文章详述互联网公司的现金流多么匮乏，以及它们的股票如何被高估。（旁注：时任科技股分析师亨利•布洛格特表示，这篇文章的数学计算是错误的。）当时还没有人意识到互联网企业以及安然（Enron）等其他公司正在玩什么会计把戏。

    那时我们根本不需要知道这些东西便恐慌了。在《巴伦周刊》的文章发布后三天内，纳斯达克指数大跌近500点，跌幅10%。

    希基说，金融危机前也是同样的状况。将股票与企业销售额或净利润相比，很显然2007年10月的市场估值已经过高了。今年早些时候，如果结合当期增长预期，科技股的市盈率也已超出2000年的水平。

    “为什么人们认为绝不会下跌，这一点我不能理解，”希基表示。

    虽然市场价值说对于华尔街人士意义重大，但从未对金融教授产生多少影响。十年多前，诺贝尔奖经济学奖得主爱德华•普雷斯科特在当年股市大崩盘前对1929年的股价进行了研究。他的结论是：1929年的股灾并不是因为股票估值过高。更何况，基于我们现在了解的信息，当时的股价还是挺便宜的。

    而且，当互联网泡沫破灭时，有太多股票变得一文不名，但也有一些股票即使在市场峰值时期也非常便宜。比方说，亚马逊（Amazon.com）2000年的最高股价经拆股调整后为89美元。该股当前交易价格为328美元。

    科技股同样也和几个月前一样昂贵。不妨来看看顺风车应用软件Uber。6月初，该公司估值达到惊人的170亿美元，这还是在该公司募得另外一笔12亿美元风险投资之前的估值。这使得该股的虚拟纸面价值超过了市值135亿美元的亨氏公司（Hertz）。这听上去全是泡沫，但没人着急卖。

    2000年那篇《巴伦周刊》文章并非财经媒体首次对科技股发出警示。但不知什么原因，这篇文章引起了重视。

    “我们最多只能说，是投资者的情绪变化导致了抛盘，”哈佛大学（Harvard）教授马科姆•贝克表示，“但我们真的不知道，投资者情绪为什么会变化。”

    In this same vein, I dove in to a mystery that has been stumping market-watchers and financial journalists—myself included—for ages: What causes investors to go from optimistic to nervous to panicked and back?

    The answers that economists have come up with have been mostly unsatisfying or disproven. For a while, many settled on the notion that the market was essentially random, and left it at that.

    But then came the financial crisis, which nearly swallowed the entire economy. Again the search for the causes of financial contagions, and how to contain them, became a hot topic.

    The good news is that we have new research on what causes market panics, including a major study that came out in just the past few months. The bad news is this likely won’t end the debate either. Here’s what we know, and don’t know, about investors and their freak-outs.

    Ask most professional investors and market strategists why stock panics happen and you will mostly get the same answer: Stock prices get too high.

    “It’s called exhaustion in market terminology,” says Fred Hickey, who is the long-time editor of the widely followed newsletter, The High-Tech Strategist.

    The realization that stocks were significantly overvalued appears to be what led to the 2000 tech bust. In March 2000, Barron’s published an article detailing how fast dotcom companies were running out of cash, and their stocks were overvalued. (Fun side note: Henry Blodget, then a technology stock analyst, said the math of the article was wrong.) That was long before anyone realized the accounting tricks that dot.com companies, and others, like Enron, were playing.

    But we didn’t need to know any of that stuff to panic. In the three days following the Barron’s article, the Nasdaq index fell nearly 500 points, or 10%.

    Hickey says the same was true in the run up to the financial crisis. Compare stocks to corporate sales or earnings, and it was clear that the market was overvalued in October 2007. And earlier this year, the prices of technology stocks, when you factor in their current growth projections, were trading at higher multiple of earnings than they were back in 2000.

    “Why anyone would expect anything other than a decline is beyond me,” says Hickey.

    Market value explanations, while a big deal for Wall Streeters, never really held much sway with financial professors. A little over a decade ago, Nobel Prize-winning economist Edward Prescott did a study of stock prices in 1929, before that year’s giant stock market crash. His conclusion: The market didn’t crash in 1929 because stocks were overvalued. If anything, based on what we know now, the market was cheap.

    And while there were plenty of stocks that turned out to be worthless when the dotcom market crashed, others were cheap even at the peak. Amazon.com’s stock , for instance topped out at a split adjusted $89 in 2000. It now trades for $328.

    Technology stocks, too, are just as expensive as they were a few months ago. Witness the phenomenon of hitcher app Uber, which was valued in early June at an eye-popping $17 billion—and that was before it raised another $1.2 billion in venture funding. That would make it worth more than Hertz (market capitalization: $13.5 billion), on virtual dotcom paper, that is. This all sounds bubbly, and yet, no one is running to sell.

    The Barron’s article in 2000 was not the first time the financial media had raised the warning sign about tech stocks. Yet, for some reason that article stuck.

    “The best we can say is that what causes selloffs is changes in investor sentiment,” says Harvard professor Malcolm Baker. “But why that change occurs we really don’t know.”