????同樣，我也試圖深入挖掘多年來一直困擾市場觀察人士和財經記者（包括我自己）的一個現象：是什么導致投資者從樂觀轉向焦慮，再轉向恐慌，然后又重新樂觀起來？

????經濟學家們給出的答案大多無法令人滿意，或被證明是錯誤的。有一段時間，很多人接受這樣的觀點：市場本質上是隨機的，僅此而已。

????但隨后發生了幾乎吞噬整個美國經濟的金融危機。尋找金融傳染的起因以及如何控制傳染，成為熱門話題。

????好消息是，我們對什么造成市場恐慌進行了新的研究，包括幾個月前剛剛發布的一項重要研究。壞消息是，這些研究可能不會平息爭論。關于投資者和他們的非理性反應，我們知道一些，但也有沒搞明白的地方。

????詢問大多數專業投資者和市場策略師“為什么會發生市場恐慌”，你多數情況會得到同樣的回答：股價漲得太高了。

????“用市場術語講，就是疲竭，”熱門新聞簡訊《高科技策略師》（The High-Tech Strategist）資深編輯弗雷德?希基說。

????意識到股票被顯著高估，看起來導致了2000年科技泡沫的破裂。2000年3月，《巴倫周刊》（Barron’s）發表了一篇文章詳述互聯網公司的現金流多么匱乏，以及它們的股票如何被高估。（旁注：時任科技股分析師亨利?布洛格特表示，這篇文章的數學計算是錯誤的。）當時還沒有人意識到互聯網企業以及安然（Enron）等其他公司正在玩什么會計把戲。

????那時我們根本不需要知道這些東西便恐慌了。在《巴倫周刊》的文章發布后三天內，納斯達克指數大跌近500點，跌幅10%。

????希基說，金融危機前也是同樣的狀況。將股票與企業銷售額或凈利潤相比，很顯然2007年10月的市場估值已經過高了。今年早些時候，如果結合當期增長預期，科技股的市盈率也已超出2000年的水平。

????“為什么人們認為絕不會下跌，這一點我不能理解，”希基表示。

????雖然市場價值說對于華爾街人士意義重大，但從未對金融教授產生多少影響。十年多前，諾貝爾獎經濟學獎得主愛德華?普雷斯科特在當年股市大崩盤前對1929年的股價進行了研究。他的結論是：1929年的股災并不是因為股票估值過高。更何況，基于我們現在了解的信息，當時的股價還是挺便宜的。

????而且，當互聯網泡沫破滅時，有太多股票變得一文不名，但也有一些股票即使在市場峰值時期也非常便宜。比方說，亞馬遜（Amazon.com）2000年的最高股價經拆股調整后為89美元。該股當前交易價格為328美元。

????科技股同樣也和幾個月前一樣昂貴。不妨來看看順風車應用軟件Uber。6月初，該公司估值達到驚人的170億美元，這還是在該公司募得另外一筆12億美元風險投資之前的估值。這使得該股的虛擬紙面價值超過了市值135億美元的亨氏公司（Hertz）。這聽上去全是泡沫，但沒人著急賣。

????2000年那篇《巴倫周刊》文章并非財經媒體首次對科技股發出警示。但不知什么原因，這篇文章引起了重視。

????“我們最多只能說，是投資者的情緒變化導致了拋盤，”哈佛大學（Harvard）教授馬科姆?貝克表示，“但我們真的不知道，投資者情緒為什么會變化。”

????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.”????