????同樣，我也試圖深入挖掘多年來(lái)一直困擾市場(chǎng)觀察人士和財(cái)經(jīng)記者（包括我自己）的一個(gè)現(xiàn)象：是什么導(dǎo)致投資者從樂(lè)觀轉(zhuǎn)向焦慮，再轉(zhuǎn)向恐慌，然后又重新樂(lè)觀起來(lái)？

????經(jīng)濟(jì)學(xué)家們給出的答案大多無(wú)法令人滿意，或被證明是錯(cuò)誤的。有一段時(shí)間，很多人接受這樣的觀點(diǎn)：市場(chǎng)本質(zhì)上是隨機(jī)的，僅此而已。

????但隨后發(fā)生了幾乎吞噬整個(gè)美國(guó)經(jīng)濟(jì)的金融危機(jī)。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(duì)什么造成市場(chǎng)恐慌進(jìn)行了新的研究，包括幾個(gè)月前剛剛發(fā)布的一項(xiàng)重要研究。壞消息是，這些研究可能不會(huì)平息爭(zhēng)論。關(guān)于投資者和他們的非理性反應(yīng)，我們知道一些，但也有沒(méi)搞明白的地方。

????詢問(wèn)大多數(shù)專業(yè)投資者和市場(chǎng)策略師“為什么會(huì)發(fā)生市場(chǎng)恐慌”，你多數(shù)情況會(huì)得到同樣的回答：股價(jià)漲得太高了。

????“用市場(chǎng)術(shù)語(yǔ)講，就是疲竭，”熱門新聞簡(jiǎn)訊《高科技策略師》（The High-Tech Strategist）資深編輯弗雷德?希基說(shuō)。

????意識(shí)到股票被顯著高估，看起來(lái)導(dǎo)致了2000年科技泡沫的破裂。2000年3月，《巴倫周刊》（Barron’s）發(fā)表了一篇文章詳述互聯(lián)網(wǎng)公司的現(xiàn)金流多么匱乏，以及它們的股票如何被高估。（旁注：時(shí)任科技股分析師亨利?布洛格特表示，這篇文章的數(shù)學(xué)計(jì)算是錯(cuò)誤的。）當(dāng)時(shí)還沒(méi)有人意識(shí)到互聯(lián)網(wǎng)企業(yè)以及安然（Enron）等其他公司正在玩什么會(huì)計(jì)把戲。

????那時(shí)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(fā)布后三天內(nèi)，納斯達(dá)克指數(shù)大跌近500點(diǎn)，跌幅10%。

????希基說(shuō)，金融危機(jī)前也是同樣的狀況。將股票與企業(yè)銷售額或凈利潤(rùn)相比，很顯然2007年10月的市場(chǎng)估值已經(jīng)過(guò)高了。今年早些時(shí)候，如果結(jié)合當(dāng)期增長(zhǎng)預(yù)期，科技股的市盈率也已超出2000年的水平。

????“為什么人們認(rèn)為絕不會(huì)下跌，這一點(diǎn)我不能理解，”希基表示。

????雖然市場(chǎng)價(jià)值說(shuō)對(duì)于華爾街人士意義重大，但從未對(duì)金融教授產(chǎn)生多少影響。十年多前，諾貝爾獎(jiǎng)經(jīng)濟(jì)學(xué)獎(jiǎng)得主愛(ài)德華?普雷斯科特在當(dāng)年股市大崩盤前對(duì)1929年的股價(jià)進(jìn)行了研究。他的結(jié)論是：1929年的股災(zāi)并不是因?yàn)楣善惫乐颠^(guò)高。更何況，基于我們現(xiàn)在了解的信息，當(dāng)時(shí)的股價(jià)還是挺便宜的。

????而且，當(dāng)互聯(lián)網(wǎng)泡沫破滅時(shí)，有太多股票變得一文不名，但也有一些股票即使在市場(chǎng)峰值時(shí)期也非常便宜。比方說(shuō)，亞馬遜（Amazon.com）2000年的最高股價(jià)經(jīng)拆股調(diào)整后為89美元。該股當(dāng)前交易價(jià)格為328美元。

????科技股同樣也和幾個(gè)月前一樣昂貴。不妨來(lái)看看順風(fēng)車應(yīng)用軟件Uber。6月初，該公司估值達(dá)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(fēng)險(xiǎn)投資之前的估值。這使得該股的虛擬紙面價(jià)值超過(guò)了市值135億美元的亨氏公司（Hertz）。這聽(tīng)上去全是泡沫，但沒(méi)人著急賣。

????2000年那篇《巴倫周刊》文章并非財(cái)經(jīng)媒體首次對(duì)科技股發(fā)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(shuō)，是投資者的情緒變化導(dǎo)致了拋盤，”哈佛大學(xué)（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(huì)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.”????