????大多數(shù)經(jīng)濟(jì)學(xué)家傾向于從供需出發(fā)來(lái)解釋股市拋盤(pán)，或許是因?yàn)檫@是他們最擅長(zhǎng)的手法。

????這可以解釋發(fā)生在許多新技術(shù)身上的市場(chǎng)泡沫膨脹和破裂。通常情況下，當(dāng)投資者剛剛注意到一項(xiàng)令人興奮的新科技，比如互聯(lián)網(wǎng)或者當(dāng)今的社交傳媒和電動(dòng)汽車(chē)時(shí)，市場(chǎng)中鮮有已公開(kāi)上市交易的此類(lèi)公司。

????如果這是趕上大好趨勢(shì)的唯一途徑，投資者會(huì)加碼購(gòu)入這些股票。但隨著更多同類(lèi)公司公開(kāi)上市，或者已經(jīng)上市的公司發(fā)售更多股票，可獲得的股票供應(yīng)增加。隨著供應(yīng)增加，價(jià)格會(huì)下跌。

????今年早些時(shí)候的一些科技股走勢(shì)提供了一些證據(jù)。2013年和2014年初有超過(guò)45家科技公司上市，其中包括Twitter、很多其他的社交傳媒公司以及熱門(mén)游戲《糖果傳奇》（Candy Crush）的開(kāi)發(fā)商King.com。這些公司每一家都在IPO后設(shè)有鎖定期（通常為6個(gè)月），內(nèi)部人士隨后可以?huà)伿酃善?。這增加了投資者必須吞下的股票數(shù)量。僅Twitter的鎖定期滿(mǎn)之后，就可以為市場(chǎng)增加5億股社交傳媒股票。

????但這不能解釋像我們今年早些時(shí)候看到的市場(chǎng)恐慌。如果這完全是因?yàn)楣┬桕P(guān)系，你可以預(yù)計(jì)拋售是有控制的，逐步的。這也不能解釋為什么Twitter或King.com的流通股增加會(huì)促使投資者拋售特斯拉或一些生物科技公司，這些公司在今年春天也遭到拋售。（實(shí)際上，Twitter的鎖定期在5月初到期，當(dāng)時(shí)科技股正在恢復(fù)。）

????這一理論也不能解釋上一次金融危機(jī)。住房和抵押債券不是新鮮事物，盡管在房地產(chǎn)泡沫破裂前，兩者的供應(yīng)肯定增加了。

????但是，無(wú)論是估值說(shuō)，還是供應(yīng)說(shuō)，都不能解釋為什么今年的科技股拋售沒(méi)有蔓延。畢竟，非科技股看起來(lái)也很貴。而且，過(guò)去幾年很多大公司一直在出售債券。不過(guò)，今年除科技股外，其他股票持續(xù)上漲，幾乎沒(méi)有任何停頓。

????供職于Firsthand基金的科技投資者凱文?蘭蒂斯表示，部分原因是沒(méi)有明顯的標(biāo)桿股供科技投資者參照。十年前，投資者會(huì)看著微軟(Microsoft)或英特爾(Intel)。如今，微軟已步履蹣跚，英特爾也不復(fù)昔日雄風(fēng)?！拔蚁?，你不會(huì)說(shuō)什么只要特斯拉沒(méi)事，市場(chǎng)就沒(méi)事，”蘭蒂斯表示，“Facebook將來(lái)會(huì)成為標(biāo)桿，但現(xiàn)在還談不上。”

????這就涉及到了解釋市場(chǎng)恐慌的最新理論。至少在學(xué)術(shù)圈，人們開(kāi)始對(duì)于十年前流行的一種理論重拾興趣，這種理論曾被效率市場(chǎng)理論的信奉者摒棄。但既然金融危機(jī)讓人們對(duì)效率市場(chǎng)假設(shè)產(chǎn)生懷疑（住房和抵押債的定價(jià)顯然有誤），其他理論正在卷土重來(lái)。

????The theory that most economists prefer to explain stock market selloffs—probably because it comes from their own playbook—starts with supply and demand.

????And it explains many market bubbles and busts in new technologies. Often when investors catch wind of an exciting new technology like, say, the Internet—or, today, social media and electric cars—there are few, if any, publicly traded companies.

????Investors will pay up for those shares if it’s the only way to get in on the trend. But as more companies that do the same thing go public, or the ones that are public sell more shares, the supply of available shares increases. And as supply rises, prices tend to fall.

????And there’s some evidence that’s what happened with technology stocks earlier this year. More than 45 technology companies went public in 2013 and early 2014, including Twitter, a number of other social media companies and game maker King.com , which owns the obsessively popular Candy Crush. What’s more, each one of these companies have lockup periods—a time, usually six months, after the IPO—after which insiders can sell shares. That increases the number of shares investors have to gobble up. The expiration of Twitter’s lockup alone made another 500 million shares of social media stock available for trading.

????But that doesn’t explain market panics, like we saw earlier this year. If it were all about supply and demand, you would expect the selloffs to be measured and gradual. It also doesn’t explain why more shares of Twitter or King.com would cause investors to dump their holdings of Tesla or a slew of biotech companies, which also sold off in the spring. (In fact, Twitter’s lock-up expired in early May, when technology stocks were recovering.)

????Nor does the theory really explain the financial crisis. Houses and mortgage bonds weren’t new, though the supply of both definitely increased during the run-up to the housing bust.

????And neither the valuation explanation nor the supply argument really explains why this year’s tech stock selloff didn’t spread. Non-tech stocks, after all, look expensive, too. And many large companies have spent the past few years selling debt. Yet, outside of tech, stocks have continued to rise without barely a hiccup this year.

????Tech investor Kevin Landis of Firsthand funds says part of the problem is there’s no obvious leader for tech investors to use as an anchor. A decade ago, investors would look to movements in Microsoft or Intel . But Microsoft has stumbled and Intel isn’t the powerhouse it used to be. “I don’t think you would say as Telsa goes, so goes the market,” says Landis. “Facebook is going to get there but it’s not there yet.”

????That leads us to the latest theory of why market panics happen. In academic circles, at least, there’s been resurgence in interest in a theory that was popular a decade ago but had been dismissed by believers in efficient market theory. But now that the financial crisis has discredited the efficient market hypothesis—clearly houses and mortgage bonds were mispriced—alternative theories are making a comeback.