????大多數經濟學家傾向于從供需出發(fā)來解釋股市拋盤，或許是因為這是他們最擅長的手法。

????這可以解釋發(fā)生在許多新技術身上的市場泡沫膨脹和破裂。通常情況下，當投資者剛剛注意到一項令人興奮的新科技，比如互聯網或者當今的社交傳媒和電動汽車時，市場中鮮有已公開上市交易的此類公司。

????如果這是趕上大好趨勢的唯一途徑，投資者會加碼購入這些股票。但隨著更多同類公司公開上市，或者已經上市的公司發(fā)售更多股票，可獲得的股票供應增加。隨著供應增加，價格會下跌。

????今年早些時候的一些科技股走勢提供了一些證據。2013年和2014年初有超過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游戲《糖果傳奇》（Candy Crush）的開發(fā)商King.com。這些公司每一家都在IPO后設有鎖定期（通常為6個月），內部人士隨后可以拋售股票。這增加了投資者必須吞下的股票數量。僅Twitter的鎖定期滿之后，就可以為市場增加5億股社交傳媒股票。

????但這不能解釋像我們今年早些時候看到的市場恐慌。如果這完全是因為供需關系，你可以預計拋售是有控制的，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。（實際上，Twitter的鎖定期在5月初到期，當時科技股正在恢復。）

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

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

????供職于Firsthand基金的科技投資者凱文?蘭蒂斯表示，部分原因是沒有明顯的標桿股供科技投資者參照。十年前，投資者會看著微軟(Microsoft)或英特爾(Intel)。如今，微軟已步履蹣跚，英特爾也不復昔日雄風。“我想，你不會說什么只要特斯拉沒事，市場就沒事，”蘭蒂斯表示，“Facebook將來會成為標桿，但現在還談不上。”

????這就涉及到了解釋市場恐慌的最新理論。至少在學術圈，人們開始對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理論的信奉者摒棄。但既然金融危機讓人們對效率市場假設產生懷疑（住房和抵押債的定價顯然有誤），其他理論正在卷土重來。

????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.