市场抛盘是怎样发生的？

    大多数经济学家倾向于从供需出发来解释股市抛盘，或许是因为这是他们最擅长的手法。

    这可以解释发生在许多新技术身上的市场泡沫膨胀和破裂。通常情况下，当投资者刚刚注意到一项令人兴奋的新科技，比如互联网或者当今的社交传媒和电动汽车时，市场中鲜有已公开上市交易的此类公司。

    如果这是赶上大好趋势的唯一途径，投资者会加码购入这些股票。但随着更多同类公司公开上市，或者已经上市的公司发售更多股票，可获得的股票供应增加。随着供应增加，价格会下跌。

    今年早些时候的一些科技股走势提供了一些证据。2013年和2014年初有超过45家科技公司上市，其中包括Twitter、很多其他的社交传媒公司以及热门游戏《糖果传奇》（Candy Crush）的开发商King.com。这些公司每一家都在IPO后设有锁定期（通常为6个月），内部人士随后可以抛售股票。这增加了投资者必须吞下的股票数量。仅Twitter的锁定期满之后，就可以为市场增加5亿股社交传媒股票。

    但这不能解释像我们今年早些时候看到的市场恐慌。如果这完全是因为供需关系，你可以预计抛售是有控制的，逐步的。这也不能解释为什么Twitter或King.com的流通股增加会促使投资者抛售特斯拉或一些生物科技公司，这些公司在今年春天也遭到抛售。（实际上，Twitter的锁定期在5月初到期，当时科技股正在恢复。）

    这一理论也不能解释上一次金融危机。住房和抵押债券不是新鲜事物，尽管在房地产泡沫破裂前，两者的供应肯定增加了。

    但是，无论是估值说，还是供应说，都不能解释为什么今年的科技股抛售没有蔓延。毕竟，非科技股看起来也很贵。而且，过去几年很多大公司一直在出售债券。不过，今年除科技股外，其他股票持续上涨，几乎没有任何停顿。

    供职于Firsthand基金的科技投资者凯文•兰蒂斯表示，部分原因是没有明显的标杆股供科技投资者参照。十年前，投资者会看着微软(Microsoft)或英特尔(Intel)。如今，微软已步履蹒跚，英特尔也不复昔日雄风。“我想，你不会说什么只要特斯拉没事，市场就没事，”兰蒂斯表示，“Facebook将来会成为标杆，但现在还谈不上。”

    这就涉及到了解释市场恐慌的最新理论。至少在学术圈，人们开始对于十年前流行的一种理论重拾兴趣，这种理论曾被效率市场理论的信奉者摒弃。但既然金融危机让人们对效率市场假设产生怀疑（住房和抵押债的定价显然有误），其他理论正在卷土重来。

    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.