Charlie Munger: Ponzi scheme

“What makes common stock prices so hard to predict is that a general liquid market for common stocks creates, from time to time, either in sectors of the market or in the whole market, a Ponzi scheme. In other words, you have an automatic process where people get sucked in and other people come in because it worked last month or last year. And it can build to perfectly ridiculous levels, and the levels can last for considerable periods. Trying to predict that kind of thing, sort of a Ponzi scheme which is, if you will, accidentally thrown into the valuation of common stocks by just the forces of life, by definition that’s going to be very, very hard to predict. But that’s what makes it so dangerous to short stocks, even when they’re grossly overvalued. It’s hard to know just how overvalued they can become in addition to the overvaluation that exists. “—Charlie Munger (2002 Berkshire Hathaway Annual Meeting).