I don't believe stock movement is as "random" as the EMH and random walk theorists would have you believe. Stocks, and any market for that matter still behave according to the laws of supply and demand and more subtly to the "laws" of standard deviation. If you pull up a chart of any stock you like and add Bollinger Bands, you'll find that stocks stay inside the bands about 95% of the time (set at 2 standard deviations away from the mean). Very rarely will stocks go more than 3 standard deviations from the mean. Once in a blue moon (like the crash of '87), stocks fall much further than 3 standard deviations. But on average, I think its remarkable that stocks (or any traded security) stay so confined to a seemingly random number such as 2 standard deviations. My question is, what are the psychological principles behind the standard deviation numbers? There must be some. There are statistical principles behind it (Central limit theorem). But pyschologically speaking, why would stock prices "stop" at 2-3 standard deviations when a stock is going up. Why not 8 or 10? Perhaps human emotions are tied to the same standard deviation numbers. Greed and hysteria reach a certain point and then stop. Fear on the other hand can be much greater than greed. Thus, stocks can go down much faster than they go up. The market can fall 20% in one day, but it's never gone up even close to 20% in a day.