Take, for example, the fact of high trading volume in security markets. In a rational world there would not be very much trading—in fact, hardly any. Economists sometimes call this the Groucho Marx theorem. Groucho famously said that he would never want to belong to any club that would have him as a member. The economist’s version of this joke—predictably, not as funny—is that no rational agent will want to buy a stock that some other rational agent is willing to sell.
