Book Reviews

Traditional capital market theory says that markets are efficient because investors are rational. The new school of behavioral finance says the opposite. Rather than solving problems "rationally," individuals tend to make biased decisions using pattern recognition techniques. However, what is rational and irrational may depend upon the type of problem we wish to solve and the method we use to solve it. If the market inefficiency is a simple objective problem, then "cool reason" should prevail. However, if the market is a complex system, then the value of data would be ambiguous making it more rational to use pattern recognition techniques. In this article we will find that rational investors would indeed keep certain types of mispricing from happening. Likewise, human behavior and the market complexity cause mispricing that cannot be arbitraged away. In the end, investors are irrational if they use the wrong method to solve a particular type of problem. By examining method and object we can find when investors are rational, when they are irrational. A non-mathematical model integrating efficient markets, behavioral finance, and complex systems is presented.