Randomness and the madness of crowds

Human interaction often appears to be random and at times even chaotic. We use game theory, the mathematical study of interactive decision making, to explain the role of rationality and randomness in strategic behavior. In many of these situations, humans deliberately create randomness as a best response and equilibrium strategy. Moreover, once out of equilibrium, individual beliefs about the real intentions of others introduce signi cant randomness into otherwise quite simple and deterministic situations of interaction. In a second step we discuss the role of randomness on nancial markets, which are prototypical institutions for the aggregation of individual behavior. As in certain simple games, nancial markets can produce outcomes that are close to perfect randomness. In fact, random walks in nancial returns are considered by most scholars to be e cient and desirable. Finally, we apply game theoretical insights to behavior on nancial markets and show how strategic speculation on `greater fools' can create a `madness of crowds' that often ends in chaotic swings, bubbles and crashes. Chapter in `The Challenge of Chance', eds. Klaas Landsman & Ellen van Wolde (Springer-Verlag, Heidelberg, 2016). Written for a general audience. ∗Weitzel (corresponding author): Radboud University, Department of Economics, IMR, Thomas van Aquinostraat 5.1.26, 6525GD Nijmegen ([email protected]) and Utrecht University School of Economics, Kriekenpitplein 21-22, 3584EC Utrecht ([email protected]). Rosenkranz: Utrecht University School of Economics, Kriekenpitplein 21-22, 3584EC Utrecht ([email protected])