Herd behavior and contagion in financial markets

Imitative behavior and contagion are well-documented regularities of financial markets. We study whether they can occur in a two-asset economy where rational agents trade sequentially. When traders have gains from trade, informational cascades arise and prices fail to aggregate information dispersed among traders. During a cascade all informed traders with the same preferences choose the same action, i.e., they herd. Moreover, herd behavior can generate financial contagion. Informational cascades and herds can spill over from one asset to the other, pushing the price of the other asset far from its fundamental value

Herding and price convergence in a laboratory financial market

We study whether herding can arise in a laboratory financial market in which agents trade sequentially. Agents trade an asset whose value is unknown and whose price is efficiently set by a market maker. We show that the presence of a price mechanism destroys the possibility of herding. Most agents follow their private information and prices converge to the fundamental value. This result contrasts with the case of a fixed price, where herding and cascades arise. When the price moves, however, agents may behave as contrarian, i.e., they may trade against the market, something not accounted for by the theory. Finally, we study whether informational cascades arise when trade is costly (e.g, because of a Tobin tax). With trade costs, most subjects rationally decided not to trade and the price was unable to aggregate private information efficiently

Transaction costs and informational cascades in financial markets: theory and experimental evidence

We study the effect of transaction costs (e.g., a trading fee or a transaction tax, like the Tobin tax) on the aggregation of private information in financial markets. We analyze a financial market à la Glosten and Milgrom, in which informed and uninformed traders trade in sequence with a market maker. Traders have to pay a cost in order to trade. We show that, eventually, all informed traders decide not to trade, independently of their private information, i.e., an informational cascade occurs. We replicated our financial market in the laboratory. We found that, in the experiment, informational cascades occur when the theory suggests they should. Nevertheless, the ability of the price to aggregate private information is not significantly affected

Herd behavior and contagion in financial markets

We study a sequential trading financial market where there are gains from trade, that is, where informed traders have heterogeneous private values. We show that an informational cascade (i.e., a complete blockage of information) arises and prices fail to aggregate information dispersed among traders. During an informational cascade, all traders with the same preferences choose the same action, following the market (herding) or going against it (contrarianism). We also study financial contagion by extending our model to a two-asset economy. We show that informational cascades in one market can be generated by informational spillovers from the other. Such spillovers have pathological consequences, generating long-lasting misalignments between prices and fundamentals

When half the truth is better than the truth: A Theory of aggregate information cascades

We introduce a new model of aggregate information cascades where only one of two possible actions is observable to others. When called upon, agents (who decide in some random order that they do not know) are only informed about the total number of others who have chosen the observable action before them. This informational structure arises nat- urally in many applications. Our most important result is that only one type of cascade arises in equilibrium, the aggregate cascade on the observable action. A cascade on the unobservable action never arises. Our results may have important policy consequences. Central agencies, for example in the health sector, may optimally decide to withhold in- formation from the public.

Teoria dei Processi Imitativi e Applicazioni Economiche

This paper provides a survey of recent theories of herding behaviour, bridging two rather distants strands of literature (roughly, American and European). In the first part of the paper the explanation is based on the idea of asymmetric information and principal-agent approach; these could lead to an over-estimation of public information and under- estimation of private information, leading to informational cascades and interruption of social learning. The second part reviews the second strand of literature on herding, where transition probabilities from one strategy to another, which are stochastic at intividual level, give rise to quasi-deternimistic paths at aggregate level. The concept of self- referential and hetero-referential systems are introduced. Feb. 2004Herd behaviour, informational cascades, asymmetric information, learning, self-organisation, exchange rate, bubbles, financial crises

Information Aggregation with Random Ordering: Cascades and Overconfidence

In economic models, it is usually assumed that agents aggregate their private and all available public information correctly and completely. In this experiment, we identify subjects' updating procedures and analyze the consequences for the aggregation process. Decisions can be based on private information with known quality and observed decisions of other participants. In this setting with random ordering, information cascades are observable and agents' overconfidence has a positive effect on avoiding a non-revealing aggregation process but it reduces welfare in general.

Information Aggregation with Random Ordering: Cascades and Overconfidence

In economic models, it is usually assumed that agents aggregate their private and all available public information correctly and completely. In this experiment, we identify subjects' updating procedures and analyze the consequences for the aggregation process. Decisions can be based on private information with known quality and observed decisions of other participants. In this setting with random ordering, information cascades are observable and agents' overconfidence has a positive effect on avoiding a non-revealing aggregation process but it reduces welfare in general.