Epidemics of rules, information aggregation failure and market crashes

This short paper argues that rationally motivated coordination between agents is an important ingredient to understand the current economic crisis. We argue that changes in parameters that model the structure of a macro-economy or financial markets are not exogenous but arise as agents adopt rules that appear to be the norm around them. For example, if a rule is adopted by the majority of ones' neighbors it will become acceptable or, alternatively, if agents learn that changing their rule leads to greater gains, they will modified their rules. However, as rules develop and spread they may have consequences at the aggregate level which are not anticipated by individuals. These rules may be adopted by implicit consensus as they turn out to be profitable for individuals, but they may also weaken the constraints imposed by regulators. Indeed, the emergence of new rules or the modification of old ones may render the whole system more fragile, which may then cease to function. To illustrate this we develop a simple model, motivated by the 2007-2008 crisis in credit derivatives markets, to show how coordination on simple and apparently profitable rules may cause a market to collapse.