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This paper suggests a class of stochastic collective learning processes exhibiting very irregular behavior. In particular, there are multimodal long run distributions. Some of these modes may vanish as the population size increases. This may bethought of as bubbles" persistent for a nite range of population sizes but disappearing in the limit. The limit distribution proves to bea discontinuous function of parameters determining the learning process. This gives rise to another type of bubbles": limit outcomes corresponding to small perturbations of parameters are di erent.Since an agent's decision rule involves imitation of the majority c hoice in a random sample of other members of the population, the resulting collective dynamics exhibit herding" or epidemic" features. JEL Classi cation: C72, C73, D83.

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