Absolute Expediency and Imitative Behaviour

This paper analyses a model of learning by imitation, where besides the decision maker, there is a population of individuals facing the same decision problem. We analyse a property called Absolute Expediency, which requires that the decision maker's expected payo¤ increases from one round to the next for every decision problem and every state of the population. We give a simple characterisation of the expediency property and show that its basic feature is proportional imitation: the change in the probability attached to the played action is proportional to the di¤erence between the received and the sampled payo¤ (the sampled payo¤ plays the role of an aspiration level).