Résumé

In most fields such as biology, statistical physics and sociology for example, the behaviour of a system as a whole is not considered to be the same as that of an individual nor of some “representative” or average component of the system. Thus the passage from micro behaviour to macro behaviour is far from simple and additive. Yet, in macroeconomics the economy as a whole, or, at least a whole sector of the economy is depicted as behaving like a “representative agent”. The purpose of this talk is to suggest that we should cease to insist on the idea that the aggregation of the choices and actions of individuals who directly interact with each other can be captured by the idea of the aggregate acting as only one of these many individuals. The gap between micro and macrobehaviour is real and important. The main determinant of aggregate behaviour is the interaction between the individuals and the influence that this has on them. We can learn a lot from social insects who operate with very simple and local rules but who achieve a great deal of coordination and, as a result, produce aggregate outcomes which could not be predicted from the behaviour of the average individual. Furthermore, the behaviour of these insects is far from optimal or efficient, but they get the collective job done. To understand this we should recognise that some of the characteristics of aggregates are caused by aggregation itself. To taked a simple example, as Cournot already observed the continuous reaction of the aggregate may be the result of individuals making simple, binary discontinuous choices. For many phenomena, it is much more realistic to think of individuals as having thresholds - which cause them to react - rather than reacting in a smooth, gradual fashion to changes in their environment which, of course, includes their “neighbours”. Indeed, the aggregate itself may also have thresholds which cause it to react. When enough individuals make a particular choice, the whole of society may then move. When the number of individuals is smaller, there is no such movement. One has only to think of the results of voting. Systems which function in this way will be subject to sudden and important upheavals which come from within the system. Indeed major crises are in fact frequent and inherent in our economic system. What we should do is to think of the economy as a non equilibrium system which self organises in such a way that it experiences these sudden and large changes from time to time. The sort of economic models that we could be using do, at least, allow us to address this sort of problem. We have too long been preoccupied by illusory questions of “efficiency” and “competitiveness”. What we need to look at is the central problem of economics which is how the disparate activities of all the participants in the economy come to be coordinated or in bad times, how this coordination breaks down – and what effects this can have on society and its well being.

Référence

Alan Kirman (GREQAM), « Mind the Gap; Individual and Collective Rationality in Economics », IAST General Seminar, Toulouse : IAST, 20 décembre 2012, 15h30–16h30, salle MF323.