ABSTRACT
The economical and social systems whose behaviour depends on the aggregation of a large number of individual decisions are naturally embedded in a statistical description which tries to find laws or regulations which do not appear when looking at the units of the system considered separately. Mathematical models are used more and more frequently to study such social systems, and have given rise to a large literature. Two different points of view are considered: on the one hand, the normative approach examines socio-economical systems in relation to value (normative) criteria selected a priori * . On the other hand, the descriptive approach explains the evolution of such systems on the basis of the behaviour of their units.
