On the stability of procedures and constitutions in social choice theory.

Summary This thesis addresses a logically fundamental question, although not much addressed by collective choice theories. These theories generally consider the following categories: individual preferences, which deal with possible choices (preference objects), and collective choice functions, which define choices from individual preferences. But, in reality, the principles, rules and methods of collective choice are invoked and applied at different levels: the objects of choice are rarely complete descriptions of the state of the world including all possible details, they are often intermediate decisions that actually describe the modalities of future decisions. The standard theory thus crushes different levels. It allows itself this shortcut by referring to the consequentialist principle according to which an individual compares choice rules by simply comparing the choices induced by these rules: it is therefore unnecessary to introduce level 2 preferences (concerning the rules themselves), and there is no need to speak of choices about choice rules. However, the consequentialist principle applied to the question of collective choice is quite questionable. There is obviously a valorisation of choice procedures in themselves, and the attachment of many citizens to democratic principles, as well as their rejection of the very idea of dictatorship, is flagrant. It is therefore relevant to ask what happens to the theory of collective choice if we abandon this consequentialist shortcut. This is what is done in this thesis, by adopting the completely opposite point of view, which renounces imposing any connection whatsoever between level 2 preferences (or even higher level preferences) and initial level preferences. Choice methods are themselves choice objects. We must therefore deal with a preliminary problem of circularity of notions: in what sense can we say that a method of choice is or is not self-chosen? A function cannot belong to its arrival set, and we are led to define a generalized social choice correspondence (or constitution) as a sequence of choice correspondences of successive levels. With this difficulty solved, the thesis addresses another problem, not a logical one but one that is described as "positive". A method of choice may not choose itself: suppose, for example, that individual 1 prefers the dictatorship of individual 2 and that individual 2 prefers the dictatorship of individual 1. The two constitutions (dictatorship of 1 and dictatorship of 2) are in this case unstable. The major part of the thesis is devoted to variations on this theme of the stability of choice procedures, which also corresponds to the title of the thesis, and to the content of the first three chapters. ...