Sistemas de votação e o teorema da impossibilidade de Arrow.

Abstract

Throughout history, several researchers have suggested different electoral systems that were intended to define a fair or ethical way of expressing the "popular will" over a certain set of choices. The focus of this work is on ranking polling systems where, instead of each citizen voting simply for the best candidate (on his / her opinion), he / she should vote in an order on all possible candidates (or options) in an election. The system must then, following a predetermined set of rules, produce an "elected order" by the population. In this work, we study some properties that are usually expected, or if desired, for such a system to be considered fair. We note that the outcome of an election can change dramatically when using different systems, even though both systems seem, a priori, to return fair choices. This confirms that the outcome of an election is directly linked to the type of electoral system that is adopted. We also make an analysis of several influential systems throughout the history of the so-called "Social Choice Theory". Finally, we end by showing a demonstration of Arrow's influential Theorem of Impossibility, which says that in ranking voting systems certain important conditions and can not be met simultaneously. We conclude, then, that in democratic polls it is necessary to give up one of these properties (or not to use a system by rank). In this way, the study of the most common electoral systems has a sophisticated role in helping us to define, criticize or influence the electoral system used in an election, helping us to perceive the strengths and weaknesses of each one of them.