When voting becomes paradoxical Condorcet, Arrow, and democracy

One of the fundamental assumptions in economics and decision theory is that individuals—or “agents,” as they are often called—are rational. This means, in particular, that their preferences obey certain intuitive properties, called axioms. For example, if an agent prefers A to B (we can write A > B) and B to C (B > C), it is natural—and rational—to assume that they also prefer A to C (A > C): this is the axiom of transitivity. But what is true at the individual level suddenly becomes much less obvious at the collective level. When several agents make a decision together, their aggregate preferences may violate these natural principles, leading to surprising or even absurd results. This is where the great paradoxes of voting and social choice arise.

A brief article on Condorcet’s paradox, written as a book chapter (for a book that will be published in French in the Fall, or in the Winter), is  now available.