Probabilistic Aspects of Voting, Intransitivity and Manipulation Elchanan Mossel∗ Abstract Marquis de Condorcet, a French philosopher, mathematician, and political scientist, studied mathematical aspects of voting in the eighteenth century. Condorcet was interested in studying voting rules as procedures for aggregating noisy signals and in the paradoxical nature of ranking 3 or more alternatives. These notes will survey some of the main mathematical models, tools, and results in a theory that studies probabilistic aspects of social choice. Our journey will take us through major results in mathematical economics from the second half of the 20th century, through the theory of boolean functions and their influences and through recent results in Gaussian geometry and functional inequalities. ∗MIT, Cambridge MA, USA. E-mail:
[email protected]. Partially supported by NSF Grant CCF 1665252 and DOD ONR grant N00014-17-1-2598 1 1 Introduction Marquis de Condorcet, a French philosopher, mathematician and political scientist, studied mathematical aspects of voting in the eighteenth century. It is remarkable that already in the 18th century Condorcet was an advocate of equal rights for women and people of all races and of free and equal public education [79]. His applied interest in democratic processes led him to write an influential paper in 1785 [16], where in particular he was interested in voting as an aggregation procedure and where he pointed out the paradoxical nature of voting in the presence of 3 or more alternatives. 1.1 The law of large numbers and Condorcet Jury Theorem In what is known as Condorcet Jury Theorem, Condorcet considered the following setup.