Consider the following two seemingly unrelated questions. First, why does Rousseau (1993 [1762]) believe that the formation of factions or partial associations is not conducive to the general will in Du Contrat Social, II, 3? Second, why do federal assemblies typically strive for some form of degressive proportionality, i.e. a balance between equal and proportional representation, for the countries in the federation? We will show that there is a surprising connection between these questions. We turn to our first question. It (...) is often thought that Rousseau’s opposition to factions can be interpreted in reference to the Condorcet Jury Theorem. The Condorcet Jury Theorem states that if voters are more likely to be right on some issue than not and cast their votes independently, then the chance that the majority is correct converges to one as the number of voters goes to infinity. This interpretation is tempting, but it is inconsistent with some crucial textual evidence. We turn to our second question. Consider a federation of countries with a decision-making assembly in which each country casts a block vote. On equal representation, the vote of each country has the same weight. On proportional representation, the weights of the votes are proportional to the countries’ population sizes. In between these extremes, we can let the weights increase as a function of population sizes, but smaller countries receive greater weights and larger countries receive lesser weights than proportionality would warrant. Such weightings are called degressively proportional weightings. How can degressive proportionality be justified? And does this justification provide us any guidance as to how these weights should be set? A correct understanding of Rousseau’s misgivings about the formation of factions will hold the key to these questions. (shrink)