Spurious Unanimity Revisited

Spurious unanimities threaten the possibility of social choice under uncertainty. This has been argued to reveal that collective evaluations require knowing more than preference data. Such a conclusion may seem like a severe overreaction to the impossibility result in question. Yet this interpretation admits of a principled refinement: preference information may suffice but not, contrary to what the standard IIA condition requires, be restricted to a given pair of options independently of all others. Thus, in the sense associated with IIA, collective evaluations require knowing more than binary preference data. I argue that this account both illuminates recent developments in the literature and clarifies their conceptual significance. Simply put, the consideration of uncertainty reveals that collective rationality demands departing from (contrast the weaker, more familiar claim: it does not demand abiding by) the classical IIA condition of social choice theory.