Preference Monotonicity and Information Aggregation in Elections

If voter preferences depend on a noisy state variable, under what conditions do large elections deliver outcomes "as if" the state were common knowledge? While the existing literature models elections using the jury metaphor where a change in information regarding the state induces voters to switch in favour of only one alternative, I allow for more general preferences where a change in information can induce switch in favour of either alternative. I show that information is aggregated for any voting rule if and only if the probability of switch in favour one alternative is strictly greater than the probability of switch away from that alternative for any change in information. In other words, unless preferences closely conform to the jury metaphor, for large classes of voting rules, there are equilibria that produce outcomes different from the full information outcome with high probability. This condition is very fragile and may be easily violated in spatial elections if the policy space is multidimensional. I conclude that state-contingent conflict in voter preferences may often lead to failure of information aggregation.