A Theory of Momentum in Sequential Voting

Elections with sequential voting, such as presidential primaries, are widelythought to possess momentum effects, where the choices of early voters influence the behavior of later voters. Momentum may take time to build, and can depend on how candidates perform in each stage relative to expectations. This paper develops a rational theory of behavior in sequential elections that accounts for these phenomena. We analyze an election with two candidates in which some voters are uncertain about which candidate is more desirable. Voters obtain private signals and vote in a sequence, observing the history of votes at each point. We show that, regardless of the voting rule, voters can herd on a candidate with positive probability, and such a “bandwagon” can occur with probability approaching one in large electorates. Our theory is distinct from the standard information cascades literature because voting is a collective decision problem, and consequently voters have forward-looking incentives to consider the actions of those after them.