Why judges always vote

This paper provides the first account of the practice of universal voting on the Supreme Court - that is, why justices never abstain, unlike voters in other committee contexts. Full participation among justices is explained using models of spatial competition, showing that two features particular to the Court encourage full participation. First, the doctrine of stare decisis makes the resolution of future cases in part dependent on the resolution of present ones. This raises the cost of abstention, particularly to risk-averse justices. Second, the so-called narrowest grounds or Marks doctrine enforces the logic of the median voter theorem in cases presenting more than two options. This makes voting by otherwise indifferent or alienated justices rational, where it otherwise would not be. Although these explanations may not exhaust the multi-causal factors behind the robust phenomenon of zero abstention, they are the first attempt to rigorously analyze how two unique institutional judicial rules mitigate the incentive to abstain.