Manipulation of social choice rules by strategic nomination of candidates

It often happens that the entry of a third candidate into a race for legislative office is supported by a group of voters who favour, not the new candidate, but one of the two initially declared candidates. It is hoped that the third candidate will draw votes away from a particular one of the other two, changing the course of the election. This phenomenon is examined at a general level and the set of social choice rules which are invulnerable in this sense is characterized.This paper was written while I was a Canada Council Research Fellow visiting at Stanford University. I am grateful to Professor W. Leinfellner for helpful comments on a first draft and I assume full responsibility for errors.