  In social choice theory with ordinal preferences, a voting method satisfies
the axiom of positive involvement if adding to a preference profile a voter who
ranks an alternative uniquely first cannot cause that alternative to go from
winning to losing. In this note, we prove a new impossibility theorem
concerning this axiom: there is no ordinal voting method satisfying positive
involvement that also satisfies the Condorcet winner and loser criteria,
resolvability, and a common invariance property for Condorcet methods, namely
that the choice of winners depends only on the ordering of majority margins by
size.
