Sequential Voting and Agenda Manipulation

We provide characterizations of the set of outcomes that can be achieved by agenda manipulation for two prominent sequential voting procedures, the amendment and the successive procedure. Tournaments and supermajority voting with arbitrary quota q are special cases of the general sequential voting games we consider. We show that when using the same quota, both procedures are non-manipulable on the same set of preference profiles, and that the size of this set is maximized under simple majority. However, if the set of attainable outcomes is not single-valued, then the successive procedure is more vulnerable towards manipulation than the amendment procedure. We also show that there exists no quota which uniformly minimizes the scope of manipulation, once this becomes possible.