The ACO Seminar (2012-2013)

In this paper, I study implementation by agenda voting, a straightforward mechanism that is widely used in practice. The main result establishes that any candidate neutral majority voting rule which satisfies the two necessary conditions identified in prior work (McKelvey and Niemi [1978]; Moulin [1986]) as well as a restricted version of Sen's α [1971] can be implemented by sophisticated voting on an agenda.

The sufficient conditions are satisfied by almost every majority voting rule discussed in the literature, including the Uncovered Set, the Banks Set, the Minimal Covering Set, the Bipartisan Set, the Tour- nament Equilibrium Set, and the Slater Set. This effectively shows that the Copeland Set is the only popular majority voting rule which is not agenda implementable. Having said this, the Copeland Set can be implemented "approximately" in the sense that there exists an agenda such that the Copeland score of any sophisticated voting outcome is at least two-thirds that of the Copeland winner(s).

The main result also clarifies what can be implemented via dominance solvable voting and backward induction, two appealing yet poorly understood solution concepts related to agenda voting.