Agenda-manipulation in ranking

Abstract

A committee ranks a set of alternatives by sequentially voting on pairs, in an order chosen by the committee's chair. Although the chair has no knowledge of voters' preferences, we show that she can do as well as if she had perfect information. We characterise strategies with this 'regret-freeness' property in two ways: (1) they are efficient, and (2) they avoid two intuitive errors. One regret-free strategy is a sorting algorithm called insertion sort. We show that it is characterised by a lexicographic property, and is outcome-equivalent to a recursive variant of the much-studied amendment procedure.