Parliamentary Voting Procedures: Agenda Control, Manipulation, and Uncertainty

TL;DR

This paper analyzes the computational complexity of manipulation and control in parliamentary voting procedures, revealing that while some problems are tractable, real-world elections are often resistant to manipulation and control.

Contribution

It provides a detailed complexity analysis of amendment and successive procedures, including experimental insights and complexity results for incomplete preferences.

Findings

Manipulation is generally hard in real-world elections.

Agenda control is typically impossible in practice.

Deciding possible winners with incomplete preferences is NP-hard.

Abstract

We study computational problems for two popular parliamentary voting procedures: the amendment procedure and the successive procedure. While finding successful manipulations or agenda controls is tractable for both procedures, our real-world experimental results indicate that most elections cannot be manipulated by a few voters and agenda control is typically impossible. If the voter preferences are incomplete, then finding which alternatives can possibly win is NP-hard for both procedures. Whilst deciding if an alternative necessarily wins is coNP-hard for the amendment procedure, it is polynomial-time solvable for the successive one.