Coalition Structure and Polarization in Parliamentary Voting Networks: Evidence from the Italian Parliament

TL;DR

This paper introduces a network-based framework for analyzing parliamentary voting behavior, revealing how coalition structures influence polarization and uncovering implicit alliances using public voting data from the Italian Parliament.

Contribution

The study develops a novel quantitative methodology combining network analysis and voting metrics to analyze legislative dynamics and coalition structures at multiple scales.

Findings

System-level polarization varies with coalition structure, not size.

Technical governments show lower polarization despite broader support.

Mediator roles are concentrated among about 2% of parliamentarians.

Abstract

Ensuring legislative accountability in multi-party systems requires quantitative tools that reveal actual voting behavior beyond formal party affiliations. We present a network-based framework for analyzing parliamentary dynamics at multiple scales, capturing coalition structure, group coherence, and individual influence. Applied to over 4 million vote expressions from the Italian Parliament across three government formations (2018-2021), the methodology combines network modularity, voting distance metrics, and betweenness centrality to map the structure of collective decision-making. Using this framework, we show that system-level polarization, as captured by network modularity, varies systematically with coalition structure rather than coalition size. Technical governments display paradoxically lower global polarization despite broader formal support, reflecting structurally mixed voting patterns rather than unified blocs. On polarizing issues such as immigration, network polarization depends strongly on the fragmentation or cohesion of the opposition, even when the governing coalition votes cohesively. Betweenness analysis reveals that mediator roles are highly concentrated, with only about 2% of parliamentarians acting as structural bridges between communities. Community detection further uncovers implicit coalitions that are not apparent from declared alliances. The framework relies exclusively on public roll-call data, enabling reproducible analysis and direct applicability to any legislature with transparent voting records. By linking individual voting behavior to emergent system-level structure, the methodology provides quantitative infrastructure for comparative analysis of legislative voting networks and coalition monitoring, enabling systematic assessment of legislative behaviour.