Community Structure in Congressional Cosponsorship Networks

TL;DR

This paper analyzes the community structure of U.S. Congress members based on cosponsorship networks, revealing sharp increases in political polarization that peaked in the 104th Congress and highlighting differences between the House and Senate.

Contribution

It introduces a network-based approach using modularity to measure political polarization and community structure in Congress, providing new insights into legislative collaboration and partisan divides.

Findings

Sharp increase in polarization before the 104th Congress

Polarization remains high in the House of Representatives

Senate polarization has decreased since the peak

Abstract

We study the United States Congress by constructing networks between Members of Congress based on the legislation that they cosponsor. Using the concept of modularity, we identify the community structure of Congressmen, as connected via sponsorship/cosponsorship of the same legislation, to investigate the collaborative communities of legislators in both chambers of Congress. This analysis yields an explicit and conceptually clear measure of political polarization, demonstrating a sharp increase in partisan polarization which preceded and then culminated in the 104th Congress (1995-1996), when Republicans took control of both chambers. Although polarization has since waned in the U.S. Senate, it remains at historically high levels in the House of Representatives.