# Detecting coalitions by optimally partitioning signed networks of political collaboration

## TL;DR

This paper introduces mathematical programming models for optimally partitioning signed networks, applied to US Congress data, revealing that ideological polarization can enhance legislative effectiveness.

## Contribution

It presents new optimization models for partitioning signed graphs and demonstrates their application to political networks, providing a globally optimal solution to a complex NP-hard problem.

## Key findings

- Polarized coalitions can increase legislative effectiveness
- The models efficiently handle dense signed networks
- Optimal partitioning reveals ideological homogeneity as protective

## Abstract

We propose new mathematical programming models for optimal partitioning of a signed graph into cohesive groups. To demonstrate the approach's utility, we apply it to identify coalitions in US Congress since 1979 and examine the impact of polarized coalitions on the effectiveness of passing bills. Our models produce a globally optimal solution to the NP-hard problem of minimizing the total number of intra-group negative and inter-group positive edges. We tackle the intensive computations of dense signed networks by providing upper and lower bounds, then solving an optimization model which closes the gap between the two bounds and returns the optimal partitioning of vertices. Our substantive findings suggest that the dominance of an ideologically homogeneous coalition (i.e. partisan polarization) can be a protective factor that enhances legislative effectiveness.