Title:      Party Polarization in Congress: A Social Networks
Approach

Authors:    Andrew Waugh, Liuyi Pei, James Fowler, Peter Mucha,
Mason Porter

Entrydate:  2009-07-23 12:36:40

Keywords:   

Abstract:   We use the network science concept of modularity to
measure polarization in the United States Congress. As a measure
of the relationship between intra-community and extra-community
ties, modularity provides a conceptually-clear measure of
polarization that directly reveals both the number of relevant
groups and the strength of their divisions. Moreover, unlike
measures based on spatial models, modularity does not require
predefined assumptions about the number of coalitions or
parties, the shape of legislator utilities, or the structure of
the party system. Importantly, modularity can be used to measure
polarization across all Congresses, including those without a
clear party divide, thereby permitting the investigation of
partisan polarization across a broader range of historical
contexts. Using this novel measure of polarization, we show that
party influence on Congressional communities varies widely over
time, especially in the Senate. We compare modularity to extant
polarization measures, noting that existing methods
underestimate polarization in periods in which party structures
are weak, leading to artificial exaggerations of the extremeness
of the recent rise in polarization. We show that modularity is a
significant predictor of future majority party changes in the
House and Senate and that turnover is more prevalent at medium
levels of modularity. We utilize two individual-level variables,
which we call "divisiveness" and "solidarity," from modularity
and show that they are significant predictors of reelection
success for individual House members, helping to explain why
partially-polarized Congresses are less stable. Our results
suggest that modularity can serve as an early-warning signal of
changing group dynamics, which are reflected only later by
changes in formal party labels.

http://polmeth.wustl.edu/retrieve.php?id=927

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