A Cluster-Based Measure of Issue Polarisation: US Trends 1988-2020 and Predictors Across 105 Nations 1999-2022

Political issue polarisation worries scholars and the public alike. To understand what drives political issue polarisation, cross-national comparative research is necessary. Yet, measures of political issue polarisation which can be meaningfully applied across different countries are lacking. We propose a new technique for measuring political issue polarisation using clustering algorithms, specifically k-means, to identify groups of citizens within multidimensional political issues spaces from the bottom up. We then analyze the separation, internal cohesion, and size parity of the resulting groups to quantify a society’s issue polarisation. Our measure converges with several alternatives in finding that US issue polarisation increased from 1988 to 2020, and particularly rapidly after 2008. We find that political issue polarisation typically takes the form of disagreement between culturally-conservative majorities and culturally-liberal minorities. We find only one of sixteen candidate predictors robustly correlates with polarisation across the world: Secular cultural values. There is a non-linear relationship between Secular cultural values and polarisation: low-Secularism countries are less polarised, primarily because their culturally-conservative majorities are larger, but above a moderate level of Secularism, the relationship levels off.