Party system ideological polarization in Western Europe: data, trends, drivers, and links with other key party system properties (1945–2021)

Unlike other key party system properties (e.g. fragmentation, volatility), polarization lacks a publicly available cross-national and longitudinal dataset that can be used as an established source among the scientific community. This article aims to fill this gap by introducing a novel publicly available dataset of party system ideological polarization in Western European party systems. The dataset relies on multiple expert surveys and provides measures of party system ideological polarization for each parliamentary election and legislature in 20 Western European countries since 1945. The article also presents party system polarization's comparative longitudinal trends, drivers, and links with other key party system properties. It finds that party system ideological polarization is on the rise in Western Europe, mainly due to a progressive shift in the electoral support from ideologically moderate mainstream parties to more extreme challenger parties. Moreover, in recent years, high party system polarization has recurred in highly fragmented and volatile contexts, thus creating a detrimental context for the working of democracy.