Towards turbulent times: measuring and explaining party system (de-)institutionalization in Western Europe (1945–2015)

Over the last decades, Western European party systems have experienced growing levels of electoral volatility and the recurring emergence of successful new parties. This evidence calls into question the issue of party system institutionalization (PSI), a topic taken for granted so far in Western Europe, following the conventional wisdom that party systems are highly institutionalized in this region. This article tackles this issue and provides some contributions: it offers a theoretical clarification of PSI and develops an index allowing for cross-country and cross-time comparability; it looks for an explanation, by testing the impact of various potential determinants and their changes over time. Covering 324 elections in 19 countries since 1945, the analysis shows that, since the 1970s, a process of de-institutionalization is going on and that PSI is mainly a function of the cleavage structure and the number of parties, with economic performance becoming relevant only in the last period.