Party system institutionalization is the process through which the patterns of inter-party competition become stable and predictable over time. Notwithstanding the widespread agreement on this definition, the operationalization of this concept is more problematic, as scholars usually measure party system institutionalization (PSI) either only through electoral volatility or by focusing exclusively on government formation. Moreover, they hardly take into account the crucial dimension of time, intimately connected with PSI as a processual phenomenon. This paper proposes a new approach for measuring party system institutionalization in Western Europe, a region where high levels of institutionalization have been taken for granted for a long time, but where party competition has become increasingly unstable and unpredictable over the last years, especially since the post-2008 economic crisis. We extend the index of volatility and its internal components of regeneration and alteration, originally devised for the electoral arena (Emanuele 2015; Chiaramonte and Emanuele 2017), also to the study of the other two arenas, the parliamentary and the governmental ones (i.e., we focus also on the parliamentary and the governmental volatility). By doing so, we offer a comprehensive account of the patterns of (de-)institutionalization in Western European countries since 1945, their cross-national and cross-time variation and the interactions among the three arenas of competition.
