Despite a great flourishing of studies about Latin America and Central and Eastern Europe, the issue of party system institutionalization has been widely neglected in Western Europe, where the presence of stable and predictable patterns of interactions among political actors has been generally taken for granted for a long time. Nevertheless, party system institutionalization is not something that can be gained once and for all. This article proposes a theoretical reconceptualization and a new empirical operationalization of party system (de-)institutionalization. Furthermore, it tests the presence of patterns of de-institutionalization in Western Europe from 1945 to (March) 2015 (336 elections in 19 countries) by using an original database of electoral volatility and of its internal components (regeneration and alteration). Data analysis shows that Western Europe is facing great electoral instability and party system regeneration and that many countries have experienced sequences of party system de-institutionalization, especially in the last two decades

Italy went to the polls on 4 March 2018 in a general election whose outcome was highly uncertain until the very last day of the campaign. The three main contenders were the centre-right coalition led by Berlusconi and Salvini, the centre-left coalition headed by Renzi, and the Movimento 5 Stelle (M5S, Five Star Movement)-a populist, anti-establishment party founded by comedian Beppe Grillo and now led by Luigi Di Maio. Uncertainty surrounded the election outcome not only due to the employment of a new, untested electoral system, but also as a result of the very high percentage of undecided voters and the competitiveness of the main political groupings. According to the polls, none of them would get the majority of seats in Parliament required to form a government outright.

We compare two tools for displaying, in graphical form, information about vote outcomes in multiparty elections at the constituency level. One was recently proposed by Nagayama and introduced to the English-speaking world by Reed, who applied this method to Japanese and Italian election data. Reed labels the method Nagayama diagrams. Recently, Taagepera has shown how the domain of potential uses of Nagayama diagrams can be expanded significantly. A second graphical device has been used by a number of authors for various types of election analyses, but is not that well known in the comparative parties literature. This method, which uses barycentric coordinates (i.e. triangular) rather than the more familiar rectangular coordinates, has gone under a variety of names (e.g. trilinear plot, toroidal diagram and simplex representation), but we have chosen to use the last of these labels. We make use of both methods to visually present election data (by constituency) for the Italian national elections of 1994, 1996 and 2001. We show how different types of information may be readily gleaned from the two types of graph, and, perhaps most importantly, illustrate how we may improve the ready intuitive interpretability of each type of graph by specifying boundary constraints to define particular regions of the graph – a technique we call ‘segmentation’.

The literature on party system change and electoral volatility in post-communist Europe tends to make a clear-cut distinction between Central and Eastern European (CEE) party systems and Western European (WE) ones. The former are unstable and unpredictable and electoral volatility is driven by the continuous emergence of new political parties. Conversely, electoral stability is the rule in the latter, and volatility is associated with electoral shifts among established parties. This conventional wisdom suffers from three potential sources of bias: case selection, time coverage and method. By correcting these biases, this article investigates whether the traditional division between CEE and WE party systems has been levelled as regards volatility. To do so, it presents evidence based on an original data set of electoral volatility and its internal components covering 31 WE and CEE party systems since 1990. It finds that a process of asymmetric convergence in the levels of electoral volatility is taking place between the two regions, with Western Europe approaching Central and Eastern Europe with increasing electoral instability.

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