The Origins of Party System Polarisation: How Parties and Voters Shape the Ideological Spread in Party Systems

Party system polarisation is an essential concept to describe the quality of party competition in comparative politics. Both a very high and a very low level of polarisation are associated with dysfunctional party systems. Polarised competition results in political conflicts, while the opposite dynamic leads to a lack of alternatives for the voter and reduces parties’ accountability. Because of its key role in characterising democratic systems, party system polarisation is a frequently used independent variable in comparative politics. Nevertheless, there are relatively few studies of its causes; in particular, we see a research gap in the analyses of direct effects due to parties and voters. We therefore ask in the following contribution: How do interactions within the party system and the electorate affect polarisation? We argue that current explanations are complementary, and we integrate them into a single theoretical framework. Applying time-series cross-section analyses to manifesto data and survey data, we show that — besides the well-known fragmentation effect, especially the patterns of government — opposition and party identification matter. We conclude that, for the long-term stability of a party system, the strength of the opposition is as important as the stability of the government.