What Makes Party Systems Different? A Principal Component Analysis of 17 Advanced Democracies 1970-2013

Party systems, that is, the number and the size of all the parties within a country, can vary greatly across countries. I conduct a principal component analysis on a party seat share dataset of 17 advanced democracies from 1970 to 2013 to reduce the dimensionality of the data. I find that the most important dimensions that differentiate party systems are: the size of the biggest two parties and the level of competition between the two biggest parties. I use the results to compare the changes in electoral and legislative party systems. I also juxtapose the results to previous party system typologies and party system size measures. I find that typologies sort countries into categories based on variation along both dimensions. On the other hand, most of the current political science literature use measures (e.g., the effective number of parties) that are correlated with the first dimension. I suggest that instead of these, indices that measure the opposition structure and competition could be used to explore problems pertaining to the competitiveness of the party systems.

Automated summary

This study examines party systems in 17 advanced democracies and finds that the size of the biggest two parties and the level of competition between them are the most important dimensions. The author suggests using indices that measure the opposition structure and competition to explore problems related to the competitiveness of party systems, instead of using traditional measures such as the effective number of parties.