Adapt or perish? How parties respond to party system saturation in 21 Western democracies, 1945-2011

This study examines whether (and how) parties adapt to party system saturation (PSS). A party system is oversaturated when a higher effective number of parties contests elections than predicted. Previous research has shown that parties are more likely to exit when party systems are oversaturated. This article examines whether parties will adapt by increasing the nicheness of their policy platform, by forming electoral alliances or by merging. Based on time-series analyses of 522 parties contesting 357 elections in twenty-one established Western democracies between 1945 and 2011, the study finds that parties are more likely to enter - and less likely to leave - electoral alliances if PSS increases. Additionally, a small share of older parties will merge. The results highlight parties' limited capacity to adapt to their environments, which has important implications for the literature on party (system) change and models of electoral competition.