How do parliamentary party switches affect party systems? We analyse the patterns of party switching in eight democracies (Estonia, Ireland, Italy, Lithuania, Norway, the Netherlands, Poland and Romania) in 1990s and 2020s. We measure the switches along two dimensions. First, we consider the number of MPs involved, the coordination of moves (individual vs. collective switching) and whether the switches are simultaneous or staggered. Second, we take account of the origin and destination of switchers: unaffiliated MPs, one or several parliamentary groups (existing or new), and combinations thereof. Elucidating these types of party instability sheds new light on the complexity of party instability and allows us to examine conditions under which actors engage in the various types. For example, when are we likely to see “collective defection” (coordinated exit from one parliamentary group to join another), “fission-fusion” (coordinated exit from several groups to form a new parliamentary group) or “exit” (exiting a parliamentary group to become independent)? Examining diverse types of instability gives us a more complete picture of parliamentary party instability than is provided in the current literature. The motivations for and types of party switching impact party support and thus affect party systems generally.
