Cleavage (politics)  In political science and sociology, a cleavage is a historically determined social or cultural line which divides citizens within a society into groups with differing political interests, resulting in political conflict among these groups. Social or cultural cleavages thus become political cleavages once they get politicized as such. Cleavage theory accordingly argues that political cleavages predominantly determine a country's party system as well as the individual voting behavior of citizens, dividing them into voting blocs. It is distinct from other common political theories on voting behavior in the sense that it focuses on aggregate and structural patterns instead of individual voting behaviors. Classical cleavage theories have generally been focused on the persistence of dominant conflicts within national political systems over the course of history. Political sociologists Seymour Martin Lipset and Stein Rokkan (1967) for example used the term in their often cited essay on cleavage structures in West European politics. In their essay, the authors argue how the European party systems at their time of writing were still largely based on the social and cultural cleavages that characterized European societies a century earlier. They therefore argue that these 'frozen party systems' can be seen as political expressions of historically determined societal divisions. Although some authors have claimed that the cleavages in Lipset and Rokkan's theory are still dominant for contemporary voting behaviors in Western Europe, others have argued that these traditional cleavages have become less important and new conflict lines have emerged. Conflicts that have emerged around several new political cleavages are for example cultural, such as conflicts over integration and multiculturalism, or environmental, such as ongoing politics over climate change. The cleavages in Lipset and Rokkan's classical theory have its origins in two developments in 19th century Western Europe. On the one hand European societies at the time saw a period of so-called national revolutions. These events were revolutionary as the centralized state came to take over political roles that had formerly been assigned to decentralized and/or religious communities. According to Lipset and Rokkan, these historical national revolutions gave rise to the following two societal and political cleavages:   On the other hand, the authors claim that the Industrial Revolution also generated two persistent cleavages:   Lipset and Rokkan claim that the political parties that emerged in Western Europe during the end of the 19th and beginning of the 20th century were formed on the base of these structural cleavages in societies. At the time of writing their essay (1967), they moreover observed that these party systems had for the biggest part not changed.