Seymour Martin Lipset in the context of "Political opposition"

Modernization theory or modernisation theory holds that as societies become more economically modernized, wealthier, and more educated, their political institutions become increasingly liberal democratic and rationalist. The "classical" theories of modernization of the 1950s and 1960s, most influentially articulated by Seymour Lipset, drew on sociological analyses of Karl Marx, Emile Durkheim, Max Weber, and Talcott Parsons. Modernization theory was a dominant paradigm in the social sciences in the 1950s and 1960s, and saw a resurgence after 1991, when Francis Fukuyama wrote about the end of the Cold War as confirmation of modernization theory.

The theory is the subject of much debate among scholars. Critics have highlighted cases where industrialization did not prompt stable democratization, such as Japan, Germany, and the Soviet Union, as well as cases of democratic backsliding in economically advanced parts of Latin America. Other critics argue the causal relationship is reverse (democracy is more likely to lead to economic modernization) or that economic modernization helps democracies survive but does not prompt democratization. Other scholars provide supporting evidence, showing that economic development significantly predicts democratization.

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. These blocs are distinguished by similar socio-economic characteristics, who vote and view the world in a similar way. 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.