 At the time of writing their essay (1967), they moreover observed that these party systems had for the biggest part not changed.Parties then still seemed to be based on the aforementioned four cleavages, making many European party systems seem 'frozen'.The main explanation they give for this is that with the development of labor parties as well as the introduction of universal suffrage at the beginning of the 20th century all groups of civil society were in a way represented in the political arenas of these states.