The idea of the five-party model is built on the seminal work of Lipset and Rokkan, in which they launched what they called the "frozen hypothesis." According to Lipset and Rokkan, major social cleavages since the 1920s, when parliamentary democracy was introduced, produced a party system that remained frozen during several decades. In the Nordic countries, this heritage was interpreted through the five-party-system. Challenges to this system now come from the new forces that have affected other European countries, namely populist and green parties. These challenges will have different impacts because the Nordic party systems have diverged during the last decades.