‘Party cohesion’ is a central concept in the analysis of agenda-setting, veto players and coalition-building as well as in the analysis of policy efficiency and party responsiveness. However, there is no indicator to measure party cohesion in a systematic manner over time and across parties. As a consequence, most established studies treat political parties as unitary actors although from an analytical point of view they should be considered collective actors. In order to overcome this deficiency, in this article a time-variant and party-specific index of party cohesion is developed which can be used in macro-comparative statistical analysis. The concept of ‘ideological cohesion’ is developed along the Left–Right dimension. This index is applied in order to compare the party cohesion of Nordic social democratic parties (SDs) with their counterparts in 17 additional countries. The results show that the myth of the cohesion of Nordic SDs is only true for the golden age of the welfare state. Currently, most of the Nordic SDs actually have a lower party cohesion than their counterparts in many other countries.