The extreme right (e.g. Golden Dawn) and radical right or far right parties (e.g. FN) that have experienced their electoral breakthrough in Europe since the early 1980s share significant similarities and are therefore often depicted as either a single or two unified party families. Within these families, parties resemble each other to a lesser or greater extent when it comes to their ideology, their leadership, and organisation. They are also alike in their electoral and office strategies, as well as in the composition of their electoral constituencies. Moreover, it appears that, despite different genealogies, they might have become more similar in recent years.

The similarities between extreme/radical/far right parties exceed the resemblances that could be expected on the basis of the “common-response-to-common-challenges” hypothesis, and probably cannot be explained by the influence of identical demand-side conditions in the various European countries alone. Instead, they seem to indicate that the interdependence of or linkages between extreme/radical/far right parties play an important role in their development, for example in the form of learning processes and transnational cooperation. Given the increasing importance of cross-border processes like Europeanisation, globalisation and internationalisation (e.g. the formation of various radical right-wing groups in the EP 2014), the question of interdependence and diffusion becomes even more pertinent. Yet, this particular research topic remains underdeveloped. With party likenesses and interdependence as a common theme, we seek to bring together a number of insightful papers that theorise and investigate the diffusion process, and highlight the inherent methodological challenges involved in studying interdependence.