This study examines the diffusion of party entry decisions within ideological families. The question of why new parties enter has received ample attention in cross-national (Harmel and Robertson 1985; Hug 2001; Tavits 2006) and case study research (Kitschelt 1988; Kitschelt and McGann 1997; Rydgren 2004). The theory of strategic entry is the most successful cross-national attempt towards a generic theory on party entry (Cox 1997; Tavits 2006). It argues that a party will enter if the benefits of political office times the probability of victory is high enough to justify the costs. Variables tapping into the costs of entry include the number of signatures required to start a party. A party can infer its likelihood of success from the institutionalization of the party system and also the heterogeneity of a society matters. Last, the benefit of political office is captured by a country’s degree of corporatism. More recent research has also started to incorporate the political environment in which parties operate, like for instance, the presence of likeminded parties (Van de Wardt et al. 2016). What the current literature fails to consider, however, is the presence of likeminded parties abroad. This study fills this void by examining the diffusion of party families across 19 Western European countries between 1945-2015. Specifically, we examine how the presence of the same party family in a foreign country affects entry decisions. In so doing, we examine whether diffusion effects are conditioned by geographical proximity and whether it is merely the presence or electoral success of the party family abroad that matters. The hypotheses are tested by means of spatial econometric analyses on a new dataset on party families providing the most fine-grained classification of party families up to date.
