List Openness and Intra-Party Ideological Diversity in Proportional Representation

Electoral system theory suggests that open-list proportional representation systems strengthen incentives for candidates to cultivate a personal vote, thereby encouraging differentiation from co-partisans and increasing within-party ideological heterogeneity. These effects are expected to be conditioned by party ideology: left-wing parties are typically characterised by stronger programmatic cohesion and centralised organisation, which constrain ideological differentiation, whereas right-wing parties tend to exhibit weaker ideological discipline and greater internal heterogeneity, making them more responsive to personal vote incentives. However, comparative research has paid limited attention to whether and how these incentives are reflected in candidates’ individual ideological position-taking. In particular, it remains unclear whether more candidate-centred electoral environments are associated with greater ideological divergence from party positions and how such effects vary across party families. This paper examines whether list openness in proportional representation systems affects individual candidates’ ideological positioning and the extent of ideological dispersion within parties, and whether these effects are moderated by party ideology. The analysis draws on comparative candidate-level data from 28 elections in 15 European countries that vary in list openness. The results indicate, in line with theoretical expectations, that candidates’ ideological distance from their party mean is, on average, larger in more open-list systems, and that this association is stronger among right-wing parties than among left-wing parties. Taken together, the findings suggest that electoral system design—particularly list openness—is systematically associated with the degree of intra-party ideological heterogeneity observed at election time.