Conditional Effect of Interest Group Endorsements on Voter Choice: A Conjoint Experiment in Japan

Interest groups play a pivotal role in democratic elections by providing heuristic cues that help voters infer candidate positions. While studies in Western democracies show endorsements can influence voter preferences, less is known about their effects in non-Western contexts characterised by weaker voter attachments to organised interests. This study examines the electoral impact of interest group endorsements in Japan, a system characterised by public skepticism and voter detachment from organised interests. Using randomised conjoint experiments across three survey waves in 2022–2023, it evaluates how endorsements from various organisations affect voter preferences in House of Councillors elections through a framework focusing on congruence. The results show that average endorsement effects are limited, not due to a uniform absence of influence, but because directional cues generate heterogeneous and offsetting responses across voter subgroups. Conditional analyses reveal that voter–organisation congruence is the primary driver of endorsement effectiveness: while policy interest provides little evidence of alignment, organisational affiliation and, to a lesser extent, trust significantly increase the likelihood of supporting an endorsed candidate. This conditional pattern suggests that endorsement cues can operate in both positive and negative directions depending on the relationship between voters and endorsing organisations. The study underscores how public distrust and organisational detachment shape the communicative role of interest groups in contemporary democracies.