Extracting ideological dimensions from legislative speeches in the Japanese Diet

Abstract Understanding the ideological positions of political parties is crucial for empirically testing various theories in political science. In Japan, some studies have assessed each party’s policy outlook through electoral platforms, candidate surveys, or expert surveys, with most studies focusing on the past thirty years. However, limited efforts have comprehensively and longitudinally quantified the conflicts between political parties. Although previous studies have used election-related data to reveal interparty conflicts during elections, interparty conflicts between elections remain unclear. This study determines the stance of Japanese political parties during parliamentary debates. We collected text data from committee speeches from the Diet Proceedings Database from 1959 to 2019 and conducted a quantitative text analysis. By applying the Wordshoal method, we first estimated political party positions on specific policy topics in the House of Representatives and the House of Councillors. Then, we aggregated these policy-specific positions yearly via dynamic factor analysis. Consequently, we succeeded in discerning the axis of conflict in Japanese parliamentary debates and evaluated the longitudinal one-dimensional ideal points for political parties. Additionally, we visualized the changes in each party’s ideological position, identified controversial topics, and explored how these topics changed over time, providing new insights into the study of Japanese politics.