Assessing the Conservative Nature of the Supreme Court of Japan via Ideal Point Estimation of Justices

ABSTRACT Japan is an intriguing case in the literature of judicial independence because of the Liberal Democratic Party's (LDP) long‐lasting one‐party dominance in government—which, in theory, leads to low independence. Although scholars have found plentiful anecdotal observations implying the LDP's judicial control, quantitative evidence remains scarce. Focusing on the Supreme Court as the first step, this study provides a new dataset on justices' ideal points through an extensive compilation of justices' voting data and application of the dynamic item response theory model. I validate the estimates' interpretability as ideological positions. The results present several novel findings: justices with a career‐judge background are relatively conservative; the Chief Justice tends to be particularly conservative; the conservative camp has tended to hold on to a court majority since the 1960s; and the government's partisanship, to some degree, influences appointed justices' positions. These results reinforce scholars' views that Japan's court leans conservatively and aligns with the LDP, with implications for judicial independence in Japan.