  Political polarization in public space can seriously hamper the function and
the integrity of contemporary democratic societies. In this paper, we propose a
novel measure of such polarization, which, by way of simple topic modelling,
quantifies differences in collective articulation of public agendas among
relevant political actors. Unlike most other polarization measures, our measure
allows cross-national comparison. Analyzing a large amount of speech records of
legislative debate in the United States Congress and the Japanese Diet over a
long period of time, we have reached two intriguing findings. First, on
average, Japanese political actors are far more polarized in their issue
articulation than their counterparts in the U.S., which is somewhat surprising
given the recent notion of U.S. politics as highly polarized. Second, the
polarization in each country shows its own temporal dynamics in response to a
different set of factors. In Japan, structural factors such as the roles of the
ruling party and the opposition often dominate such dynamics, whereas the U.S.
legislature suffers from persistent ideological differences over particular
issues between major political parties. The analysis confirms a strong
influence of institutional differences on legislative debate in parliamentary
democracies.
