The Polarizing Effect of Partisan Echo Chambers

We are witnessing increasing partisan polarization across the world. It is often argued that partisan "echo chambers" are one of the drivers of both policy and affective polarization. In this article, we develop and test the argument that the political homogeneity of people's social environment shapes polarization. Using an innovative, large-scale pre-registered "lab-in-the-field" experiment in the United Kingdom, we examine how polarization is influenced by partisan group homogeneity. We recruit nationally representative partisans and assign them to discuss a salient policy issue, either with like-minded partisans (an echo chamber) or in a mixed-partisan group. This allows us to examine how group composition affects polarization. In line with our expectations, we find that partisan echo chambers increase both policy and affective polarization compared to mixed discussion groups. This has important implications for our understanding of the drivers of polarization and for how out-group animosity might be ameliorated in the mass public.