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 ar- gument 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 UK, we examine how polarization is influenced by parti- san group homogeneity. We recruit nationally representative partisans and assign them to discuss a salient policy issue, either with like-minded par- tisans (an echo chamber) or in a mixed-partisan group. This allows us to examine how group composition affects polarization. In line with our ex- pectations, we find that partisan echo chambers increase both policy and affective polarization compared to mixed discussion groups. This has im- portant implications for our understanding of the drivers of polarization and for how outgroup animosity might be ameliorated in the mass public
