  Political polarization plays a pivotal and potentially harmful role in a
democracy. However, existing studies are often limited to a single country and
one form of polarization, hindering a comprehensive understanding of the
phenomena. Here we investigate how affective and interactional polarization are
related across nine countries (Canada, France, Germany, Italy, Poland, Spain,
Turkey, UK, USA). First, we show that political interaction networks are
polarized on Twitter. Second, we reveal that out-group interactions, defined by
the network, are more toxic than in-group interactions, meaning that affective
and interactional polarization are aligned. Third, we show that out-group
interactions receive lower engagement than in-group interactions. Finally, we
show that the political right reference lower reliability media than the
political left, and that interactions between politically engaged accounts are
limited and rarely reciprocated. These results hold across countries and
represent a first step towards a more unified understanding of polarization.
