  While political polarization has increased as measured through surveys,
currently we lack comprehensive, longitudinal, and ecologically valid
measurement of the polarization of online political speech that spans social
media platforms. Using language models, we analyze ~2.5 billion comments on
Reddit and Twitter across ~1.7 million accounts from 2007-2023 and find that
polarized speech has been rising on both platforms since their inception, with
outgroup polarization levels higher on Twitter than Reddit. On Twitter, while
U.S. politicians on the left have been consistently more polarized than
everyday users, politicians on the right experienced the highest growth in
polarization, overtaking journalists, media, and everyday users over the past
four years. Today, politicians, the group listened to the most for their
political rhetoric, are far more polarized than everyday users. Additionally,
while polarized speech is typically lower for accounts with more followers,
right-leaning political influencers are an exception to this trend, which may
influence perceptions of polarization on the left versus the right.
Polarization is more diffuse across users on Twitter than on Reddit, where it
is localized to a few communities. Polarization also varies by topic, with
right-leaning users twice as likely to use polarized rhetoric about immigration
as left-leaning users while left-leaning users are somewhat more likely to be
polarized around healthcare. Our large-scale analysis reveals previously
unknown patterns of polarization across platforms, groups, and topics that will
help us better contextualize polarized content and potentially intervene to
reduce it.
