  We present the first large-scale measurement study of cross-partisan
discussions between liberals and conservatives on YouTube, based on a dataset
of 274,241 political videos from 973 channels of US partisan media and 134M
comments from 9.3M users over eight months in 2020. Contrary to a simple
narrative of echo chambers, we find a surprising amount of cross-talk: most
users with at least 10 comments posted at least once on both left-leaning and
right-leaning YouTube channels. Cross-talk, however, was not symmetric. Based
on the user leaning predicted by a hierarchical attention model, we find that
conservatives were much more likely to comment on left-leaning videos than
liberals on right-leaning videos. Secondly, YouTube's comment sorting algorithm
made cross-partisan comments modestly less visible; for example, comments from
conservatives made up 26.3% of all comments on left-leaning videos but just
over 20% of the comments were in the top 20 positions. Lastly, using
Perspective API's toxicity score as a measure of quality, we find that
conservatives were not significantly more toxic than liberals when users
directly commented on the content of videos. However, when users replied to
comments from other users, we find that cross-partisan replies were more toxic
than co-partisan replies on both left-leaning and right-leaning videos, with
cross-partisan replies being especially toxic on the replier's home turf.
