  Political polarization appears to be on the rise, as measured by voting
behavior, general affect towards opposing partisans and their parties, and
contents posted and consumed online. Research over the years has focused on the
role of the Web as a driver of polarization. In order to further our
understanding of the factors behind online polarization, in the present work we
collect and analyze Web browsing histories of tens of thousands of users
alongside careful measurements of the time spent browsing various news sources.
We show that online news consumption follows a polarized pattern, where users'
visits to news sources aligned with their own political leaning are
substantially longer than their visits to other news sources. Next, we show
that such preferences hold at the individual as well as the population level,
as evidenced by the emergence of clear partisan communities of news domains
from aggregated browsing patterns. Finally, we tackle the important question of
the role of user choices in polarization. Are users simply following the links
proffered by their Web environment, or do they exacerbate partisan polarization
by intentionally pursuing like-minded news sources? To answer this question, we
compare browsing patterns with the underlying hyperlink structure spanned by
the considered news domains, finding strong evidence of polarization in
partisan browsing habits beyond that which can be explained by the hyperlink
structure of the Web.
