  We evaluate homophily and heterophily among ideological and demographic
groups in a typical opinion formation context: online discussions of current
news. We analyze user interactions across five years in the r/news community on
Reddit, one of the most visited websites in the United States. Then, we
estimate demographic and ideological attributes of these users. Thanks to a
comparison with a carefully-crafted network null model, we establish which
pairs of attributes foster interactions and which ones inhibit them.
  Individuals prefer to engage with the opposite ideological side, which
contradicts the echo chamber narrative. Instead, demographic groups are
homophilic, as individuals tend to interact within their own group - even in an
online setting where such attributes are not directly observable. In
particular, we observe age and income segregation consistently across years:
users tend to avoid interactions when belonging to different groups. These
results persist after controlling for the degree of interest by each
demographic group in different news topics. Our findings align with the theory
that affective polarization - the difficulty in socializing across political
boundaries-is more connected with an increasingly divided society, rather than
ideological echo chambers on social media.
  We publicly release our anonymized data set and all the code to reproduce our
results: https://github.com/corradomonti/demographic-homophily
