The next speaker in this session at the AoIR 2025 conference is my great colleague Ehsan Dehghan, whose focus is on sedimented polarisation across Reddit collectives. To what extent is there cross-ideological interaction on the platform? What forms does such interaction take: how deliberative or antagonistic is it?
Indeed, what do we mean by cross-ideological interaction? This could mean information sharing, cross-posting, cross-commenting, cross-linking between subreddits, or other discursive patterns. The project drew on a dataset of activity over 16 years across 11 subreddits, containing 6.2 million submissions from 801,000 authors and nearly 200 million comments from 5.5 million authors attached to those submissions.
From these, the project generated a normalised similarity score (NSS) that built on the contribution ratio of each subreddit to calculate the expected overlap between any two subreddits. This enabled a pairwise comparison of subreddits with each other, examining whether the actual overlap between subreddits is above or below what was expected. This can be calculated for each of the distinct activity types.
For domain sharing, there is clear ideological clustering amongst the chosen subreddits; right-wing and left-wing subreddits have strong intra-group overlap and low inter-group overlap, for instance. Patterns for specific URL sharing are broadly similar, though overlap between any two specific subreddits is necessarily substantially lower overall.
User contribution overlaps show considerably different patterns. There is still broad ideological clustering, but also strong distinction between apparently very similar subreddits such as /r/conservative and /r/Conservative: these have distinct and different participant communities, largely because of their own history – one subreddit was created response to concerns with the other, and the community split in two.
In terms of topics, right-leaning subreddits had various common and fairly predictable topics and antagonists; some of these have particular features and focal points, while a number of them also have specific rules around trigger topics and incivility. Left-leaning topics have more diverse and specialised or niche topics, and more diverse antagonists.
Overall, this paints a picture of minimal cross-interactions even between discursively resonant subreddits; there is fragmentation amongst as well as polarisation between subreddits representing specific ideological positions. Generic subreddits such as /r/Politics too do not function as a public sphere or town square; it is not a space where users participating in ideological subreddits mingle with each other.
Polarisation on Reddit is therefore sedimented, and assumed as a starting-point for participation – it is not a result of activities on the platform.
