Brexit Means Brexit: Selection Bias, Echo Chambers, and Entrenched Opinion on Reddit
This research provides insights into the dynamics of political polarization on structured discussion platforms like Reddit, which is crucial for understanding how online communities contribute to opinion entrenchment.
This paper investigates political polarization on Reddit, specifically within the r/Brexit subreddit, using a new framework and a BERT classifier trained on a crowd-annotated dataset of 5,895 submissions. The study found that persuadable users disengage, leading to entrenched opinions among remaining users, and that echo chambers are dominant, accounting for nearly 40% of interactions between like-minded individuals.
Political polarisation on structured discussion platforms such as Reddit differs fundamentally from that on broadcast platforms such as Twitter/X, yet most prior work targets the latter. We present an end-to-end framework for measuring and analysing polarisation dynamics, applied to the r/Brexit subreddit (871K submissions, November 2015 -- February 2021). We construct r/Brexit, a crowd-annotated stance dataset of 5,895 labelled submissions (inter-annotator agreement = 0.804), and train a domain-adapted BERT classifier. We introduce a continuous polarity metric that replaces discrete stance categories, revealing fine-grained opinion spectra across 27 politically-defined periods. Our analysis yields three key findings: (a) future stance prediction is confounded by survivorship bias: persuadable users disengage, and those who remain are already entrenched; (b) echo chambers are quantifiably dominant, with nearly 40% of interactions between like-minded users; (c) user current polarity is the dominant predictor of future polarity, with echo-chamber immersion as the secondary predictive signal. These findings reveal that Reddit's partisan core is entrenched by self-selection, not softened by cross-cutting exposure.