Brexit Means Brexit: Selection Bias, Echo Chambers, and Entrenched Opinion on Reddit

TL;DR

It is revealed that Reddit's partisan core is entrenched by self-selection, not softened by cross-cutting exposure, which means that Reddit's partisan core is entrenched by self-selection, not softened by cross-cutting exposure.

Abstract

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.

Figures (7)

• Figure 1: Temporal density of posts and comments in r/Brexit (November 2015 -- February 2021). Activity surges coincide with the original Brexit date (29/03/2019), the second attempt to call elections (09/09/2019), and the UK's final separation (31/12/2020).
• Figure 2: MTurk annotation quality.(a) Step 1: parameter sweep over worker qualifications (MHC: minimum HITs completed; AR: approval rate; full sweep in Online Appendix B appendix); the best configuration yields IAA $= 0.798$ (four English-speaking countries; UK-only is higher but causes unfinished batches). (b) Step 3: per-class IAA before ("all", post Step 2 allowlist, overall $0.740$) and after ("filt.") the low-confidence filter (overall $0.804$).
• Figure 3: Stance classification results (stratified five-fold cross-validation). BERT-Reddit achieves the best macro-F1 ($0.555$) and accuracy ($0.783$).
• Figure 4: (a) PDF of the continuous polarity distribution across all user--period pairs. (b) CDF of absolute polarity for pro-leaning ($>0$) and anti-leaning ($<0$) users.
• Figure 5: (a) Most frequent user presence patterns ($\blacksquare$ = present; $\square$ = absent): 70.5% of users appear in a single period, and fewer than 1% sustain engagement across three or more consecutive periods. (b) Proportion of users who remain in the next period, binned by activity and degree percentiles. Higher engagement strongly correlates with staying. The exception cluster ($>$96th percentile degree, 44th--64th percentile activity) contains two highly polarised anti-Brexit broadcasters who accumulate replies without posting frequently.