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

TL;DR

This study develops a framework to measure and analyze political polarization on Reddit, revealing entrenched echo chambers, survivorship bias in user engagement, and the dominance of current polarity in predicting future opinions.

Contribution

It introduces a novel continuous polarity metric, a large annotated stance dataset, and an end-to-end analysis framework for polarization dynamics on structured discussion platforms.

Findings

Echo chambers dominate interactions, with nearly 40% between like-minded users.

Survivorship bias affects future stance prediction, as disengaged users leave.

Current user polarity is the strongest predictor of future stance.

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.