Reddit's leftward political bias

tl;dr I combined data from the reddit API and AllSides Media to measure the political bias of six popular political subreddits. I found that the supposedly neutral r/politics is significantly more left-leaning than the explicitly left-leaning subreddits r/democrats and r/liberal, but that political bias has not been increasing over time.


The general sentiment is that political discussion has become increasingly polarized in recent years, especially on social media sites like reddit. To test this hypothesis, I examined 6 popular political subreddits (r/politics, r/worldpolitics, r/democrats, r/liberal, r/republican, and r/conservative)I used the data dumps available at I looked for trends in the number of posts per month and measured the bias of posts using data from AllSides media.

Trends in Posting Activity

This first plot shows the total number of posts each month for the six subreddits.Reddit was founded in 2005, but the first political subreddit r/politics was created in August 2007. Therefore, the plot begins at this date. The subreddits are stacked based on founding date, with the youngest subreddits on the bottom.

Large spikes in the graph correspond to the US presidential elections of 2008, 2012, and 2016. There was relatively little political posting in the non-election timeperiods before President Trump was elected; but since his election, political posting has remained high.

Bias in Subreddits

To measure the political bias of subreddits, I used the AllSides media bias ratings. This dataset labels domains on a scale from "strongly left" to "strongly right" leaning. I converted these labels into numbers using the following conversion table:

bias label political leaning number
strongly left-2
strongly right2

Then for each url in each subreddit, I assigned it the corresponding political leaning number.

The plot below shows a three-month moving average of the political leaning of links in each subreddit.

What stands out most about the political leaning of linked media is how much more left-leaning the media links from r/politics posts are compared to the other subreddits. r/politics is often criticized for claiming to accept all political values while actually being a hub for liberal views, and this graph supports such notion. Leading up to the 2016 election, all non-GOP subreddits generally shifted more conservative, while r/conservative and r/republican became more left-leaning. This may reflect political parties linking news websites associated with the opposite party, in order to criticize the bias of said party before an election.


The political leaning of linked media in the six political subreddits studied has not become more extreme over time. In fact, it has historically been less extreme during elections, and is currently on a moderating trend as we approach the 2020 election season.

The frequency of posts, however, has increased dramatically recently. This may account for the public perception of increased extremism.