How Reddit Atlas works

What the atlas measures

The atlas measures overlapping participation. If many of the same people are active in two subreddits during the same year, those communities are treated as related. It does not read titles, comments, or post bodies, so closeness means “used by the same people,” not necessarily “about the same thing.”

That distinction explains some of the map’s stranger neighbors. A city and its sports team may sit together because they share a local audience. Two communities with similar names may sit far apart because their audiences barely overlap.

The source records are yearly Arctic Shift archives of Reddit posts and comments. They are reduced to one row per author, subreddit, and year. Deleted and removed accounts, AutoModerator, and malformed records are excluded. An active author is anyone with at least one remaining post or comment in that subreddit-year.

A subreddit needs at least 100 active authors to enter the map. For years with more than 10,000 eligible communities, the atlas starts with the 10,000 largest by active authors; communities without a qualifying relationship are then dropped.

Colors and compass directions do not carry fixed meanings from one year to the next. Each year is clustered and laid out independently.

Similarity model

Raw shared-user counts favor Reddit’s largest communities, so the atlas normalizes overlap by the size of both audiences. For subreddits A and B, the score is:

shared qualifying authors ÷ √(active authors in A × active authors in B)

“Qualifying” authors participated in between 2 and 100 subreddits that year. The lower bound removes people who cannot connect two communities; the upper bound limits extremely broad accounts that would otherwise tie much of Reddit together. The denominator still uses each subreddit’s full active-author count.

A pair must share at least 25 qualifying authors before it can become a relationship. The model then ranks every subreddit’s strongest matches. Most current years retain a union of each node’s top 15 relationships; the 2008–2010 builds retain 30. The interface keeps a longer ranked neighbor list for inspection while drawing only mutually strong relationships, which prevents the canvas from becoming a hairball.

Communities are detected with Louvain clustering on this weighted graph. Most years are placed in two dimensions with an igraph DrL layout; the 2008–2010 builds use UMAP over the neighbor graph. Both methods compress a complicated network onto a flat screen, so local neighborhoods are more reliable than long-range distance or direction.

The public atlas contains subreddit-level counts and relationships, not usernames, text, or individual activity histories. Usernames exist only in private intermediate build data as join keys.

Data volume by year

The activity totals below come from the yearly aggregate rows after the record exclusions described above. “Eligible subreddits” is the number with at least 100 active authors that year, before the 10,000-node cap and relationship filter are applied.

Together, the listed aggregates contain 24,877,666,863 posts and comments. Coverage reflects what is present and parseable in the source archives; it should not be read as a guarantee that every Reddit record from every year is available.