Reverse-engineering Hacker News: how votes, time decay, and hidden penalties rank stories

Hacker News scores each submission from its upvotes, the elapsed time since posting, and a set of penalties. Because time carries a larger exponent than votes — the so-called ‘gravity’ factor — every story’s score inevitably decays to zero, which is why nothing lingers on the front page. Rather than re-sort the whole list on each page view, HN reranks a story only when it gets upvoted, plus one randomly chosen top-50 story every 30 seconds, with pages cached for up to 90 seconds. This means a story can sit in the ‘wrong’ slot for minutes at a time.

The author’s key finding, drawn from crawling the /news and /news2 pages once a minute and comparing observed rankings against scores computed from the formula, is that the #1 spot is heavily engineered by penalties rather than raw popularity. Anomalies where a story ranks well below its score reveal penalty factors in action. Some are automatic: any title containing ‘NSA’ took a 0.4 penalty, and a long list of popular domains — Ars Technica, Business Insider, GitHub, Medium, Reddit, The Verge, YouTube and others — were docked 0.25 to 0.8, likely to counteract the vote inflation that comes from big-name sources being submitted many times over. The impact is steep: a 0.4 penalty makes each vote count as roughly a third of a vote and accelerates a story’s decline by about two-thirds, while a 0.1 penalty is near-fatal.

The most dramatic mechanism is the ‘controversy’ penalty, meant to suppress flamewars. Once a post has more comments than upvotes and crosses roughly 40 comments, its score is scaled down sharply (the published Arc code uses a squared votes-to-comments ratio; the author suspects the live version cubes it). The effect is abrupt — heavily discussed stories can drop dozens of places or vanish from the top 60 the instant they hit that comment threshold, explaining the familiar experience of a hot thread disappearing from the front page. The broader takeaway: HN’s front page is a curated, continuously penalized artifact, not an organic reflection of votes.