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We ran an international competition to design new social media algorithms, then used a browser extension to intercept users' feeds and re-ranked them in real time. Each algorithm could re-order, add, and/or delete posts and comments on Facebook, X/Twitter, and Reddit. 2/

The result: affective polarization dropped by an average of -0.03 SD (p<0.05), a 1.5 point shift on the 100-point feeling thermometer. By comparison, US polarization has risen ~0.6 points/year, so this is like reversing roughly 2.5 years of increase. 3/

The two strongest algorithms: Uprank Bridging + Downrank Toxic, and Add News. Notably, upranking constructive content alone wasn't enough, we also needed to reduce the prominence of outrage and toxicity. But merely adding diverse news, without any re-ranking, also worked! 4/

Interesting work! I appreciate the effort to make RSs less manipulative. Lower UX is not necessarily bad if it means users see relevant content outside their bubble. The real concern is whether bridging ranking ends up tone-policing by downranking truthful, justified but emotionally forceful speech.