YouTube says it will no longer be recommending videos claiming September 11 was an inside job or that the Earth is flat, not round.
"We'll begin reducing recommendations of borderline content and content that could misinform users in harmful ways," the Google-owned site said in a blog post.
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Videos that don't break the site's guidelines will still be there for viewing, and will still come up in searches if that's what you're looking for, but they won't appear as suggestions alongside unrelated content.
"We think this change strikes a balance between maintaining a platform for free speech and living up to our responsibility to users," YouTube said, claiming fewer than 1 percent of the site's content would be affected by the change.
Initially the change will only affected viewers in the US, before being rolled out around the world - assuming it's a globe.
Sites hosting user-generated content like YouTube, Facebook, Twitter and Tumblr have been looking at ways to curb the growth of fringe content in recent years, cracking down on fake news (Facebook, Twitter) and adult content (Tumblr).