Did you know that Hillary Clinton sold weapons to ISIS? Or that Mike Pence called Michelle Obama “the most vulgar First Lady we’ve ever had”? No, you didn’t know these things. You couldn’t know them, because these claims are false. But many American voters believed them.

One of the most distinctive features of the 2016 campaign was the rise of “fake news,” factually false claims circulated on social media, usually via channels of partisan camaraderie. Media analysts and social scientists are still debating what role fake news played in Trump’s victory. But whether or not it drove the outcome, fake news certainly affected the choices of some individual voters.

Why were people willing to believe easily dis-confirmable, often ridiculous, stories? In this paper, I will suggest the following answer: people believe fake news because they acquire it through social media sharing, which is a peculiar sort of testimony. Social media sharing has features that reduce audience willingness to think critically or check facts. This effect is amplified when the testifier and audience share a partisan orientation. Shared partisan affiliation encourages testimony recipients to grant more credibility to testifiers than would otherwise be warranted.

So far these points may seem familiar. But the deeper aim of this paper is to normatively evaluate how fake news is transmitted and here my answer may be less expected. I will argue that fake news transmission is often individually reasonable. That is, individual people typically act reasonably when they grant greater credibility to fellow partisans, even if this sometimes leads to the acquisition of false beliefs.

This normative analysis generates a further claim about the remedy for fake news: it will not be solved by focusing on individual epistemic virtue. Rather, we must treat fake news as a tragedy of the epistemic commons, and its solution as a coordination problem. Fake news exploits otherwise reasonable practices of information transmission. Ending it will require institutional change.