Judith Donath is a Harvard Berkman Faculty Fellow and formerly director of the MIT Media Lab's Sociable Media Group. She is the author of The Social Machine: Designs for Living Online (MIT Press, 2014). The opinions expressed in this commentary are hers.

"Tim Kaine will ban the Catholic Church from the US if they don't change their stance on same sex marriage."

And while the election has come and gone, fake news stories continue to proliferate on social media. Indeed, they are often shared more than real news is.

Fake news is harmful. Many untrue stories are believed by the people who post them. Sometimes a story that was intended as satire circulates as fact. Others are deliberately deceptive. A report in the Denver Guardian headlined "FBI Agent Behind Clinton Email Leaks Found Dead in Murder-Suicide" was shared widely on Facebook, with comments such as "The Clinton Crime Syndicate has more lies and blood on their hands than any previous gangster in history." This wholly fabricated story appeared on November 5, a few days before the election. (This story, along with the rest of the Denver Guardian, has disappeared in the aftermath of recent scrutiny of fake news. ) As Martin Baron, the executive editor of the Washington Post. said , "If you have a society where people can't agree on basic facts, how do you have a functioning democracy?"

Combating the spread of false stories is important but difficult. The obvious solution -- fact-checking -- is ineffective : Many fake news consumers are suspicious of mainstream media and dismiss as partisan and deceptive any statistics or evidence that counter their beliefs. Worse, as we'll discuss below, demonstrating the factual flaws in a story can actually increase its social value. To discourage the spread of fake news, we need to better understand why sharing these stories is so appealing.

Partisanship is part of it. People like and share news that conforms to their existing beliefs, and fake news stories are often strongly biased. Of course, people also share real news when it agrees with their views. One would think that given a real news story or a fake one, both conforming to one's beliefs, that the real news would be preferable. But fake news thrives online, frequently surpassing real news in its reach.

Recognizing the social function of news sharing is the key to understanding this seeming irrational behavior. In the world of social media, of Facebook and Twitter, news is shared not just to inform or even to persuade. It is used as a marker of identity, a way to proclaim your affinity with a particular community.

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For better or worse, we humans have a fundamental tendency to divide into groups. We bond with our own in-group and set ourselves apart from the others. Face to face, we signal this social identity in the clothes we wear, the foods we eat and more. But in the online world of information, material markers count for less. Instead, news sharing has become a prominent identity signal: We proclaim our affiliations by posting links to articles that reflect our groups' taste and beliefs.

Legitimate stories can, of course, serve this role. But for the purpose of marking social identity, fake stories can be even more powerful.

Posting any story, real or fake, that conforms to your community's viewpoint bolsters your ties with them. Even if it is false, you have still demonstrated your shared values.

The key is the difference in how outsiders respond to your posting of a fake vs. real news story. If the news is real, outsiders who recognize it as such may disagree with it, but posting it does not reflect badly on you, and it may provide a common ground for argument and discussion.

If, however, the news you post is fake, outsiders are more likely to be outraged. If you stand by it tenaciously, they may call you a fool or a liar. This infuriated response makes posting fake news a convincing signal of your allegiance to your in-group. By demonstrating that you are willing to sacrifice your ties to and your reputation among the outsiders, you prove the authenticity of your commitment.

Furthermore, stoking conflict with outsiders strengthens the in-group's cohesion. This, too, raises the status of the person who posts a hotly contested story, especially when tensions are high.

These dynamics explain why fact checking can be counterproductive . When a story that a community believes is proved fake by outsiders, belief in it becomes an article of faith, a litmus test of one's adherence to that community's idiosyncratic worldview. The two sides will perceive that they have no common ground or understanding of truth, and the story becomes an even more potent signal of identity and catalyst of discord.

This is why when signaling identity is the reason for sharing news, fake news is hard to uproot. It proves the poster's commitment to the community by demonstrating willingness to sacrifice outside relationships.

Recognizing this helps us craft more productive responses:

First, follow the now-old adage, "Don't feed the trolls." If someone posts a fake story, and you think they have simply been duped, certainly it is useful to point out the error with a more reliable source. Please do graciously. No one likes to be publicly humiliated. Sometimes a private message is better. But if you think the posting is really about proclaiming identity, ignore it. Don't amplify its value by arguing. And if you must say something, here a private message is really better -- you can convey your disapproval without providing the public display of discord that just strengthens their signal.

Second, help promote a culture that reveres veracity. Check your sources before you post anything. Support newspapers and other organizations that do good, reliable reporting. Discourage people in your own community when they promote stories that feel good to you, but are, alas, untrue.

Third, appreciate humor. Like fake news, jokes and satire are markers of identity -- funny to insiders, and often incomprehensible or offensive to outsiders. They may be tasteless, they may be divisive but unlike fake news, they are not an assault on truth.

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Information can bring people together or drive them apart. Sharing false information, or fake news, is divisive. It's about claiming a separate territory, with its own rules and logic. Sharing true information is, ideally, the opposite. It's about unifying people, not only rallying the ones who agree, but also persuading the ones who do not; it provides a common ground.

Ultimately, to remove the appeal of fake news, people need to value debate and discussion. They need to value reaching across to different communities, to discuss and debate; they need to choose not to build walls against the Other, but to engage and persuade.