We analyzed nearly 3 million U.S.-based tweets and Facebook posts to examine what social media posts that go “viral” have in common. We specifically looked at political posts, including those by members of Congress or left- and right-leaning media outlets. The results were stark. The most viral posts tended to be about the opposing political party. Facebook posts and tweets about one’s political out-group (that is, the party opposed to one’s own) were shared about twice as often as those about one’s own political group. Posts about the opposition were almost exclusively negative. Furthermore, each additional word about the opposing group (for instance, in a Republican’s post, that might include “Democrat,” “leftist,” or “Biden”) in a social media post was associated with a 67 percent increase in social media shares. These effects were much larger than other factors known to increase sharing, such as negative words (“sad” or “mad”) or moral-emotional words (such as “evil” or “hate”). The increase occurred whether posts were from Republicans or Democrats or whether they were shared through either Facebook and Twitter.