Upworthy has a good rationale for its workforce: The gut is the best way to find the kind of emotional stories that travel. “I think of it as user empathy,” says Critchfield. “Really being able to put yourself in the shoes of a user so you can make a product that appeals to them—even if that product is a headline.” Curators are encouraged to use their feelings when selecting items, the best of which come complete with personal stories. A much-touted example of the phenomenon is the video that tops the cancer section, an inspiring story of a young boy pursuing a singing career despite suffering from terminal cancer. After hearing he had passed away, Adam Mordecai, one of Upworthy’s curators, became fixated on the boy’s story, sharing his experience with staff in an email: “I googled more videos of his and fell upon the documentary. Then I hit play. Then I started crying and having flashbacks to my dad dying of pancreatic cancer and what one goes through when that happens. I wasn’t sure if I was biased but the whole thing seemed timely and wonderful.” He posted the short documentary with the title, “This Amazing Kid Just Died. What He Left Behind is Wondtacular.”

It’s one of Upworthy’s greatest hits, garnering 15 million pageviews and over 2.8 million Facebook shares. A single by the then-deceased boy briefly held the top sales slot on iTunes. While Mordecai watched the numbers rise, he was excited but not surprised. “On the second view I was still crying so I had a sense it was going to be a hit,” he wrote.

Upworthy’s techniques fit well with the research about human psychology and what arouses our emotions. In the largest study of its kind, Jonah Berger and Katherine L. Milkman, two marketing professors at the University of Pennsylvania’s Wharton School of Business, analyzed 7,000 articles appearing in The New York Times in 2008 to see which were the most shared. Controlling for other factors, like popularity of the author or prominence on the Times’ website, the two found that the most easily shared articles were ones that aroused feelings—in a positive way, such as awe, or a negative one, such as anger or anxiety. People share positive content for “self-presentation,” says Milkman. “I want to be associated with positive things; if it’s going to make you feel good it’s going to rub off and affect your opinion of me.” Milkman explains the anger-inducing content as a way for people to bond over “shared emotional experiences.” “When we see someone being fired, it’s something we can get angry over—it brings people together.” The two found the patterns consistent in a secondary test where they rejiggered the headlines of the articles to make them less intense and arousing. The subtler articles were less likely to arouse, and thus, less likely to be shared.

It’s a phenomenon that may even predate the internet: A group of Northeastern University professors analyzed 132 newspapers, published in the years before the Civil War, when it was routine for editors to lift and reprint stories from other papers—a primitive precursor to aggregating or retweeting. “Just like we do see with a lot of viral content today, [the most reprinted posts] often contain an inspirational or uplifting message,” says Ryan Cordell, a professor of English at Northeastern who led the study. One of the most popular was titled, “The following most touching fragment of a letter from a dying wife to her husband was found by him some months after her death.” If you add, “it will amaze you,” it might as well be an Upworthy post.

Upworthy’s “curiosity-gap” headlines, which entice by leaving out titillating details, emerge from research pioneered by George Loewenstein, a professor at Carnegie Mellon University in the late 1990s. In Loewenstein’s eyes, curiosity closely resembled a drive—like hunger or sex—that was triggered strongly by absence. Curiosity exists in the gap between “what one knows and what one wants to know,” he wrote in a seminal review published by Psychological Bulletin in 1996. “Such a focus on missing information is a necessary condition for curiosity,” Loewenstein continued. Missing information propels curiosity. It’s a simple point that, nonetheless, explains why mystery novels are engrossing—until one is told whodunnit—and why Upworthy’s headline’s are so eminently clickable.

‘I was talking to students a couple of days ago about the question, “How do you make something go viral?” And it really struck me that this is not a question for me, this is a question for God.’

Alexis Sobel Fitts is a senior writer at CJR. Follow her on Twitter at @fittsofalexis.