When Jonah Berger was a graduate student at Stanford, in the early aughts, he would make a habit of reading page A2 of the Wall Street Journal, which included a list of the five most-read and the five most-shared articles of the day. “I’d go down to the library and surreptitiously cut out that page,” he recalls. “I noticed that what was read and what was shared was often different, and I wondered why that would be.” What was it about a piece of content—an article, a picture, a video—that took it from simply interesting to interesting and shareable? What pushes someone not only to read a story but to pass it on?

The question predates Berger’s interest in it by centuries. In 350 B.C., Aristotle was already wondering what could make content—in his case, a speech—persuasive and memorable, so that its ideas would pass from person to person. The answer, he argued, was three principles: ethos, pathos, and logos. Content should have an ethical appeal, an emotional appeal, or a logical appeal. A rhetorician strong on all three was likely to leave behind a persuaded audience. Replace rhetorician with online content creator, and Aristotle’s insights seem entirely modern. Ethics, emotion, logic—it’s credible and worthy, it appeals to me, it makes sense. If you look at the last few links you shared on your Facebook page or Twitter stream, or the last article you e-mailed or recommended to a friend, chances are good that they’ll fit into those categories.

Aristotle’s diagnosis was broad, and tweets, of course, differ from Greek oratory. So Berger, who is now a professor of marketing at the University of Pennsylvania’s Wharton School, worked with another Penn professor, Katherine Milkman, to put his interest in content-sharing to an empirical test. Together, they analyzed just under seven thousand articles that had appeared in the Times in 2008, between August 30th and November 30th, to try to determine what distinguished pieces that made the most-emailed list. After controlling for online and print placement, timing, author popularity, author gender, length, and complexity, Berger and Milkman found that two features predictably determined an article’s success: how positive its message was and how much it excited its reader. Articles that evoked some emotion did better than those that evoked none—an article with the headline “BABY POLAR BEAR’S FEEDER DIES” did better than “TEAMS PREPARE FOR THE COURTSHIP OF LEBRON JAMES.” But happy emotions (“WIDE-EYED NEW ARRIVALS FALLING IN LOVE WITH THE CITY”) outperformed sad ones (“WEB RUMORS TIED TO KOREAN ACTRESS’S SUICIDE”).

Just how arousing each emotion was also made a difference. If an article made readers extremely angry or highly anxious—stories about a political scandal or new risk factor for cancer, for example—they became just as likely to share it as they would a feel-good story about a cuddly panda. (In this particular study, certain pieces were coded as eliciting arousing emotions; in a follow-up, arousal was further measured physiologically.)

Berger and Milkman went on to test their findings in a more controlled setting, presenting students with content and observing their propensity to pass it along. Here, too, they found the same patterns. Amusing stories that had been chosen specifically because they were positive and arousing were shared more frequently than less amusing ones. Anger-inducing stories were shared more than moderate takes on the same events. When the researchers manipulated the framing of a story to be either negative (a person is injured) or positive (an injured person is “trying to be better again”), they found that the positive framing made a piece far more popular. The findings have since been replicated by several independent research teams, who have found that videos that shock or inspire are more likely to be shared on Facebook and more likely to gain viral traction.

Positivity and arousal go a long way toward explaining the success of Web sites like Upworthy, which started in 2012 and is known for using headlines designed to make you laugh, cry, or feel righteous anger (for example, on the site right now, “A Hilarious Stand-Up Routine About How Commercials for Black People Actually Sound” and “The Struggles of Being a Woman in a Male-Dominated Field Summed Up in a Short Comic”). Even the site’s tearjerker content has a positive message: “Watch a Teenager Bring His Class to Tears Just by Saying a Few Words,” reads one. Despite launching less than two years ago, the site has steadily climbed the ranks of Internet popularity, ranking third in a December rating of Facebook shares, right behind BuzzFeed and the Huffington Post. Its posts are like the infamous cat videos on YouTube—funny, positive, and arousing—but taken to a new level. Still, as Berger points out, “There are lots of cat videos that don’t get shared”—and lots of would-be Upworthys that never quite make it. So what characterizes the ones that do?

Since his initial foray into the nature of sharing, Berger has gone on to research and test a variety of viral-promoting factors, which he details in his new book, “Contagious: Why Things Catch On.” Almost ten years in, he feels he’s discovered a formula of sorts: as sites like Upworthy or BuzzFeed would likely put it, The Six Things You Need to Know to Make Your Voice Heard. While emotion and arousal still top the list, a few additional factors seem to make a big difference. First, he told me, you need to create social currency—something that makes people feel that they’re not only smart but in the know. “Memes like LOLcats, I think, are a perfect example of social currency, an insider culture or handshake,” Berger told me. “Your ability to pass it on and riff on it shows that you understand. It’s the ultimate, subtle insider signal: I know without yelling that I know. When your mom sees an LOLcat, she has no idea what it is.” When Upworthy first started, not everyone knew what it was, and the videos seemed fresh. Now they are being derided as link bait and mocked. Other sites, including the Washington Post, are copying their formula.

The presence of a memory-inducing trigger is also important. We share what we’re thinking about—and we think about the things we can remember. This facet of sharing helps explain the appeal of list-type stories (which I wrote about in detail last month), as well as stories that stick in your mind because they are bizarre. Lists also get shared because of another feature that Berger often finds successful: the promise of practical value. “We see top-ten lists on Buzzfeed and the like all the time,” he notes. “It allows people to feel like there’s a nice packet of useful information that they can share with others.” We want to feel smart and for others to perceive us as smart and helpful, so we craft our online image accordingly.

A final predictor of success is the quality of the story itself. “People love stories. The more you see your story as part of a broader narrative, the better,” Berger says. Some cat lists are better than others, and some descriptions of crying teen-agers are more immediately poignant; the best underlying story, regardless of its trappings, will come out on top. That, in fact, is what the Upworthy editors have argued in response to their critics: the headlines may seem like link bait, but the stories, the curators promise, are worthwhile. “Coming up with catchy, curiosity-inducing headlines wasn’t the reason Upworthy had those 87 million visitors,” they write. “It was because millions of members of the Upworthy community watched the videos we curated and found them important, compelling, and worth sharing with their friends.”