Half of all YouTube videos have fewer than 500 views, but a tiny fraction of the 100 hours of video uploaded to YouTube every minute garner millions of hits, turning amateur filmmakers into stars or launching viral marketing campaigns. Recently, a video of a New York woman getting catcalled posted by the advocacy group Hollaback! became an Internet sensation. A video of men trying to lure a drunk woman home went viral last week, before it was revealed to be a hoax. A few days ago, a clip of a man singing “Blackbird” to his dying son started making the rounds.

These three videos all seem pretty different, but researchers have begun to spin out some underlying commonalities. “Emotion is a big driver of what goes viral,” says Jonah Berger, a professor at Wharton and the author of Contagious: Why Things Catch On. “Whether something pulls on our heartstrings, makes us angry, or provokes controversy, the more we care, the more we share.” “These three recent examples absolutely evoke high arousal emotions,” says Karen Nelson-Field, a marketing researcher at the University of South Australia. “In this case, these are negative emotions, [but] … it is the intensity that drives the most sharing.”

In a 2011 paper in Psychological Science, Berger argues the most important factor in transmission is physiological arousal: Emotions that increase arousal, like anxiety and amusement, will be more effective than low-arousal emotions like sadness and contentment. To test this, Berger devised an experiment in which he showed 93 students either a neutral film clip or a clip that had been proven to evoke a specific emotion—either a highly arousing one or a less arousing one. The students were then given an unemotional article and video, and asked the likelihood, on a scale of 1 to 7, that they would share them. The ones who’d been primed to feel highly aroused were more likely to say they’d pass it along—regardless of whether the emotion they were feeling was positive or negative.

In a second experiment, Berger raised participants’ arousal through a different means: exercise. Berger had half his subjects sit still, and half jog in place for a minute. Then, they read a neutral news story and were told they could email it to whoever they wanted. Exercise-induced arousal turned out to boost sharing, too: Of those who jogged in place, 75 percent chose to forward the article—compared to just 33 percent of those who sat still.

Intensity of emotion isn’t the only factor affecting shareability. Positive videos may be slightly more likely to be shared, at least on Facebook. In a 2013 experiment in the Australasian Marketing Journal, Nelson-Field and a couple of colleagues selected a wide range of 800 short videos; the number of Facebook shares for each video ranged from as few as three per day to as many as 109,000 a day. Nelson-Field then recruited 28 people to watch 50 videos each and, immediately after each one, select which of 16 emotions described their state while watching the video; each emotion was coded as either “high arousal” or “low arousal” and “positive” or “negative.” (“High arousal,” positive emotions, for instance, include inspiration, astonishment, and hilarity; on the other end of the spectrum, low-arousal, negative emotions include discomfort, boredom, and frustration.) Nelson-Field found that videos that elicit “high arousal” emotions were shared about twice as often as those eliciting “low” emotional response, and positive videos were shared 30 percent more than negative ones.

Another key element may be surprise. In a 2007 paper, Angela Dobele, a marketing researcher at the Royal Melbourne Institute of Technology in Australia, analyzed the emotional content of nine successful viral marketing campaigns, including both non-profits like “Rock the Vote” and commercials for brands like Honda and Motorola. Dobele had 20 people view each campaign, and then answer survey questions designed to gauge their level of six emotions: surprise, joy, sadness, anger, fear, and disgust. Surprise was only emotion that was present in all nine campaigns. Joy and sadness were tied for runner-up. Dobele also noticed a gender difference: Men were more likely than women to pass on a message evoking fear or disgust.

The decision to share a video can also “be driven by desire for self-authentication, in particular the desire to express one's identity or express membership of a collective,” says Dobele. Take the Hollaback! video. “Sharing the video is a way of identifying with her.… This campaign offers the basis for an ongoing conversation with likeminded others, ultimately allowing them to reaffirm collective bonds.” There will always be a place for Madison Avenue creatives, but the secret to online success may lie with scientists.