Most people will have heard a version of the story about the rat served instead of chicken. In one of the most popular accounts, a woman is eating Kentucky Fried Chicken as she watches TV at home when she notices it tastes odd. Turning up the lights, she sees it isn’t chicken, but a rat with extra-crispy coating. A kid working at KFC had fried it as a prank. Or so the story goes. The Kentucky Fried Rat yarn has become one of the more widely known and persistent urban legends. By 1980, more than a hundred versions of the tale were in circulation in the U.S. alone.

The combination of eating something inappropriate and unsuitable contact with animals is irresistible. The yuckiness of a story contributes to its appeal. The persistence of the Kentucky Fried Rat story helps to explain why some material spreads rapidly online. At the core is the emotion of disgust. It may seem odd that such a negative emotion would be one that people want to share. But disgust is a surprisingly powerful motivation for sharing.

Researchers Chip Heath, Chris Bell, and Emily Sternberg decided to test how far people would go in passing on disgusting anecdotes, no matter how far-fetched. They chose twelve disgusting urban legends and altered them to be either more or less revolting. In one example, the story of a man finding a dead rat at the bottom of his glass of soda was made more nauseating by having him ingest bits of the animal. The less repulsive version had the man notice a bad smell and spot the rat before drinking the soda. The results showed that people were far more likely to share the most disgusting account of a story, even if the tale was truly repulsive. The researchers found similar results when they looked at the most popular stories on websites that specialized in urban legends. The more disgusting a story, the more likely it was to be distributed online.

There is a science to grossness. Psychologists who have studied why some things, actions and people elicit feelings of repulsion point to several key elements. They come together to provoke a reaction of disgust. There is the inappropriate use of food, such as the Subway employee who shared a picture of himself with his penis on the restaurant’s sandwich bread on Instagram. Sometimes bodily products are involved. Or animals. Poor hygiene, sex, and death can also elicit disgust. Finally there is deviant behavior that goes beyond societal morals and practices. Disgust is an emotion that started off as a way to avoid harm to the body and has expanded to become a way to avoid harm to the soul. It explains the gut reaction to moral violations, such as when the NRA sent a pro-gun tweet at the time of the Aurora massacre, or when Home Depot posted a photo of two African Americans and a man in a monkey suit with the caption, “which drummer is not like the others?”

Disgust provokes both an intense moral and physiological reaction. Watching a video of a young woman eating a live praying mantis may leave a viewer feeling like throwing up. Yet it was also one of the videos that students taking part in a psychology experiment were most likely to forward to others. It may seem odd that people would react by turning to friends and saying, “OMG! You have to see this.” But according to researchers in Belgium and the Netherlands, the more intense an experience, the more we want to share it.

The team found that students talked the most about a clip from the notorious cult film Faces of Death. It was deliberately chosen because it usually prompts a visceral response from viewers. The excerpt showed people at a restaurant smashing a monkey’s skull and eating the brain. The scene looks real enough, but it was faked using Styrofoam hammers and cauliflower for the brains. After watching the clip, students were eager to talk about it with others and compare their feelings. Sharing disgust provides an emotional release. It also is a way of confirming with others the boundaries of what is socially acceptable.

The tale of Rose the goat

I remember the first time I realized just how much emotions could affect how news travels online. I had been the daily news editor at the BBC News website, deciding on the mix of stories on the front page. It is the most coveted real estate of the site, as millions visit the front page every day. As the daily news editor, my job was to make sure visitors to the site saw the main news of the day, as well as the amusing or entertaining. One day in September a few years back, my colleagues noticed something odd happening on the site.

A story published seven months earlier was suddenly the most read article. It continued to be popular for days, even though it was buried within the site in the Africa section. Adam Curtis, then the World editor at the BBC News website, started investigating why a 185-word story that was months old had unexpectedly become a hit with readers. “It had not been re-published, re-written or revised,” he wrote on the BBC’s editors’ blog. “So how is it that upwards of 100,000 people a day were passing it on to their friends and acquaintances?”

The clue lay in the headline: Sudan man forced to marry goat. The article told the story of a Sudanese man who had been caught having “improper relations with the beast.” He was ordered to pay a marriage dowry of $50 to the goat’s owner. “We have given him the goat, and as far as we know they are still together,” the owner of the goat was quoted as saying. The tale of one man and his goat “wife” attracted some attention at the time of publication in February 2006, but soon disappeared into the archive. When it resurfaced in September, Curtis wasn’t sure whether people were really reading the story “some crazed animal lover has been repeatedly hitting the site with fake requests.”

This was no scam. During that morning in September, readers from across the world had stumbled across the story and emailed the link their friends. The more people read it, the more they told others about it. People in the U.S., Australia, France, India, and more — even in the tiny Grand Duchy of Luxembourg — could not get enough of the story. Other news outlets reported on the travails of Rose the goat.

Rose came to an unfortunate end in May 2007. Eating scraps on the streets of Juba in southern Sudan, she died after choking on a plastic bag. The BBC carried a mock obituary, noting how, during her short marriage, “friends would joke about how she had reached the end of her tether, about whether the couple would have any kids, and if they did, whether they would employ a nanny.”

By the time of her death, Rose had become an Internet phenomenon. Even a year after publication, the original article continued to be among the BBC’s most emailed stories. It became one of the most popular stories published since the site’s launch in 1997. The goat’s tale was one of the first major examples of the impact that sharing can have on the news, taking an obscure story from a remote part of the world and propelling it to international notoriety. The short news item was surprising and unusual, but it also had several of the elements that elicit disgust: animals, sex, and moral violations. No wonder it was so widely read.

The story of Rose highlights how, in the marketplace of ideas online, some stories spread due to a gut reaction, not on the basis of its whether it is true, useful, or entertaining. It was an early sign of the power of emotions to determine the sort of news that was fit to share online. When we email a story from The New York Times, we are not just judging it on whether it is well written or whether we believe everything it says. We react to the amusing, the inspiring, the positive. More surprising is the desire to share stories, photos, or videos that arouse negative emotions of anger and disgust.

Emotions have always played a role in how information spreads. Consider the endurance of urban legends that are just too far-fetched to be true. But emotional engagement becomes even more significant in an online world. Social networks make it much easier not only to reach out to a large group of friends, but to see how they react. When we see others doing something, we tend to ape the same choices. The result is a feedback loop that validates how we feel about a particular story and reinforces our sense of belonging. There are consequences when our social circles become our editorial filters, privileging the sensational over the important, and the amusing over the earnest.