Sarah Wells makes an unlikely cyber-vigilante. But the middle-aged mother in Virginia was outraged when she read a Saint Charles Journal article on Megan Meier, a 13-year-old Missouri girl driven to suicide by relentless online bullying. The fact that the bullying appeared to be instigated by the mother of one of Megan's friends through a fake MySpace account enraged Wells all the more.

When Wells learned that the woman had filed a police report against the dead girl's father – who had destroyed the woman's foosball table in anger and grief – she resolved to take matters into her own hands. The newspaper account didn't identify the perpetrator of the deadly hoax by name, but included enough detail to track her down through online property-tax records. With a few minutes of sleuthing, Wells identified the woman as Lori Drew, of O'Fallon, Missouri. After confirming it with someone in the O'Fallon area who she says was "in a position to know," she posted the name to her blog.

"It was outrageous enough what she had done, but dragging (Megan's father) into the courts, calling the police and bringing the charges against the family whose daughter had suffered a great deal because of her ... for her to do that, it was like, OK, it's coming back to you," Wells says.

Experts say the firestorm that followed illustrates what happens when the social imperative to punish those in a community who violate social norms plays out over the internet. The impulse is human nature, say experts, and few can imagine an offense more egregious than a trusted adult preying on the emotions of a vulnerable child. Shunning wrongdoers, especially in the absence of legal redress, helps maintain order and preserve a community's moral sense of right – think church excommunications and the Amish tradition of Meidung.

But the drive for social shaming – to right a wrong and restore social balance – can run amok and create paradoxical consequences, especially on the internet where people instigate mobs in ways they wouldn't do offline.

"Internet shaming is done by people who want actually to enforce norms and to make people and society more orderly," says Daniel Solove, professor of law at George Washington University and author of The Future of Reputation: Gossip, Rumor and Privacy on the Internet. "The problem is that internet shaming actually destroys social control and makes things more anarchic, and it becomes very hard to regulate and stop it."

Wells published only Lori Drew's name, but her readers and other bloggers followed by finding and posting her husband's name, the family's address and phone number, a cellphone number, the name of the family's advertising company, and the names and phone numbers of clients with whom they worked.

As Megan's story raced across the internet and was picked up by national media, more people joined in the cause, including Wired News readers.

Many were outraged that Drew, a middle-aged woman with a teen daughter of her own, fabricated the MySpace identity of a 16-year-old named "Josh Evans" to woo the emotionally fragile Megan, in order to learn what the girl was saying about her own daughter. Once Megan was lured in, "Josh" turned on her, heaping verbal abuse until the teen went over the edge.

In retaliation, readers called Drew's advertising clients to urge them to withdraw their business from her. But it wasn't long before there were death threats, a brick through a window and calls to set the Drews' house on fire. Police have reportedly increased patrols to guard the family against attack. A peaceful protest is scheduled to occur on the family's street this weekend.

Solove isn't surprised. He points to a story two years ago about a man whose camera phone was stolen from his unlocked car. The apparent thief took pictures of himself and friends, which automatically uploaded to the camera owner's Sprint web account.

The owner discovered the thief's name and posted it online with the photos. Netizens superimposed the thief's picture onto pornographic images and posted racist remarks about him. After a Wired News story revealed that the alleged thief was 16 and that his mother claimed she'd bought the camera from a street seller, Solove says the owner regretted the fury he'd unleashed and asked the online mob to back off. Instead, the mob turned on him.

"When you have a bunch of people together all trying to achieve the same goal, the whole starts to take on a mind of its own," Solove says, "and people start to act in extreme ways that they might not otherwise act."

It's no surprise that many people who posted Drew's name and personal details online have done so under a cloak of anonymity, with the exception of Wells and a few others.

"People don't mind doing (this kind of thing) as long as it doesn’t cost them anything, as long as there's very little risk of retribution," says Robert Kurzban, professor of psychology at the University of Pennsylvania and the author of works on social exclusion and stigmatization. "But when people actually have to pay a cost to punish other people, they prefer not to do that."

Wells says she doesn't condone calls for violence and has removed any that appeared on her blog. There are many who agree that the punishment fits the crime in this case. But online vigilantes have been known to react just as strongly to lesser grievances.

Two years ago a college girl in South Korea was harassed after her tiny dog defecated on the floor of a subway car, and she ignored passengers' requests to clean it up. Someone on the train snapped her photo and posted it online. She was quickly dubbed Dog Poop Girl, and within days a cyberposse had discovered her name and was digging up information about her and her family. The public humiliation reportedly led her to withdraw from her university, and the pictures of her and the feces are still online today.

Wells acknowledges that such a case can make it seem as if the internet is "a perpetual outrage machine," where people are simply looking for the next injustice to get worked up about. But she feels the reaction to Dog Poop Girl was unique to Korean culture and wouldn't have caused the same stir here, although Americans were as amused by the story as others around the world. Still, she thinks the circumstances of Megan's story dictated the depth of the reaction here.

The fact that it involved an adult targeting a child, and that Megan's father – and not Lori Drew – was facing criminal charges, made it a clear-cut cause for cybermob outrage.

Kurzban agrees and disagrees. He says it's not really the details of a particular social violation that determine the strength of a community's reaction to it, but the degree to which a community agrees in its perception of the violation.

In other words, if many people reading the same facts didn't believe that Lori Drew violated a social norm, then the response against her would have been more muted. Indeed, a minority of people on the blogs said that Megan should have ignored the insulting messages, and that her parents should have monitored her online activity better. But they were swiftly shouted down by the mob.

In this way, the stories of Poop Girl and Megan are wildly different on the surface, yet essentially the same, Kurzban says, since both involved actions that the majority of people in their respective communities reacted to in the same way.

For her part, Wells thinks Drew deserves to be a pariah.

"I think this should follow her wherever she goes," Wells says. "There should be pressure on her to be sorry and to do something to make it better."

Wells herself felt the capricious hand of internet justice on Tuesday, when another blogger condemned her as a "vigilante" and posted her address and phone number online.