There’s something about the Internet that can bring out meanness in teenagers.

That is one finding of a study to be released Wednesday that reports that nine in 10 teenagers say they have witnessed cruelty by their peers on social networks.

For the vast majority of teens, Facebook is the social network of choice. They say that incidents of mean and cruel behavior are pervasive and cut across all ages and backgrounds on social networks, according to the study by the Pew Research Center’s Internet & American Life Project, which surveyed 800 children between the ages of 12 and 17.

The data on Internet experiences for young people is not all bad. Eight in 10 teenagers said they have developed positive feelings about themselves and forged better friendships on social networking sites, according to Pew.

Still, the prevalence of “mean” behavior — a term the center doesn’t define — raised concerns among child-safety advocates and parents who say adolescents may be subjecting themselves to unhealthy online environments.

“For teens, these are exciting and rewarding spaces. But the majority have seen a darker side,” said Amanda Lenhart, a co-author of Pew’s report, “Teens, Kindness and Cruelty on Social Network Sites.”

Of course, bad behavior among children has been around as long as youngsters have stolen milk money and scribbled insults on bathroom walls, experts say. And online bullying is not as common as what takes place on the schoolyard or in the hallway, Pew said.

But there is something about the ease of communication on the Internet that invites an abundance of commentary about peers, experts say. That escalates when people gang up on an individual. Of the teens who said they witnessed cruelty online, 21 percent said they joined in the harassment. Three out of 10 girls ages 12 to 13 said they have experienced mostly unkind treatment on social networks — the most negative response of any group of youth, according to the report.

Lenhart and other experts on social media said teenagers see themselves differently online than in the real world. Some assume a sort of “alter ego” on the Web, engaging in conversation with more bravado and taking more risks than they do when face to face with a peer, she said.

Peers can be particularly cruel on sites such as FormSpring that allow users to post comments anonymously, or on the comment boards of sites such as YouTube, according to experts.

Facebook — with 800 million global users — requires its members to use their real identities, which it thinks is one way to prevent anonymous bullying. It also allows users to block photos of and comments about themselves that they don’t like.

But that hasn’t stopped all bullying. And some experts worry that younger adolescents are particularly vulnerable. Rachel Simmons, an author and speaker on children and social media, said bullying occurs most in middle school, yet parents are often helping their children get online when they are younger than 13, the minimum age required for Facebook.

“The younger the kid, the meaner the peer group becomes, so this is an alert to parents that not every kid is ready for the independence of having their own social networking page,” Simmons said.