A new type of cyberbully known as a griefer is overtaking other forms of aggression on the Internet and in the world of online gaming, a health researcher warned.

Unlike traditional Internet bullies who work through instant messages and cell phones, griefers lurk on online multi-player videogames, harassing their victim by bullying, tormenting or thwarting other players in the game.

Griefers trap a victim and they will attack the victim through the game, Dr. Sally Black, an assistant professor in health services at Saint Joseph University in Philadelphia, said in an interview.

There's torment, humiliation and belittling, she added.

The world of online multi-player gaming has grown increasingly popular as gamers adopt new roles and personalities in 'massive multiplayer online role-playing games,' or MMORPGs such as World of Warcraft, RuneScape and Everquest.

I personally believe these are addictive games, because you have kids playing them for many, many hours a day, and putting their lives on hold basically to play some of these, like World of Warcraft and Warhammer, said Black.

They have a strong violent nature and a lot of sexuality in them, she added.

Children as young as 12 are attracted to the games and play them for hours each day. Black said the long-term effects may include aggressiveness, anti-social behavior and acting out.

The American Medical Association announced last month that it may classify videogame use as an addictive behavior. It later dropped the issue but has called for more research on the effects of video game and Internet use, as well as a review of the video game ratings system.

Previous studies have not directly linked children's exposure to media violence with increases in aggression and bad behavior.

Black's research on children and bullying includes a case study, currently in review, about a 12-year-old old boy whose behavior changed after he began playing a MMORPG.

She said she has witnessed changes in her students, her children and their friends and believes industry could start to control marketing towards children, much like tobacco products and alcohol.

It will give parents their power back, she said. Because right now these marketers are going straight after the young ones to try to get them addicted at an early age, and it's like a parent trying to fight tobacco and alcohol. It's very difficult, she said.