That is particularly true for children with more exposure to media violence, including television shows. One effect: Children will view violence as an acceptable way to settle conflicts, according to a 2000 congressional report that cites data from six medical associations.

In Chicago, the transit authority pulled ads for the new game because a previous game, "Grand Theft Auto: San Andreas," had drawn complaints from Gov. Rod Blagojevich in 2004. The CTA switched ad agencies, a representative said, and the new agency was unaware of previous complaints.

Illinois enacted a law backed by Blagojevich that banned selling violent videogames to minors, punishable by a year in jail, but a federal judge in 2005 ruled it unconstitutional.

Dan Rubin, 22, sold videogames for two big-box retailers when the "Grand Theft" games started getting popular. An avid gamer, he said the violence in "Grand Theft Auto" is over the top, but don't blame kids for wanting to play.

"Parents are not aware of how violent these games are," he said, noting that salespeople have a responsibility to better inform parents.

The 2007 study from MediaWise found that 1 in 3 retailers fail to educate customers about ratings on videogames.

"Grand Theft Auto IV" is rated M, intended for mature audiences over 17, because of blood, intense violence, partial nudity, strong language and use of drugs and alcohol.

"There are industry projections that 'Grand Theft Auto IV' will do more than $400 million in sales" in its first week, said Wes Sand, senior vice president for retailer Game Crazy. The game sells for $60 and plays on the Xbox 360 and PlayStation 3.