CHICAGO -- A new study found that most teen-rated video games contain violence, and many have content that isn't listed on the label -- including sexual themes, alcohol use and profanity.

The study, appearing in Wednesday's issue of the Journal of the American Medical Association, analyzed a random sample of 81 video games rated T -- for teens -- for violence, sexual themes, profanity, substance use and gambling. They then compared the content of the games to the industry's ratings.

In 48 percent of the games, researchers from Harvard University and Children's Hospital Boston found inappropriate content that was not noted on the game box.

The researchers found that while the presence of a "content descriptor" on the game box provided a good indication of that type of content in the game, the absence of one didn't mean the absence of content that might concern parents.

The study found that 90 percent of games rewarded or required the player to injure characters, and 69 percent of the games rewarded or required the player to kill. The researchers also found that video games were significantly more likely to depict women partially nude or engaged in sexual behaviors than men.

The researchers said parents need to be more vigilant about the games their children play, while the industry needs to be more diligent in its ratings of games suitability for children.

Kevin Haninger, the study's lead researcher, said the industry's rating system needs to be clarified and include more complete information, or else consumers will lose confidence in the system.

"All media educate children, whether intended or not," said Kimberly Thompson, of the Center for Media and Child Health at Children's Hospital Boston. "By using the (Entertainment Software Rating Board) rating information and actively engaging in their children's experiences with video games, parents can make the best choices and promote their children's healthy development."