Most video games rated "M" for mature audiences fail to disclose violent content on their labels and can easily fall into the hands of children, according to a study released on Monday.

"Parents should not interpret the absence of a content descriptor to mean the absence of content," said study author, Kimberly Thompson of the Harvard School of Public Health.

"Parents and physicians should recognize that popular M-rated games contain a wide range of unlabeled content and may expose children and adolescents to messages that may negatively influence their perceptions, attitudes and behaviors," Thompson wrote in the Archives of Pediatrics & Adolescent Medicine.

Researchers observed more than 6,000 character deaths, or 145 an hour, when they played 36 computer games randomly selected from 147 rated for players aged 17 or older.

They included popular games like "Grand Theft Auto: Vice City" and "Resident Evil 3: Nemesis" which are played on video game consoles.

In the last five years, 8- to 18-year-olds have nearly doubled the average amount of time spent playing such games from 26 to 49 minutes a day.

The study found around 80 percent of M-rated games included sex, violence, profanity, gambling or drug and alcohol use, none of which was described on the labels.

Previous studies have exposed how easy it is for children to get their hands on such games.

For example, the Federal Trade Commission reported in 2002 that 40 percent of M-rated games were purchased for children younger than 17, while 69 percent of unaccompanied children aged 13 to 16 were able to buy games intended for adults.

Other articles in the journal focused on the effects on children of watching TV violence.

Activity diaries kept by caregivers recording two days in the lives of 3,500 children aged 6 to 12 found that for each hour of violent TV watched, the children spent 20 to 25 fewer minutes with friends.

Researcher David Bickham of Harvard School of Public Health described the phenomenon as "a downward spiral from violent television viewing to aggressive behavior to social isolation to viewing more violent television.

"Exposure to violent television could, therefore, be the catalyst for a cyclical system leading toward an aggressive, socially isolated lifestyle," he wrote.