The level of gun violence in top-selling films rated PG-13 have tripled since the rating debuted in the mid-1980s, and it now exceeds the presence of gun violence in the most popular R-rated films, according to a study the New York Times reported.

Researchers at The Ohio State University, VU University - a Dutch school - and the Annenberg Public Policy Center at the University of Pennsylvania found encounters with guns occur, on average, more than twice an hour in popular movies in both rating categories.

Episodes of gun violence in PG-13 rated films has been on the rise since it debuted in 1985.

"[PG-13 rated movies] had the same level of gun violence of PG and G movies - so basically none - and then it skyrockets," Ohio State's Brad Bushman, who conceived the study with Anneberg's Dan Romer, told U.S. News And World Report. "Since 2012 gun violence in PG-13 movies is significantly higher than gun violence R rated movies ... That's a big problem for several reasons: one is PG-13 movies are especially attractive to youth."

PG-13 movies now surpasses the violence in R-rated films, which "are technically not open to young viewers unless they are accompanied by an adult," according to the Times.

However, G , PG, and R-rated films exhibited no trends over this period, a stark contrast to the PG-13 rated film findings.

The study, prompted by December's Sandy Hook school shootings, examined 945 movies. Researchers counted the appearances of gun violence in each five-minute segment of a sampling of films that ranked among the top 30 at the domestic box office from 1950 to 2012.

They found Gun violence in American films had more than doubled since 1950 - the year "Cinderella," "Sunset Boulevard," and "Annie Get Your Gun," were among the best-selling films.

Romer told the Times that by focusing only on top-selling movies each year, the study said as much about audience behavior as about the "level of violence in Hollywood's output."

"Violence sells," he said. "We recognize that, and the movie industry realizes it."

A spokeswoman for the Motion Picture Association of America, which oversees the domestic film ratings system in partnership with theater owners, declined to discuss the study, according to the Times.

In announcing the study, researchers called for changes to the ratings system, which, "according to some of its critics is tougher on sex than on violence," the Times reported.

"We treat sex as R," Mr. Romer said in the announcement. "We should treat extreme gun violence as R."