The prevalence of gun violence in top PG-13 films has more than tripled since the rating was introduced in the United States in 1984, and last year it eclipsed even the amount in R-rated movies, according to new findings. "I think most parents would be surprised to learn that," said Brad Bushman, a professor of communication and psychology at Ohio State University and one of the authors of the study due to have been published yesterday in the journal Pediatrics . "We were pretty shocked," he said. The PG-13 rating, introduced after an uproar over violence in Indiana Jones and the Temple of Doom , indicates that parents should be "strongly cautioned" that some material may be inappropriate for children not yet teenagers. The R rating means anyone under 17 must be accompanied by a parent or adult guardian. The authors said their findings were particularly troubling given considerable research into what has been called the "weapons effect", which suggests that depiction of gun violence in media could lead to more aggressive behaviour in the real world. "We know that movies teach children how adults behave, and they make gun use appear exciting and attractive," said Dan Romer, another co-author and the director of the Adolescent Communication Institute of the Annenberg Public Policy Centre in the US state of Pennsylvania. To determine how violence - and specifically gun violence - has changed in films, researchers chose 945 films sampled from the 30 top-grossing releases each year from 1950 to last year. Coders sifted through the movies, noting all the violent sequences. In the end, they found that gun violence in films had more than doubled since 1950 and had tripled in PG-13 films over the past quarter-century. In addition, while PG-13 movies initially had only about as much violence as G- and PG-rated films, since 2009 they have contained as much violence as R-rated films or more. Romer said even though some of the most popular PG-13 films in recent years, such as The Hunger Games , The Dark Knight Rises and Snow White and the Huntsman are based on comic-book heroes or other fantasy characters, "these films have a lot of violence in them". "We think that the PG-13 rating is no longer very helpful," he said. "If they're going to allow content like that in PG-13 movies … it sort of goes against the grain of how they define the difference" compared with an R-rated movie, he added. According to the Motion Picture Association of America's rating system, "there may be depictions of violence in a PG-13 movie, but generally not both realistic and extreme or persistent violence". By comparison, an R-rated movie could contain "intense or persistent violence". Its spokeswoman declined to comment.