Image caption The Dark Knight (2008) was among PG-13 movies that "had a lot of gun violence", according to the study

Levels of gun violence have more than tripled since 1985 in films rated as suitable for teenagers, according to a US study.

Gun violence in PG-13 films last year actually exceeded that in films rated R for viewers aged 17 and older, according to the research.

Overall film violence had doubled since 1950, found the study published in the American Academy of Pediatrics.

Put simply, violence "sells", one researcher told the BBC.

Dr Daniel Romer, co-author of Gun Violence Trends in Movies, said he and his colleagues had examined the top 30 grossing movies of each year from 2012 to 1950.

In a total of 945 films from the last six decades, there were 17,695 violent scenes, according to the joint study by Ohio State University and the Annenberg Public Policy Center at the University of Pennsylvania.

Some 396 of the 420 films they studied since 1985 contained violence, including 783 instances of gun violence.

Some of the recent PG-13 movies that "had a lot of gun violence", according to the study, included The Dark Knight (2008), Transformers: Dark of the Moon (2011), Captain America: The First Avenger (2011), The Avengers (2012), The Amazing Spiderman (2012) and Taken 2 (2012).

The study's authors suggested that sex scenes were more likely to incur a stricter rating than scenes of violence.

Dr Romer suggested the Motion Picture Association of America (MPAA) re-evaluate its rating system in order to better protect young viewers in the future.

"We'd like to see those films [containing gun violence] put into the R category… or [have them] rethink how much gun violence they will show," Dr Romer said.

Viewership of R-rated films is restricted to those over age 17 or accompanied by a parent; PG-13 strongly cautions parents that some material may not be appropriate for viewers under age 13.

But that rating does not preclude theatres from selling tickets to anyone of any age.