The new analysis, says study author Dan Romer, research director of the Annenberg Public Policy Center of the University of Pennsylvania, suggests that "Hollywood continues to rely on gun violence as a prominent feature in its highly popular PG-13 action-oriented films".
Romer's research is important because, while the link between watching violence on the big screen (or on TV or in video games for that matter) and real-world violence is not well understood, there is evidence that such scenes may contribute more generally to aggressive behaviour and desensitisation to violence.
Until we know more, Romer and his co-authors write, paediatricians "should consider advising parents to be cautious about exposing their children to the gun violence in PG-13 movies".
The methodology used in both studies is similar and involves splicing the films into five-minute segments and noting whether a character fired a gun and hit another character during that time.
Multiple instances of gun violence were counted only once if they happened within the same five-minute segment. Researchers noted that in G.I. Joe: Retaliation, for example, there were 22 five-minute segments for a total of 1 hour, 50 minutes, and gun violence occurred in nine of the 22 segments - equalling roughly 40 per cent of the movie.
The original study involved looking at 945 top-grossing releases from 1950 to 2012; the new one covers films out from 2013 to 2015. The data clearly show that the amount of violence per hour of PG-13 movies continues to increase as compared to their R-rated counterparts.
The researchers attributed the phenomenon to the popularity of action movies for children and teens that contain a lot of what they call "bloodless violence" by comic book figures and expressed concern about the films' "erasure of the consequences" such as blood and suffering by more realistic characters.
Chris Ortman, a spokesman for the Motion Picture Association of America, said that the voluntary, 50-year-old ratings system, the Classification and Rating Administration, has changed over time to reflect the changes in "American parents' sensitivities".
"The purpose of Cara is ... to reflect the current values of the majority of American parents, and he pointed out that the ratings are assigned by a team of raters who themselves are parents."