The research, from the Annenberg Public Policy Center (APPC) at the University of Pennsylvania, also found desensitization could lead to escalating violence in movies rated PG-13.
The findings were published online Monday in the journal Pediatrics.
Researchers gave 1,000 parents a series of graphic clips to watch from movies including:- 8 Mile.
- Die Hard.
- Taken 2.
- The Terminator.
After the first viewing, parents thought the minimum age for watching such content should be around 17, said the study. By the sixth and final viewing, the surveyed parents lowered the minimum age by three or more years.
"We know these scenes are somewhat disturbing to parents," said Dan Romer, the study’s lead author. "When parents first see them, they say you shouldn't let someone younger than 17 see them – which is comparable to an R rating. But they get more and more accepting of that content as they’re watching it."
Causing 'ratings creep'
The study's authors also contend that reviewers for the Motion Picture Association of America (MPAA), and who are also parents, are subject to the same kind of desensitization.
The report argues MPAA reviewers could be "more likely to be lenient when it comes to evaluating the appropriateness of such content for children."
The study suggests the phenomenon can explain what it described as "ratings creep": allowing more violence into films aimed at youth. According to a complimentary 2013 study also released Monday by the APPC, violence in popular PG-13 movies has tripled since 1985.
It also found that popular PG-13 films in 2012 actually contained more gun violence than R-rated movies in the same year.
The result, warns the study's authors, is that "our entire culture may be undergoing desensitization to violent movies with consequences that remain unknown."
One possible consequence is a broader acceptance of the use of guns, they say.