Movies from the 1980s like “Terminator” or “Die Hard” were rated R at the time of their release – but if they were released today, they’d probably be rated PG-13, a new study suggests.



That's because PG-13 movies today — such as “The Hunger Games” or “The Avengers” — contain more violence than the R-rated films of the 1980s, according to a new report published today in the journal Pediatrics. In particular, gun violence in PG-13 films has tripled since 1985, the year the PG-13 rating was first introduced. And overall, violence in movies has nearly quadrupled since the 1950s.

Psychologists say it’s a worrisome trend that we should take seriously, because there is evidence that watching violence on screen increases aggression in real life.

“Of course it’s not the only factor, and it may not even be the most important factor, but it isn’t a trivial factor — and it’s one we can change," says Brad Bushman, an Ohio State University psychologist and lead author of the new report.

Bushman and colleagues analyzed 945 popular films released from 1950 to 2012. Each movie was among the 30 top-grossing films of that year, and they randomly chose 15 of those top 30 movies to scrutinize. Undergrads watched every film and counted every violent act — they defined a violent sequence as “physical acts where the aggressor makes or attempts to make some physical contact with the intention of causing injury or death.”

They found that since 2009, PG-13 movies have featured as much or more violence than the R-rated films released those same years. And in 2012, there was more gun violence in PG-13 films than in the R-rated ones out that year.

Take the “Die Hard” sequels. One of the films the undergrads analyzed was 1990’s “Die Hard 2,” which was rated R. But a later sequel in the series, 2007’s “Live Free or Die Hard,” actually had more gun violence and a comparable amount of overall violence—and yet it was rated PG-13.

Same idea with the “Terminator” movies: The third one in the series, “Terminator 3: Rise of the Machines,” was included in the study— it got an R rating in 2003. But they found that it had less gun violence than 2009’s “Terminator Salvation,” which received a PG-13 rating.

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One more example that really jumped out at study co-author Dan Romer was the famously violent 1987 film “The Untouchables.”

“It had gun violence in it that was comparable to a lot of the movies we’re calling PG-13 in the last five years,” says Romer, director of the Adolescent Communication Institute at the University of Pennsylvania's Annenberg Public Policy Center.

“I wouldn’t be surprised if ‘The Untouchables’ today would get a PG-13.” He thinks the same would apply to the Eddie Murphy comedy “Beverly Hills Cop,” which was rated R in 1984, but feels more like today’s PG-13 movies in terms of violence.



“There are exceptions, but in the top-grossing films, over 90 percent of them have some violence,” Romer says. “Violence is very good for Hollywood. And PG-13 is good for Hollywood, because it doesn’t restrict anyone from going into the theater.”

There are a few things that might explain the remarkable rise in violence in PG-13 films. Ratings are determined by the Motion Picture Association of America — which means, Bushman says, they’re “assigned by the industry.” (The MPAA declined to comment on the study, but you can read more about the ratings system here.)

And a movie rated PG-13 will attract more theatergoers than an R, of course, because kids can go see it. Romer also thinks the rise in sci-fi and comic book movies has something to do with it —violence may be easier for us to handle if it’s got a fantasy element to it. And violence is understandable in every language, which means violence-fueled action movies are more marketable overseas than comedies.



The researchers also examined graphic sexual scenes in the movies they analyzed, and found that sex was much more likely to earn a film an R-rating than violence. “Take a film like ‘Ted.’ There’s hardly any violence in that. But because he has sex—not even very graphic sex, they just show him having sex — that gets an R,” Romer says.

It's worth noting that there is also a lot of crude language in that movie, which can also garner a film an R rating. “(Sex) consistently gets an R rating if it’s at all explicit, but that’s not the case with violence," Romer says.



We don’t like the idea that violent movies —or TV shows or video games — influence our behavior. But many studies have suggested that they do. One often-cited 1967 study found that the mere sight of a gun made people act with more hostility, deciding to deliver a harsher electric shock to another study participant. More than 50 other studies since have found similar evidence of the “weapons effect” — the idea that just seeing a weapon can increase aggression.

“People tell me all the time, I watch violent media and I’ve never killed anyone. Well, big deal. Nobody kills anyone; murder is a very rare event,” says Bushman. “So you’ve never murdered anyone. What I want to know is —how do you treat other people?”

Watching violent images doesn’t always make us more likely to want to punch things; it can also make us less likely to help people in need, Bushman found in a 2009 study. This is true for all of us, but psychologists agree that children are particularly vulnerable.

Bushman would like to eventually see movie ratings in the U.S. decided by a panel that includes child psychologists, with ratings that clearly spell out which ages the movie is appropriate for. But until then, parents who are worried about their kids seeing violent images on screen can take a few pages from Bushman’s book.

For one, he blocks all violent and sexual content online. He also encourages parents to do their homework – if the kids want to see a movie or buy a video game, Google it first. Scenes from many popular video games are available on YouTube, for example. And parents should also always explain their decision to their kids.



Recently, after reviewing his own son’s birthday wish list, he told him, “You listed five games, and these three are OK. These other two are not good, and let me show you why. We try to give you healthy choices, and not just for food, but for your media diet as well.”

You can follow NBCNews.com health writer Melissa Dahl on Twitter.