Parents may get so accustomed to seeing sex and violence in movies and television that they end up lowering their standards for what kids are allowed to watch, a new report suggests.

In a new study, parents who watched several clips of movie violence in succession became desensitized over time, and they relaxed their standards for what they would allow their children to see.

In particular, the researchers point out, the violence in PG-13 movies has become more graphic over the last several decades.

“We released a study last November that showed a dramatic increase in violence and in particular gun violence in popular PG-13 movies over the past 10 years,” said lead author Daniel Romer of the Annenberg Public Policy Center at the University of Pennsylvania in Philadelphia.

“There is now actually more gun violence in PG-13 than in R-rated movies,” he told Reuters Health by email.

The new study suggests that parents are now more willing to allow their children to watch these movies, he said.

“This is consistent with surveys done by the Kaiser Family Foundation showing that parents express less concern about both sex and violence in movies,” Romer said.

For the study, researchers asked 1,000 parents of pre-teens and teenagers to watch a series of movie clips one after another. For each clip, parents recorded the minimum age at which they would be comfortable allowing their child to view the movie.

The six movie clips, which ranged from 15 to 59 seconds, were arranged randomly for each parent viewer and came from popular PG-13 or R rated movies.

Violent scenes, usually involving guns, came from the films Collateral, Taken 2, Die Hard, Live Free or Die Harder and the Terminator series, while the sex scenes were taken from 8 Mile and Casino Royale.

At first, the parents rated the violent scenes and the sex scenes as appropriate for 17-year-olds, on average.

By the last clip in the series, parents deemed scenes of a similar level of violence and sex appropriate for 14-year-olds, according to results in Pediatrics.

They also reported more willingness to have their own child view the movie by the end of the sequence.

The increase in movie violence and concurrent desensitization may be working to fuel each other, Romer said.

“We potentially have a cycle of more violence in PG-13 movies (which is the most popular rating category today) leading to greater acceptance among parents, leading to the Motion Picture Association of America encountering less resistance in rating those movies PG-13, which 15 years ago would have been rated R,” he said.

There is evidence that viewing a lot of violent TV content starting in grade school can lead to more aggressive behavior as children age, he said.

“This is a difficult thing to research, and several studies have looked at this question in many different ways,” said Dr. Jeanne Van Cleave of the Pediatrics Department at Harvard Medical School and the Division of General Academic Pediatrics at MassGeneral Hospital for Children in Boston.

“Although they have their limitations, collectively, studies indicate an association between viewing violence and later tendencies toward aggression,” Van Cleave told Reuters Health by email.

She wrote a commentary accompanying the new study in Pediatrics.

“We don't know how gun violence in particular affects this process,” Romer said, and there is less evidence for how if at all sexual media affects children’s’ development.

There is a risk that kids who are vulnerable to imitate violence they see in movies and television will do so, especially if they have access to guns and a reason to use them, he said.

“My professional opinion as a pediatrician is that yes, there is a pretty important difference between seeing these things at age 14 compared to age 17,” Van Cleave said. “There is a lot of cognitive and social development that happens between these ages, such that a 14-year-old and a 17-year-old are going to come away with different impressions of the same thing.”

Parents should be aware that PG-13 movies can contain high levels of violence, Romer said.

“It is hard to be objective as a parent about deciding what’s okay for a child to watch and what isn’t,” Van Cleave said. “Being aware that PG-13 movies can be quite violent while still attaining that rating may help parents in their decision-making.”