Before you allow your children to take in a PG-13 movie at the local gigaplex, a new study suggests, you should think twice.

Recent PG-13 movies are as violent as R-rated films, and are also studded with violent lead characters who glorify other risky behaviors from drinking to sex, according to research published Monday in the journal Pediatrics.

Those depictions may lead impulsive teenagers and preteens to prematurely experiment with alcohol and sex, the study suggests.

"If the goal of the rating system is to shield youth from age-inappropriate behavior and be a tool for parents, I think its effectiveness is in doubt," said Amy Bleakley, the study's author and a senior research scientist at the Annenberg Center for Public Policy at the University of Pennsylvania.

"An R rating does at least put some limits on who can see a film. You have to be 17 - although I don't know how well theaters enforce it," said coauthor Dan Romer, the center's associate director for research. "But with PG-13, you could be 5 years old and get in if you're smart enough to buy a ticket."

The study is the second from Annenberg in as many months to depict the movie ratings, issued by the Motion Picture Association of America, as an unreliable guide for parents.

In November, the researchers found a smoking gun in "ratings creep," concluding that gun violence has tripled in PG-13 movies since the rating was introduced 25 years ago. The Terminator (1984) was released with an R rating; a more violent sequel received a PG-13. RoboCop (1987) was initially slapped with an X rating before edits brought it to an R. The Robo-remake, set for release early next year, is expected to be rated PG-13, which means parents are "strongly cautioned" because some material may be inappropriate for children under 13.

In the study released Monday, the researchers looked at five-minute segments from 390 of the top-grossing movies released from 1985 to 2010. They found that 90 percent of the films had at least one segment in which a main character was violent, and in 77 percent of those movies the character was also drinking, smoking, or engaging in sexual behavior.

This "co-occurrence" of violence and other risky behavior occurred in 82 percent of PG-13 and 88 percent of R movies. Even in G (general audiences) and PG (parental guidance) movies, 63 percent had mayhem-plus.

"We're not sure about the effects of that," Bleakley said. "But this study is a step in trying to understand what children are being exposed to."

Romer added, "You could argue that [sex, drinking, smoking] are relatively normal adult behaviors. So we're worried that by throwing violence in with it, it might make violence seem more normal."

One encouraging trend emerged: Depictions of smoking are unusual, reflecting the habit's social stigma. The percentage of movies in which main characters smoked plunged from almost 70 percent in 1985 to 21 percent in 2010.

For years, critics have lambasted the ratings system as untrustworthy or broken. But according to one director, barbs and academic studies are unlikely to change anything.

"The PG-13 rating is designed to attract the most lucrative market," said documentary filmmaker Kirby Dick, whose 2006 documentary, This Film Is Not Yet Rated, savaged the MPAA and outed its secretive board of raters by name. "Basically, they're sacrificing America's children for their own bottom line."

An MPAA spokeswoman said that the group's ratings reflect the values of the American public.

"It's important to remember that a PG-13 is a strong warning to parents about the content of a film, and it is accompanied by a descriptor that gives parents specific detail about which elements of the film warranted the rating," said Kate Bedingfield, the MPAA's chief spokeswoman and a former acting director of media affairs for the Obama White House.

"The purpose of the rating system is to reflect the standards of American parents, not set them. The rating board tries to rate a film the way they believe a majority of American parents would rate it," Bedingfield said. "Societal standards change over time and the rating system is built to change with them."

Dick dismissed Bedingfield's statements as "complete B.S.," and called the descriptors "laughable and inscrutable" because they lack standardization.

And he noted that many of the raters - who watch up to 800 movies a year - have no children or had children who were well into adulthood.

"They don't reflect the standards of American parents at all," he said.

Contact Sam Wood at 215-854-2796, samwood@phillynews.com, or @samwoodiii on Twitter.

Inquirer staff writer Marie McCullough contributed to this article.