'R' -- Hollywood's Scarlet Letter

Slamming a head into a pencil; holding a knife to a pair of scared, quivering lips; shoving a gun into a mouth and pulling the trigger.

And yet, no R rating?

It's a puzzling thing about "The Dark Knight," which is no doubt the darkest movie in the Batman franchise. The film is rife with violent scenes and drips with the Joker's yellow-toothed menace. Yet it bears only a PG-13 rating, the same sticker slapped on "Mamma Mia!," whose most offensive scenes are arguably those of Pierce Brosnan singing.

The reason? R is increasingly becoming Hollywood's scarlet letter, the rating no would-be blockbuster wants. It scares parents and cuts out the whole swath of under-18 movie goers. If it means higher profits, studios have no qualms about lobbying the Motion Picture Association of America (MPAA) for lower ratings. And often, the MPAA has no problem granting their wishes.

"Think about it: When have you seen a comic book movie about a superhero that got an R rating?," said Eddie Schmidt, who produced 2006's "This Film Is Not Yet Rated," a documentary about the MPAA's notoriously secretive rating system. "The system is self-regulated: it's set up by Hollywood for Hollywood products. So it's in their best interest for films to get PG-13 ratings."

"For PG-13, they gave the rating reason, 'intense sequences of violence and some menace.' But what does that mean?," he continued. "PG-13 is obviously stronger than PG, not as strong as R, but what's actually in the movie? What is 'some menace'? It's unlikely that you're going to get any info that's going to prevent you from seeing it, even if it's info that's helpful for parents. You might read more information about the movie and not want to go -- therein lies the issue with [the MPAA] giving out information."

The MPAA did not respond to ABCNews.com's requests for comment. Schmidt speculated they probably screened multiple versions of "The Dark Knight" before giving Warner Brothers the rating they wanted.

"Generally you submit a cut and then they come back to you with a preliminary rating," he said. "And a lot of times there is back-and-forth between the studio and the MPAA to try to get what would be a more desirable rating. Studios obviously have a lot of resources; they can resubmit multiple cuts with small changes here and there to get the rating they want. It's not unheard of for a studio to submit 9 or 10 different cuts to get the rating they want."

"Their standards are not transparent," Schmidt added. "Sexualized nudity would push a movie to get an R rating. Violence -- the more realistic it is, the more bloody, the more carnal, the more likely it is to get an R rating. But you might have instances of violence in PG-13 movie and in an R movie that seem exactly the same. So there's no way of knowing. I would think it's very maddening for a parent."

And without that PG-13 rating, it's unlikely "The Dark Knight" would've been able to break all those box office records. The best opening ever for an R-rated movie? $91.8 million for 2003's "The Matrix Reloaded," a far cry from the $155.34 million "The Dark Knight" made in its first weekend. This past weekend, it delivered the best second-weekend gross in recent Hollywood history, raking in an estimated $75.6 million.

"Certainly an R rating is not as good for box office, no question about it," said Paul Dergarabedian, head of box office analytics firm Media by Numbers. "Last year, of the top 20 movies at the box office, 11 were PG-13. In 2006, out of the top 20 movies, 13 were PG-13. It's by far the most popular rating and it's the most profitable at the box office."

There is a certain kind of movie that reaps benefits from an R rating: The raunchy comedy, a la Judd Apatow's "Superbad" and "Knocked Up," which clinched the No. 1 and 2 spots at the box office, respectively, when they opened last summer.

"There's no question that for a certain segment of the audience, the R rating actually helps. These raunchy comedies that studios traditionally thought needed a PG-13 in order to get teenagers, now, they're doing well with the R rating," said Steve Zeitchik of The Hollywood Reporter. "But for an action adventure movie, the typical thinking is that you want to get all ages or as many age groups as possible. You're looking to rationalize a fairly high budget. You don't want to cut out anyone under 18."

So what are parents to do when movies that may well warrant an R rating score a PG-13? Independent movie rating services, like Kids-in-Mind.com, attempt to bridge the gap between the MPAA's cryptic system and what's actually being shown in theaters. Kids in Mind rates movies based on a 1-10 scale of severity in three categories: Sex and nudity, violence and gore, and profanity. This way, parents can find out about that pencil-jamming scene before their 8-year-old Batman fanatic develops lifelong nightmares.

"PG-13 today is really what R used to be 10 years ago," said Aris T. Christofides, editor of Critics Inc., parent company of Kids in Mind. "We allow parents to make their own decisions based on their own sensibilities and perspectives. We don't tell them what they should be thinking."

"The MPAA ratings are not really content-based," he added. "They're supposed to be, but they don't actually tell you what the movie contains. They just give you an age-based recommendation which is the opposite of what we do -- there's no magic age, like 13, where kids suddenly become mature and can watch a certain movie. That makes no sense."