In August 2010, a documentary titled A Film Unfinished was released by Oscilloscope, a New York-based independent distributor. The film’s subject is the making of an unfinished Nazi documentary about the Warsaw ghetto that was intended to spread anti-Semitic propaganda. The director, Yael Hersonski, would like A Film Unfinished to be shown in schools to educate children about the Holocaust. But because the documentary includes some horrific footage of death camp atrocities—some of them showing Jews, both dead and alive, stripped naked—the Motion Picture Association of America’s ratings division assigned the documentary an R rating. Oscilloscope appealed the decision, pointing out that a dozen years earlier, Steven Spielberg’s Shoah Foundation had released a Holocaust documentary titled The Last Days that featured similar footage but received a PG-13 rating. It even presented the MPAA with a letter from a Warsaw ghetto survivor who urged that the film be used to educate young people. But the MPAA’s appeals board voted to maintain the R rating, 12-3.

Why the disparate treatment? It’s hard to suppress the suspicion that it had something to do with Hollywood muscle. Oscilloscope is a small independent distributor founded by Adam Yauch, who (as “MCA” of the Beastie Boys) maintains some influence within the hip-hop world but who is an outsider to the motion picture industry. By contrast, Allentown Productions, which released The Last Days, is based at Universal Studios and run by a Spielberg protégé. Spielberg is the most powerful director-producer in Hollywood—the ratings category “PG-13” was his own invention—and his name was right there above the title (“Steven Spielberg and the Shoah Foundation present”). That was probably enough to make The Last Day’s Holocaust more child-friendly than A Film Unfinished’s.

For some time, critics like Kirby Dick (director of the anti-MPAA documentary This Film Is Not Yet Rated) have argued that movie ratings are biased in favor of Hollywood insiders. Sex and violence in studio-made films, it’s been said, are given greater leeway than sex and violence in the latest sensation from Sundance. The most recent puzzler was the NC-17 rating initially given Blue Valentine, distributed by the Weinstein Co., for an emotionally intense but emphatically unerotic sex scene that wasn’t remotely explicit. (The scene is so achingly sad that its only likely effect on teenage carnality will be to discourage it.) In that instance, the MPAA came to its senses and reversed its decision, assigning the movie an R rating. Would the MPAA have ever assigned its most punitive rating to the film had it been made by a major studio? It didn’t back in 1973 when it gave an R rating to Blume in Love, a Warner Bros. release that included a scene that differed from Blue Valentine’s mainly in that the husband rapes his (ex-)wife.

Now the tools of social science have been brought to bear on the favoritism question. In a June 2010 study for the journal Organization Science (“The Ratings Game: Asymmetry in Classification“; registration required), David Waguespack, an assistant professor at the University of Maryland’s Robert H. Smith School of Business, and Olav Sorenson, a professor at Yale’s School of Management, find that the MPAA goes easier in its ratings on the major studios. This is less surprising when you take into account that the MPAA’s only members are, in fact, the six major studios (Disney, Paramount, Sony, Fox, Universal, and Warner Bros.). Still, the MPAA has always maintained that its ratings carry no bias in favor of its funders. Waguespack and Sorenson allow us to conclude with some certainty that the MPAA’s claim is false. Films distributed by MPAA members are, on average, about 7 percent less likely to receive an R rating than films that aren’t distributed by MPAA members.

Waguespack and Sorenson made their calculations by looking at all the MPAA ratings for films released in the United States between 1992 and 2006 and comparing these to the scores they received from a Web site called Kids-in-Mind. No rating system is perfect, but Kids-in-Mind, the authors argue, is fairly objective because it aims to provide dispassionate descriptions to a global and multicultural audience. Its description of a film’s homosexual content, for example, is intended to be of equal value to both gay rights liberals and religious-right conservatives. (The company’s publisher boasts he’s received favorable e-mail from both.) More important, Kids-in-Mind has no financial dependence on either the major studios or the independents.

Waguespack and Sorenson plotted the percentage of movies with an R rating against the composite Kids-in-Mind score for the same movies, broken down according to whether the distributor was an MPAA member. They found that the lines converged when the Kids-in-Mind composite score was very low (i.e., little or no child-unfriendly content) and very high (lots of child-unfriendly content). But in between, where judgments were likely to be less clear-cut, the MPAA nonmembers (i.e., independents) had a consistently greater likelihood of receiving an R rating than the MPAA members (the major studios). On average, the independents were 24 percent likelier to receive an R rating than the big studios. But that figure exaggerates the difference, because it’s uncorrected for the actual content of independent films vs. big-studio films. The big studios are more risk-averse when it comes to controversial content; independent films tend to be racier. So Waguespack and Sorenson corrected for profanity, violence, sexual content, etc. When they did, independent films were still likelier to receive an R rating than big-studio films, by the aforementioned 7 percent. That 7 percent difference matters because (within Waguespack and Sorenson’s sample) PG and PG-13 movies earned, on average, 76 percent more revenue than movies rated R.

In an e-mail, Waguespack alerted me to a calculation—one he didn’t include in the paper—about which PG-13 film released between 1992 and 2006 had been (judging solely from its Kids-in-Mind score) most likely to get an R rating. The winner was 1997’s Titanic, which includes a very excellent look at Kate Winslet’s breasts, bloody gunshots, an electrocution, many horrific drownings, many floating dead bodies, and a few obscenities. If you ignored that Titanic was distributed by Paramount and Fox (which together constitute one-third of the MPAA’s membership) then Titanic was 99 percent likely to get an R rating. And yet the movie got rated PG-13! It went on to hold the record for the highest-grossing film of all time, even though it was improbably bad. (It can’t be easy to make a stinker out of the most dramatic nautical episode in history.) Waguespack also calculated which R-rated film released during the same period was most likely (judging, again, solely on Kids-in-Mind’s content analysis) to get a PG-13 rating. The winner (or, in this case, loser, since an R rating reduces box office potential) was a 1999 Bernardo Bertolucci movie with Thandie Newton that I only dimly remember hearing about called Besieged. It was distributed by Fine Line. Apparently moviegoers glimpsed one female breast (50 percent fewer than in Titanic!), no violence or gore (though you did see a couple of body functions: urine trickling down a leg and puke spewing out a mouth), and no profanity. Based on content alone, Waguespack calculated the film’s likelihood of getting an R rating was less than 2 percent. But R is what it got, and it stiffed at the box office. (Besieged had two things going against it, ratingswise: In addition to being an independent rather than a major-studio release, it was directed by a guy who had a reputation for making sexy movies like Last Tango In Paris, Luna, and The Sheltering Sky. Bertolucci’s movie immediately prior to Besieged was the R-rated Stealing Beauty. According to Waguespack and Sorenson, a director who has gotten Rs before is likelier to get Rs in the future, regardless of what’s in the movie.)

Another interesting comparison Waguespack and Sorenson made concerned industry ratings versus government ratings. I would never suggest that the United States government take over the MPAA’s ratings function. It’s none of Uncle Sam’s business! But if it ever did, Uncle Sam would probably show less favoritism toward the big studios. Waguespack and Sorenson compared movie ratings from Australia and Ontario, Canada, which are done by governments, with movie ratings from the United Kingdom, which are done by the film industry itself. They found that the United Kingdom, like the United States, favored the big studios but that Australia and Ontario did not. An obvious explanation is that foreign governments don’t have to care what American film distributors will think.

A likelier ultimate solution than government control to the bias problem is that American society will become more depraved and fewer and fewer films will receive R ratings. According to Waguespack and Sorenson, the conservative lament that MPAA ratings have become more forgiving of sex and violence over time turns out to be true. They don’t quantify it, but “MPAA ratings have become progressively more permissive.” Eventually Americans may let their kids see everything, and the MPAA ratings board may go the way of the National Legion of Decency. That would create problems of its own that would cause me to fret about my grandchildren. But MPAA favoritism would become a thing of the past.

Like Slate on Facebook. Follow us on Twitter.