It's a reasonable thing to say that if the MPAA hadn't been formed, Congress would have tried to enact some fairly restrictive legislation in its stead. For the most part, they leave Hollywood alone, because not enough people complain that Hollywood is trying to Corrupt Our Children. Hollywood self-regulates enough that society leaves them to it.
Not that the MPAA ratings are really of much use. By their very nature, they represent the opinions of a small number of people whose job it is to watch these movies and rate them. True, they try to rate according to the mores of the day... but there's ultimately only five categories: G, PG, PG-13, R, and NC-17. There's also a line of text that tries, in a paltry handful of words, to convey what it is that might have earned a movie the rating it got -- "Some sexual content" says the R rating stamp for Casanova. That doesn't tell me much. Is that a handful of female nipples? A single female frontal nudity shot? Is there any man-ass? No visible genitalia, but maybe lots of scenes of people discreetly screwing?
How about Hostel? It's rated R, "for brutal scenes of torture and violence, strong sexual content, language and drug use." You know, "brutal scenes of torture and violence" could apply to Schindler's List, which was rated R "for language, some sexuality and actuality violence." The violence in Schindler's List isn't as visually graphic as the violence in Hostel, but Hostel's violence is horror-movie-esque random excessive slaughter, whereas the violence in Schindler's List is based on actual atrocities committed here in the real world, motivated by racial hatred and with the intent of wiping out entire ethnic groups. I find that kind of violence a lot more objectionable than the random, cartoonish "horny backpackers get punished for their lust" violence shown in gorefests like Hostel. But does the MPAA agree? Noooo, so Schindler's List gets "actuality violence" whereas Hostel gets "brutal scenes of torture and violence," which sounds a lot worse, if you don't know that Schindler's List is about the (actual, real) Holocaust and Hostel is gross-out fiction.
And then there's the fact that Hostel and Casanova are both rated R, even though Casanova features less sexual content than Hostel, and apparently no violence, language, or drug use worth mentioning. R is a broad, broad category. (Except, as everyone "knows," any sexual or language content will get you an R long before an equivalent amount of violent content.)
Okay, I don't really put that much stock in the MPAA's ratings. I don't think most people do; to give the cynic in me equal time, I'll say that the MPAA is primarily a tool of the movie industry that lets it dodge heat from moral watchdogs. There's nothing fundamentally wrong with this; the MPAA is a corporation and a tool of other corporations (the studios), none of whom have any morals or scruples, because, hey, they're corporations. Nobody expects corporations to have a conscience. The MPAA exists because society demands it of the studios, not because someone had an attack of moral conscience and thought, "We must protect the children!"
Before I turned 17, I paid close attention to the rating each movie received. For any movie that looked vaguely interesting, I always made sure to check the rating as soon as possible. Nothing was more frustrating than learning that "THIS FILM HAS NOT YET BEEN RATED." I checked the newspaper daily to find out when a rating had been assigned. Amusingly, I never once got asked for ID to see a rated R movie before I was 17, and only once after I was 17. (The Puppet Masters, in case you care, which you don't.)
Practically the day I turned 17, I suddenly found that I no longer cared about the MPAA rating, since I was now allowed to see any MPAA-rated film. (This was back when NC-17 meant "seventeen and up OK." By the time they changed it to mean "eighteen and up OK," I was already 18.) These days, I never pay attention to a film's rating unless it comes up in conversation, or there's some other cultural significance to it -- I'm well aware that Revenge of the Sith is rated PG-13, mostly because there was so much coverage of that fact. But otherwise, I don't really care, because the MPAA rating of a film gives me no useful information -- I never use the rating of a film in deciding whether I want to see it.
But now I have an eighteen-and-a-half-month-old son, who will probably soon be able to focus his attention long enough to actually sit down and watch a movie. Am I going to start paying attention to the MPAA ratings? Hell no. They're still of no use to me, because they don't tell me anything about a film. I'm of the strong conviction that there is very little a child should be forbidden from seeing, as long as there is a responsible adult around to explain it to them and answer any questions. My parents were always there to discuss movies afterward. My father made the point many times that violence should always be an absolute last resort.
Does this mean I'll let my son see any movie he wants? For the most part. There's always the chance that something will come along that I don't think he should see, but then if we raise him right, anything I don't think he should see, he probably won't want to see either, so the issue may be obviated.
There's a website called ScreenIt that "rates" movies by thoroughly listing every conceivably objectionable thing the movie contains. It's got the obvious categories like "Violence," "Sex/Nudity," and "Profanity," but also categories that are presumably of interest to the modern parent -- "Disrespectful/Bad Attitude," "Music (Scary/Tense)," "Imitative Behavior," "Tense Family Scenes," and so on. Each movie's rating page has extensive lists of every event in the movie that can fall into one of those categories. By casting a wide net, they ensure that virtually everyone will be able to tell whether or not the movie contains material they'd be okay with their kids seeing. Here's their review of Brokeback Mountain, for example. (Warning: Contains man-ass.)
Of course, there's a problem. The categories they use are intended to be so broad as to cover everything that any significant number of people might object to their kids seeing. (Or even seeing themselves -- not all adults can comfortably watch gay cowboy sex.) But they can never cover everything that anyone might find objectionable. They don't have a category for, oh, I don't know, "Kindness Toward Animals," which would no doubt offend certain fundamentalist segments of society who think that Man's dominion over the world precludes kindness or sympathy toward the lesser beasts.
Okay, that's a stretch, but the point is made: No rating system can ever completely substitute for seeing a movie yourself and deciding whether it's appropriate for your kids.