It’s been more than a century since Anton Chekhov noted that if one introduces a gun in the first act of a play, it had better go off by the final curtain. The Hollywood corollary holds that if you don’t introduce guns at all, you might as well kiss your box office goodbye.

Violence is the backbone of American drama, and in the past few months, family audiences have been treated to the sight of a Batman villain snapping the neck of a nuclear physicist before an arena full of football fans in “The Dark Knight Rises”; a jittery James Bond trying to shoot a Scotch glass without accidentally snuffing his latest shag in “Skyfall”; and a clan of sexually repressed vampires ripping the heads off of their rivals in a bloody battle royale in “The Twilight Saga: Breaking Dawn — Part 2.”

It all made for damn good entertainment, but that doesn’t excuse the fact that such scenes appear in movies that are rated PG-13, meaning kids can see them if accompanied by an adult. And such scenes will continue as long as the MPAA ratings system permits egregious depictions of violence in PG-13-rated films. It’s easy to point the finger at the Classification and Ratings Administration (CARA), and every few months some distributor or filmmaker does exactly that when a rating doesn’t go their way — but nearly always because they desire a lower rating that will broaden the film’s commercial potential (as when the Weinstein Co. appealed the R ratings of “The King’s Speech” and “Bully,” both triggered by the use of profanity).

What most of the journalists who rush to the defense of such sob stories fail to realize is that CARA is not a censorship organization but a parental recommendation board. In fact, Jack Valenti established CARA to replace the openly prohibitive Production Code Administration, which could demand cuts in exchange for its seal and expected that every film had to be suitable for all audiences, with no distinction made for “adults-only” fare.

The current system allows American filmmakers the right to put whatever they want on film, so long as they agree to submit their work to CARA. As such, the ratings — G, PG, PG-13, R and NC-17 — and the accompanying descriptors (short, often unintentionally hilarious haikus describing the potentially objectionable content) are merely signposts, indicating what the average American parent would deem acceptable for their children to see.

The trouble comes in how those ratings are interpreted in the marketplace. An R rating doesn’t mean kids can’t see a film, for instance, but simply that they can’t do so without a parent or guardian present. That’s a reasonable request. I’m 35 years old, and after seeing “The Dark Knight Rises’ ” depiction of a successful terrorist plot at the hands of the scary-named and even scarier looking Bane, I was traumatized enough to want a grown-up to talk me through it. Director Christopher Nolan’s vision of the Joker was no less upsetting in “The Dark Knight,” and yet these images manage to permeate our culture, first seen on posters and later even on dollar bills, with the Joker’s face replacing George Washington’s (which just goes to show that while some people in these pages are debating whether films inspire violence, it’s clear that they inspire imitators to some degree).

If CARA is true to its aim — using a board of anonymous parents to evaluate upcoming releases according to feedback they’re getting directly from parents all over the country — then I suppose my complaint is with the public at large, because I want stiffer ratings than they do. Sure, I saw all the Bond movies as a kid, but my folks were sitting beside me for every one. When I saw Daniel Craig pull that William Tell trick in “Skyfall,” only to have Javier Bardem shoot the girl anyway, I would’ve preferred to know that all the teens in the audiences had someone older and wiser to explain why Bond’s retort — “a waste of good Scotch” — is more tragic than funny.

Still, a palpable fear of bad onscreen examples underlies many of the calls CARA’s senior raters receive. For example, one angry parent called in to complain about the level of sexuality in a PG-13-rated film. When asked about the violence in the same movie, she said, “My husband and I don’t worry too much about violence. We don’t expect our children to be violent people. We do expect them to engage in sex and to pick up bad language.” That attitude goes a long way to explain the odd paradox between CARA’s perceived over-sensitivity to sex and language, as opposed to what seems like leniency toward violence, the enforcement of which materially affects the content of American movies — and frequently penalizes the market potential of foreign and independent films not conceived with ratings standards in mind. It may also explain why a lobby concerned about underage smoking recently raised a cry to make any film depicting cigarettes an automatic R (the movement didn’t carry, though CARA does mention smoking in its descriptors now), while no one seems to feel the mere presence of guns should restrict access to a film.

Remember, CARA was established by the majors to avoid censorship. The studios need PG-13 ratings to earn back their enormous budgets. Filmmakers are contractually required to deliver a certain rating, whether or not their vision fits that same audience, and responsibility starts there, with the content producers.

(CARA often permits carnage if it occurs in the realm of fantasy, as in the bloody orc battles of “The Lord of the Rings” — another series where I would advocate parental accompaniment — while computer graphics have expanded filmmakers’ ability to represent such violence, but the last three Batman and Bond movies were clearly aiming for a certain real-world plausibility.)

Besides, trying to cram such material into a PG-13 rating waters it down anyway, as we saw with 2007’s “Live Free or Die Hard,” the series’ top-grossing entry. This year’s “Die Hard” sequel is not yet rated, but will presumably be trimmed to squeeze just under the R bar.

There was a time when things flowed the other way, back before CARA added the PG-13 — first given to the original “Red Dawn,” in which heavily armed teens stave off a Soviet-backed invasion. For fear of being given G ratings and therefore having their films misconstrued as “kiddie pics” in the marketplace, producers added profanity to the likes of “Harry and the Hendersons” and “Star Wars” to earn a PG. (When I asked then-Fox president Alan Ladd Jr. whether this was true, he recalled putting an expletive in a “Star Wars” storm trooper’s mouth to get the rating they wanted, then cutting it before the film was released.)

Now, it’s the opposite. The suits would rather release everything as PG-13 or lower in order to tap into every possible ticketbuyer, and so they find ways to color within the lines, whether or not the underlying material is suitable for teenagers — which is technically what the PG-13 attempts to distinguish. Instead of having five distinct ratings, everything clusters around the limits, so PG-13 movies are barely distinguishable from R, save for the amount of F-bombs they drop (two pushes you over the limit — one of the only firm rules the group has in applying its otherwise subjective ratings). As standards creep, so does the amount of violence (or sex) permitted.

I’m not arguing for milder movies or a complete overhaul of the existing ratings system, but a more meaningful interpretation of the one we have. Unfortunately, R is often misinterpreted to mean the way NC-17 was designed. The latter firmly states that children aren’t allowed, while R allows parents to watch alongside their children — and in a perfect world, discuss the more morally complex aspects afterwards.

But ever since the MPAA botched its X rating, declining to trademark the designation and therefore letting the porn industry tout it as a badge of honor, the prospect of a working “adults only” rating has been a lost cause. Many exhibitors won’t show NC-17 films, just as many media outlets refuse to carry advertising for them, which means R becomes the de facto adult rating, and PG-13 swells to accommodate material that would really benefit by precisely what the “PG” stands for (but has rarely actually meant): some plain old “parental guidance.” Ironically, the only rating that requires parental guidance is the R. More films should use it, and if teenagers really want to see what amounts to ruthless violence in “The Dark Knight Rises,” “Skyfall” or the latest “Twilight” picture, they need only grab a responsible adult.