If Shakespeare’s plays had gotten ratings in the 16th and 17th centuries, some might have been too sexually daring or violent to be labeled PG-13 or even R.

But back then, bans and tossed tomatoes were about the only way audiences and leaders could show their disapproval of explicit scenes and risqué plots.

With a modern-day Shakespearean doomed-love tragedy called "Brokeback Mountain" opening this weekend, the debate over this country's ratings system — handled by the Motion Picture Association of America (MPAA) — has once again bubbled over like the witches’ cauldron in “Macbeth.”

The much-blogged-about, critically acclaimed Ang Lee film starring Jake Gyllenhaal and Heath Ledger is rated R. But its storyline of two male ranch hands who fall for each other in the early ‘60s and keep their relationship a secret has led some to wonder why the movie didn’t get a more restrictive NC-17.

“I think the only reason it got an R is because Ang Lee directed it,” said film critic Anderson Jones, referring to the clout of the director behind “Crouching Tiger, Hidden Dragon,” “The Ice Storm” and “The Hulk.” “I don’t believe it deserved an NC-17 rating, but I’m surprised it didn’t get one considering the storyline.”

It doesn’t hurt that the movie's studio is Focus Features, the "indie" division of mammoth Universal Pictures. Focus Features co-president James Schamus, the film's producer, said there was no discussion of an NC-17 rating (meaning viewers 17 and under aren't admitted) — or a lesser one like PG-13. He said nothing was cut to get it an R.

“We assumed it would be R; it was R. It was totally fair,” Schamus said at a press roundtable discussion about the film. “It’s an adult, grown-up movie. It’s a movie I think young people could see or should see in the context of their parents talking to them about it. That’s an R rating to me.”

But some have blanched at the R as too strict for "Brokeback," since there are only two scenes where the main characters kiss and one short sex scene between them in which more is implied than shown.

"I don't believe it would be inappropriate [as PG-13]," Jones said. "The ratings system continues to prove how flawed it is. It's flawed, and it's controlled by larger studios."

In fact, today’s MPAA guidelines have been scrutinized since they were established — garnering grumblings from some that they're too harsh, not harsh enough, too vague or inconsistent, and praise from others who find them helpful indicators about whether movies are kid-friendly.

The MPAA defends its ratings system, saying it was established simply as a basic film-content guide for parents.

"If you took any movie, you'd probably find very few where every single person would agree," said MPAA spokeswoman Gayle Osterberg. "But 80 percent of parents approve of the ratings system and believe it's a helpful tool."

The board that chooses ratings consists of 10 to 13 people whose only requirement is to have parenting experience. They watch a film, discuss it and then give a rating based on content elements like sex, violence, nudity and profanity.

But the system is industry-driven, and the MPAA won't comment on why specific films get their ratings or who exactly decides what labels to dole out.

There are currently five MPAA ratings: G, PG, PG-13, R and NC-17. A sixth, unofficial category is no rating, or NR, for films that haven't yet received one, reject the MPAA stamp (usually when it's NC-17) or are not submitted to the MPAA at all.

"Brokeback Mountain" isn't the only movie whose rating has been analyzed in recent months. Criticism arose back in October with release of the originally NC-17 "Where the Truth Lies" starring Kevin Bacon, Colin Firth and Alison Lohman.

By the time the film hit theaters, it was without its MPAA label — given for an interrupted threesome scene and other sexual explicitness — and instead went out unrated.

"If you get slapped with an NC-17, it's often the kiss of death, at least in terms of box office," said Paul Dergarabedian, president of the box office tracking firm Exhibitor Relations Co. "Movies that are NC-17 are created to push the envelope, not make money. They're never going to be huge blockbusters. [Filmmakers] want to make a movie that's important — and profits be damned."

Since NC-17 is widely seen as a black mark because many theaters won't screen those films and most media won't accept ads for them, certain movies (including a lot of foreign ones) choose the unrated option.

"Y tu mamá también" is another film that went that route, though the "not rated" strategy frustrates some people who say it's too confusing.

Ratings advocates point to NC-17 as a worthy stamp, if marketers and theaters were less squeamish about it and ticket-takers stuck to the policy of keeping kids out.

"In this age where theaters are the size of football stadiums, they can't and won't enforce [ratings]," said Jones, who writes for AMC's Movie Club. "It's not because they're prudish. They're short-staffed. ... But somebody has to take responsibility. First it's the filmmaker, then it's the ratings system, then it's the parents, then it's the theater."

Ratings critics like Roger Ebert have lambasted the existing MPAA system as inconsistent when it comes to sex and nudity vs. violence, saying the industry is more lax on the latter than the former.

Exceedingly violent R-rated films like "Saving Private Ryan" and "The Passion of the Christ" have come under fire for getting an R when some think they should have gotten an NC-17.

Meanwhile, films like Bernardo Bertolucci's "The Dreamers" and David Mackenzie's "Young Adam" were labeled NC-17 for explicit sex and nudity.

But there have also been some sexually explicit films to squeak by with an R, including "Boogie Nights," "Kinsey" and "The Crying Game."

Harvard University recently released a study about a phenomenon dubbed "ratings creep," arguing that in the last decade, more R-rated material has "crept" into PG-13 films, PG-13 content into PG films and so on.

"I think ratings creep has happened, but it's not only because of the MPAA," said James P. Steyer, CEO of Common Sense Media, a nonpartisan group that offers extensive information about all types of media content, including movies. "Society has changed, the media industry has changed. More risqué stuff is now being seen at a younger and younger age. Parents can be blindsided."

In the case of "Brokeback," most other countries that have screened it gave it a less strict rating than the U.S. Australia labeled it an M (roughly equivalent to a PG-13), the UK a 15 (no children under 15 allowed), Ireland a 16 (no children under 16 admitted), Norway an 11 (suitable for children 11 and above), Finland a K-11 (suitable for children 11 and above) and Canada a 13+ in Quebec (children 12 and under may be admitted with an adult 16 or older) and 14A in Alberta, British Columbia and Ontario (suitable for kids 14 or older; under 14 must be accompanied by an adult).

"I think America is the only country that has given it an R," remarked Ledger at the roundtable. He said he doesn't choose acting projects based on expected ratings, even though they are linked to audience size. Because they're restricted to a narrower viewership, R films tend not to make as much money as those rated PG-13, PG or G.

Another "Brokeback" cast member, Anne Hathaway (of "Princess Diaries" fame), said she once did a film based on how big her audience might be and what the movie might be rated, but won't again.

"The one time I kind of did that, where I was just like 'Oh, this movie will have such mass appeal — wouldn’t it be fun to do?' was 'Ella Enchanted' ... a $6 million opening weekend, baby," she said at the roundtable, chuckling at the modest early returns. "It’s a wonderful movie ... but you can’t do that. At the end of the day, you need to be proud of your work, not proud of the business decisions you made. I’m not a businesswoman; I’m an actress."

"Brokeback" filmmakers have said they would like Middle America and "Bush voters" to have access to it, not just coastal audiences and "blue states." But they've been cautious about how they're rolling it out, starting in only five theaters in New York, San Francisco and Los Angeles this weekend and widening the scope from Dec. 16 through January.

"I want to make sure they have the chance to see it," Schamus said of Middle American viewers. "Whether people see it or not, that's their business. ... A lot of [theater] chains were nervous we were going to skirt some smaller communities because we were self-censoring. ... There's been a lot of fighting over who's going to get the movie."

Madonna reportedly fanned the "Brokeback" flames with her recent comment that the movie is “shocking.” The Material Girl-turned-rigid-mom arguably did more to outrage audiences in her “Justify My Love” video and "Truth or Dare" docudrama than these filmmakers did, however.

"Madonna said it was shocking? Madonna saw this film?" said Hathaway. "I don't think there's anything shocking about it. It's very real and tastefully done. I think it's over-hyped."

The MPAA ratings system was adopted in 1968, the year after Hollywood’s Motion Picture Production Code — dictating what could and couldn't be said and shown onscreen — was abolished by the motion picture industry. It followed years of erosion of the code beginning in the 1950s.

The PG-13 label was born in 1984 when two popular Steven Spielberg films, “Indiana Jones and the Temple of Doom” and “Gremlins” (the first he directed, the second he produced), were called into question for their violent content; it prompted Spielberg to suggest that MPAA head Jack Valenti add the PG-13 rating — which he did.

The rare X rating was never trademarked by the MPAA, but a few acclaimed films did receive the adult-content stamp — including "Midnight Cowboy" in 1969 (which starred Dustin Hoffman and Jon Voight and was the only X-rated movie to win the Best Picture Oscar) and "Last Tango in Paris" in 1972 (which starred Marlon Brando and was directed by Bertolucci; it had an X rating for graphic sex but got actor and director Oscar nods).

Gradually, the X rating became more frequently used by and associated with pornographic films, and in 1990, the Motion Picture Association formally adopted and trademarked NC-17 instead.

Meanwhile, the battle rages on and the ratings issue pervades, not just in film but in other media industries with their own systems — like video games, TV and the Internet.

"The tough thing about ratings is that they're in the eye of the beholder," said Exhibitor Relations' Dergarabedian. "It's a system that judges art, so you're going to have controversy. It's the nature of the beast."