History of MPAA ratings

Replacement of Hays Code
Jack Valenti, who had become president of the Motion Picture Association of America in May 1966, deemed the Motion Picture Production Code – in place since 1930 and rigorously enforced since 1934 – as out of date and bearing "the odious smell of censorship". Filmmakers were pushing at the boundaries of the Code, and Valenti cited examples such as Who's Afraid of Virginia Woolf?, which contained the expressions "screw" and "hump the hostess"; and Blowup, which was denied Code approval due to nudity, resulting in the MPAA member studio releasing it through a subsidiary. He revised the Code to include the "SMA" (Suggested for Mature Audiences) advisory as a stopgap measure. To accommodate "the irresistible force of creators determined to make 'their films', and to avoid "the possible intrusion of government into the movie arena", he developed a set of advisory ratings which could be applied after a film was completed. On November 1, 1968, the voluntary MPAA film rating system took effect, with three organizations serving as its monitoring and guiding groups: the MPAA, the National Association of Theater Owners (NATO), and the International Film Importers & Distributors of America (IFIDA).

The ratings used from 1968 to 1970 were
 * Rated G: Suggested for General Audiences
 * Rated M: Suggested for Mature Audiences – parental discretion advised
 * Rated R: Restricted – persons under 16 not admitted, unless accompanied by parent or adult guardian.
 * Rated X: Persons Under 16 Not Admitted

This content classification system originally was to have three ratings, with the intention of allowing parents to take their children to any film they chose. However, the National Association of Theater Owners urged the creation of an adults-only category, fearful of possible legal problems in local jurisdictions. The "X" rating was not an MPAA trademark and would not receive the MPAA seal; any producer not submitting a film for MPAA rating could self-apply the "X" rating (or any other symbol or description that was not an MPAA trademark).

From M to GP to PG
In 1970 the ages for "R" and "X" were raised from 16 to 17. Also, due to confusion over whether "M"-rated films were suitable for children, "M" was renamed to "GP" (for General audiences, Parental guidance suggested), and in 1971 the MPAA added the content advisory "Some material not generally suitable for pre-teenagers".

The ratings used from 1970 to 1972 were:
 * Rated G: All Ages Admitted – General Audiences
 * Rated GP: All Ages Admitted – Parental Guidance Suggested [Sometimes a disclaimer would say "This film contains material which may not be suitable for pre-teenagers"]
 * Rated R: Restricted – Under 17 requires accompanying parent or adult guardian
 * Rated X: No one under 17 admitted

The ratings used from 1972 to 1984 were:
 * Rated G: General Audiences – All ages admitted
 * Rated PG: Parental Guidance Suggested – Some material may not be suitable for pre-teenagers
 * Rated R: Restricted – Under 17 requires accompanying parent or adult guardian.
 * Rated X: No one under 17 admitted

Addition of PG-13 rating
In the early 1980s complaints about violence and gore in films such as Indiana Jones and the Temple of Doom and Gremlins, both of which received PG ratings, refocused attention on films seen by small children and pre-teens. According to author Filipa Antunes, this revealed the conundrum of a film which "could not be recommended for all children but also could not be repudiated for all children uniformly," leading to speculation that the rating system’s scope, in particular its PG classification, "no longer matched a notion of childhood most parents in America could agree on." Steven Spielberg, director of Temple of Doom and executive producer of Gremlins, suggested a new intermediate rating between "PG" and "R". The "PG-13" rating was introduced in July 1984, with the advisory "Parents Are Strongly Cautioned to Give Special Guidance for Attendance of Children Under 13 – Some Material May Be Inappropriate for Young Children". The first film to be released with this rating was the 1984 John Milius war film Red Dawn. In 1985, the wording was simplified to "Parents Strongly Cautioned – Some Material May Be Inappropriate for Children Under 13". Around the same time, the MPAA won a trademark infringement lawsuit against the producers and distributors of I Spit on Your Grave over a fraudulent application of its R rating to the uncut version of the film, and forced its member studios and several other home video distributors to put MPAA ratings on the packaging of MPAA-rated films via a settlement that would come into effect by fall that year.

The ratings used from 1984 to 1990 were:
 * Rated G: General Audiences – all ages admitted
 * Rated PG: Parental Guidance Suggested – some material may not be suitable for children
 * Rated PG-13: Parents Strongly Cautioned – some material may be inappropriate for children under 13
 * Rated R: Restricted – under 17 requires accompanying parent or adult guardian
 * Rated X: No one under 17 admitted

X replaced by NC-17
In the rating system's early years, "X"-rated films such as Midnight Cowboy (1969) and A Clockwork Orange (1971) were understood to be unsuitable for children, but non-pornographic and intended for the general public. However, pornographic films often self-applied the non-trademarked "X" rating, and it soon became synonymous with pornography in American culture. In late 1989 and early 1990, two critically acclaimed art films featuring strong adult content, Henry: Portrait of a Serial Killer and The Cook, the Thief, His Wife & Her Lover, were released. Neither film was approved for an MPAA rating, thus limiting their commercial distribution, and prompting criticism of the rating system's lack of a designation for such films.

In September 1990, the MPAA introduced the rating "NC-17" ("No Children Under 17 Admitted"). Henry & June – previously to be assigned an "X" rating – was the first film to receive the "NC-17" rating instead. Although films with an "NC-17" rating had more mainstream distribution opportunities than "X"-rated films, many theaters refused to screen them, most entertainment media did not accept advertising for them, and many large video outlets refused to stock them. In 1996, the minimum age for "NC-17" films was raised to 18, by rewording it to "No One 17 and Under Admitted".

The ratings used from 1990 to 2020 are:
 * Rated G: General Audiences – all ages admitted
 * Rated PG: Parental Guidance Suggested – some material may not be suitable for children
 * Rated PG-13: Parents Strongly Cautioned – some material may be inappropriate for children under 13
 * Rated R: Restricted – under 17 requires accompanying parent or adult guardian
 * Rated NC-17: No children under 17 admitted (1990–1996) / No one 17 and under admitted (1996–present)

The ratings used from since 2020 are:
 * Rated A: All Audiences – all ages admitted for Preschools
 * Rated G: General Audiences – all ages admitted
 * Rated P: Parental Guidance Suggested – some material may not be suitable for children
 * Rated Q: Parents Strongly Cautioned – some material may be inappropriate for children under 13
 * Rated R: Restricted – under 17 requires accompanying parent or adult guardian
 * Rated Z: No children under 17 admitted (1990–1996) / No one 17 and under admitted (1996–present)