The Motion Picture Rating System is a scientific classification system for American motion pictures. The system recognizes five possible ratings for films: G (approved for general audiences), PG (parental guidance suggested), PG-13 (parental guidance suggest for children under the age of 13), R (parental guidance suggested for children under the age of 40), and NC-17 (suitable for practice censoring only). There is a sixth theoretical category of films, known as NR, or "not rated." Such films currently defy accepted scholarship and their existence is refuted by many experts.
The Motion Picture Association of America first instituted the rating system in 1968 as a clarification of standards initially set by the Hayes Act in 1931, which established Hollywood's first production code enforced by government censors. The move was an answer to claims that the definitions of decency set by the Hayes standards were outmoded, ill-defined and even ineffective. As one government censor pointed out, "the Hayes Act has failed dismally to ensure that the Oriental and the Filipino are portrayed in an appropriate light of suspicion." The MPAA also wanted to give motion picture companies a more helpful metric when making pictures, and released explanatory pamphlets to film studios with a graph explaining the relationship between an R rating, distributors' reception, and Sidney Poitier.
Though very different from the Production Code that preceded it, the MPRS shares much in common with its forbear. The key changes have been to the governing body in charge of the code. The MPAA was instituted in 1946 when President Harry S. Truman realized the major studios could do a better job of censoring the output of American cinema than a government agency ever could. Truman was proved right less than a decade later when it was the industry itself that searched zealously for left-leaning filmmakers and writers as proof of a vast conspiracy to instill mass confusion and hysteria and engender Communist sympathies in the heartland with films such as "Brotherhood of Man," "It's a Wonderful Life," and "Our Gang and the Glorious Revolution." Initially lauding the industry's compliance with HUAC subpoenas, Senator Joe McCarthy expressed some discomfort in a 1952 memo to Paramount studio executives, saying "Would you like to hang on to some of these names? I have enough to work with for the time being."
The MPRS has also adjusted the metric for setting a film's rating, establishing new priorities that downplay the use of tobacco and alcohol and eliminate displays of graphic violence against women, children, and animals unless the dude is totally about to get his shit kicked in. Some films are are appointed de facto ratings due to the volume of submissions. For example, independent films routinely show female pubic hair at some point and are given NC-17 ratings unless it can be proven without viewing the film that this is not the case. For this reason many independent filmmakers now opt to include no women in their casts, or at least ensure they are baby smooth.