The MPA Rating System began its life in 1934 as the Motion Picture Production Code. This self-censoring venture was undertaken by the Motion Picture Producers and Distributors of America (later the Motion Picture Association of America & finally just the Motion Picture Association) to avoid the passage of state-level censorship that would've created a nightmarish web of regulatory approval processes for film distribution. The Supreme Court had decided in 1915 that motion pictures were not protected as free speech, & a number of states had since set out to censor the content of films shown inside their jurisdiction. So rather than recut each film several times for different markets, the industry hired Presbyterian elder & former head of the Republican National Committee Will H. Hays to rehabilitate the its image & define the first content code.
Hays' conservative, religious background would drive the regulations, seeking to "eliminate vulgarity & suggestiveness" & to "emphasize good taste." And the single code would ensure that any movie released would have to be found acceptable in even the most conservative markets, leading to an era of censorship that would coincide with cultural shifts from the excess of the roaring '20s to the puritanism of the conservative 1950s. Lines like Dr. Frankenstein's "Now I know what it feels like to be God!," for examples, in response to giving life to his monster in 1931's Frankenstein were banned as "pointed profanity" by the time it's sequel, the Bride of Frankenstein, was released in 1935. It was no longer acceptable to compare oneself to God on film.
The restrictions would finally loosen in the late 1960s, with the MPA Rating System--featuring it's G, M, R, & X ratings--replacing the Motion Picture Production Code in 1968. This new code moved the focus away from dictating content & toward implementing age requirements for screenings, balancing censorship concerns with the perceived need to shelter minors from adult language, nudity, & depictions of violence, substance abuse, & sex. The new rating system would allow directors the leeway to release more explicit films & the X-Rated Midnight Cowboy even won an Academy Award for best picture in 1969. But the rating quickly became associated with pornography, which forced many directors to cut content in order to qualify as R-rated. Films like Sam Raimi's Evil Dead, which was originally given an X rating, received a re-cut in order to qualify for the R rating to reach a larger audience.
The X rating was renamed NC-17 in 1990, but the stigma stuck. And commercially viable films still sometimes go through multiple rounds of re-cuts in order to avoid the NC-17 rating. Rob Zombie's House of 1,000 Corpses, for instance, had to cut a full 15% of its runtime over 3 years in order to qualify for an R-rated theatrical release in 2003.
The complicated legacy of the R rating endures today, at once a symbol of late '60s cultural liberation, attached to many of the most adventurous films of the post-studio era, & also an artifact of the unavoidable consequences of marrying artistic & commercial interests. But regardless of the censorship performed in its name, the R rating will be forever associated with many of the most lurid & horrifying movies ever captured on film. And thank God for it.