American cinema can be divided into three eras of censorship, and William H. Hays played a part in all of them. The first era, wild and sexy, stirred a backlash to young Hollywood that allowed Hays to leave his political career and consolidate power in film. The second, from 1934 to 1968, saw the industry policed by the Motion Picture Production Code, colloquially known as the Hays Code. During the third period Hays’ ideas evolved into the MPAA rating system, when his influence could be felt in almost every undeserved R and bizarre NC-17.
Source: Sex in Cinema: The Hays Code, Censorship, and Film’s First Culture War