Hays Code
The Hays Code, or Motion Production Picture Code, was introduced in 1930. Regular enforcement of the Code began in 1934, as a way to combat the perceived problems with "talkies." The general principles of the code were as follows:

1.No picture shall be produced that will lower the moral standards of those who see it. Hence the sympathy of the audience should never be thrown to the side of crime, wrongdoing, evil or sin.

2. Correct standards of life, subject only to the requirements of drama and entertainment, shall be presented.

3.Law, natural or human, shall not be ridiculed, nor shall sympathy be created for its violation.

The full text of the Hays Code may be read here.