I've learned of some very interesting things that happened in this country recently as a result of researching about The Dark Knight and the very different character of the Joker (Heath Ledger) in this film than the previous Joker played by Jack Nicholson.

Comics Code Authority

The Comics Code Authority (CCA) is part of the Comics Magazine Association of America (CMAA), and was created to regulate the content of comic books in the United States...

...The CCA was created in 1954 as part of the CMAA in response to public concern about what was deemed inappropriate material in many comic books. This included graphic depictions of violence and gore in crime and horror comics, as well as the sexual innuendo of what aficionados refer to as good girl art...

...the CCA prohibited the presentation of "policemen, judges, government officials, and respected institutions ... in such a way as to create disrespect for established authority." But it added the requirements that "in every instance good shall triumph over evil" and discouraged "instances of law enforcement officers dying as a result of a criminal's activities."...

...Depictions of "excessive violence" were forbidden, as were "lurid, unsavory, gruesome illustrations." Vampires, werewolves, ghouls and zombies could not be portrayed. In addition, comics could not use the words "horror" or "terror" in their titles. The use of the word "crime" was subject to numerous restrictions....

And then of course I did research about the further back-in-time censorship of the film industry.

United States Motion Picture Production Code of 1930

The Production Code (also known as the Hays Code) was the set of industry censorship guidelines governing the production of United States motion pictures. The Motion Pictures Producers and Distributors Association (MPPDA), which later became the Motion Picture Association of America (MPAA), adopted the code in 1930, began effectively enforcing it in 1934...

The Production Code enumerated three "General Principles" 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.

And then of course the more previous unanimous ruling of the Supreme Court of the United States that the First Amendment of the United States Constitution did not extend to motion pictures:

Mutual Film Corporation v. Industrial Commission of Ohio

The Court described movies in some technical detail and noted their popularity, but wrote "they may be used for evil," and for this reason, "We cannot regard [the censorship of movies] as beyond the power of government."