The latest issue of reason–if you were a subscriber, you'd be done reading it already!–has an excellent piece by Thomas Doherty about the ugly birth of the motion-picture code. (The current ish has a ton more in it too, all of which makes the $19.97 sub price an incredible bargain).
A recent paper from the Competitive Enterprise Institute takes a long look at how well various ratings systems work. A snippet:
While no media ratings system can or will ever achieve perfection, the best rating systems have three attributes: They attempt to describe, rather than prescribe, what entertainment media should contain; they are particularly suited to their particular media forms; and they were created with little or no direct input from government. We also find that ratings systems collapse, it simply results in the creation of better ratings systems….
Ultimately, ratings systems cannot influence the content of what gets produced in the long run. Even the highly prescriptive Comics Code did nothing to stop the emergence of graphic novels with adult themes and situations. Those who want to "clean up" media without unconstitutional government censorship will likely do best to simply avoid buying cultural products they dislike.