Fan fiction, what is allowed?

I'm curious about the recent regulation or whatever it is, on fan fiction. What is this new law; is it in the US or in the UK and what does it mean? I have not heard of anything similar over here, but I might have missed it completely. EU directive? I ask, because I notice that there are a multitude of American based web-sites that more or less pump out superheroine porn (of which some to my delight actually involve spanking), with actors often wearing exact copies of the heroine they are supposed to impersonate. They however often have a slightly different name, but the disclosure for the movies often states the name of the character imitated and adds that the movie is a parody and that the real character is the copyright of Marvel/DC or whichever universe she comes from. Also, Vivid pumps out hardcore copies of almost every major superhero movie there is, advertising all over the Internet, and they consistently seem to get away with it. There are a multitude of examples, but I only pick some porn ones here, as I assume that they are the most sensitive to the trademark owners. So, what is this fan fiction restriction that has been mentioned in some of the threads on site? Btw, completely off topic, the only "fan-fiction" I know that has been taken down here in Sweden was a quite amusing comic book clone of Donald Duck in the 1980s, in which he jumps the Berlin wall in his typical red, roofless car, to paint offending insults to Honecker on the occupied side and eventually gets shot dead by East German border guards. It was quite a hilarious comic really. It didn't make it past Disney's lawyers though. Neither did the Danish rock band "Disneyland After Dark". But they easily solved it by renaming the band to D.A.D. which everyone knows what it stands for, without anyone being able to say that they are infringing on the Disney trademark. So, Supergirl, becoming Superior Girl and Wonder Woman becoming Wondrous Woman, just happening to wear the exact same outfit and problem solved, or...?

Disney can be quite litigious. I remember they threatened Marvel (before they bought them) with legal action, because the character of Howard the Duck was too much like Donald. Marvel solved it by putting pants on Howard. Marvel themselves served a cease and desist order on independent publisher Dave Sim over his Wolveroach character who was a direct parody of Wolverine.

I am not a laywer, but at least in the US my general understanding is that case law has established that parody is fair use, and that's why all the porn stuff is called parodies.

That being said, what is and isn't fair use isn't obvious in many cases, and standard fan fiction *definitely* doesn't fall under it. So these porn companies are probably at risk of being served a cease and desist letter, and perhaps even sued (though it's not clear to me which side would win). Same for fan fiction authors, except the case would be much more clear cut (in favor of the copyright holder).

Could be why you see a lot of DC "parodies" (Superior Girl, and Wondrous Woman), but not a whole lot of Marvel parodies. Perhaps DC is much less litigious than Disney.

For an earlier example of Disney's legal actions against parodies, you might want to read Bob Levin's book, the Pirates and the Mouse, which chronicles Disney's war against the creators of the Air Pirates comic books. Disney won a judgement against the Air Pirates creators well beyond their means to pay.