Spider-Man: Sorry, we're not allowed to accept unsolicited material.
This trope has been in effect since well before the internet made it much easier to proliferate fanfic. Studies of early Star Trek fanfiction showed as many as 90% of authors were female in the 1970s, even though at the time such stories could only be shared through fanzines or through sending self-addressed-stamped-envelopes to the authors and having them mail you a manuscript.
Various theories abound as to why this trope has persisted so prominently, such as the idea that men prefer visual porn while women prefer written porn, but this idea is easily disproven by the thriving yaoi doujinshi culture in Japan and written porn sites for men like nifty.org (not to mention the fact not all fanfic is pornographic). More likely is simply that reading prose fiction is simply a more common pastime for women: men account for only 20% of the fiction market, and similarly, nearly 80% of publishing industry staff are female. It's simply that books written by men get far more media coverage, leading to the distorted perception that the industry is not as female-dominated as it actually is.
Also a reason why Rape Is a Special Kind of Evil is frequently in play throughout fanfics intended for mature audiences (i.e. in fandoms with multiple villains, rape will frequently be one of, if not the, key issue that divides the invoker of Even Evil Has Standards from the very worst).
This phenomenon has been subjected to academic analysis by ethnographer Camille Bacon-Smith and MIT's Henry Jenkins. Jenkins suggests in Textual Poachers that fanfiction is a reaction on the part of a female audience trying to find their own pleasures in media that caters mostly to males.
See also Shipping, Slash Fic, Everyone Is Gay, Estrogen Brigade, Het Is Ew, Mary Sue. Compare Most Writers Are Male and Most Tropers Are Young Nerds. Combine with The Internet Is for Porn to get All Women Are Lustful.
Faced with the trope There Are No Girls on the Internet, this leads to the absurd conclusion that the massive numbers of Internet fanfic writers are all male, and are but a small minority of all fanfic writers. Either that, or they aren't the true authors of the fanfics they post.
Only examples where a character in the work writes a fanfic are to be added. Parodies and deconstructions should not be added.
- Mahou Sensei Negima! Haruna draws her own Doujinshi which is based of Mahou Shoujo Biblion and sells her works at Comiket.
- Genshiken: Ogiue also draws Yaoi Doujinshi, much to the great trouble of her Editor boyfriend who is...not a fan. Well, Sasahara gets the appeal of yaoi to her... it's just more than a little awkward when she draws him and ships him with one of his best friends. Other than being featured himself, he's surprisingly accepting.
- Harumi Fujiyoshi of Sayonara, Zetsubou-Sensei draws Doujinshi as well — one of them about her Teacher Itoshiki-sensei. As well as a pairing of Apple × Pine when everyone else likes Pine × Apple (the characters in question are expies of Coco and Nuts). In the third series she is seen drawing a Doujinshi starring Mobile Suit Gundam 00's Tieria (also voiced by Hiroshi Kamiya).
- Darker Than Black had an entire OVA episode devoted to a character who writes in-universe slash fanfics about the protagonist.
- Nobuko Yokokawa of Ojamajo Doremi writes stories loosely based on her friends and classmates with her best friend Aiko she idolizes as the Mary Sue protagonist in most of them. Later joined by the shy artist girl Miho Maruyama, who starts creating similar comics with her. With their stories involving romantic plots with their own self-insert characters along with Aiko's, it results in awkwardly heavy doses of Les Yay for a kids' anime.
- Hikaru Amano of Martian Successor Nadesico portrays this in the English dub. She writes Self Insert Slash Fics.
- Erika from Durarara!! openly discusses writing Slash Fics about her friends. She also ships Izaya/Shizuo.
- Tsuruko, a Canon Foreigner in Haiyore! Nyarko-san's second season, wrote a Yaoi Doujinshi about the series' male lead, Mahiro Yasaka. Her #1 ambition is to meet him face-to-face, blissfully unaware that he was utterly pissed when he found out about the Doujin and in fact bought a copy just to burn it (and keep it out of Nyarko's hands).
- Ban of Heartcatch Pretty Cure writes and draws doujinshi of the Cures. The episode we're introduced to him, he's stuck with Writer's Block over a scene involving Blossom, Marine and a guy who looks like a fusion of himself and Itsuki.
- Aya from Strawberry 100%. She is secretly writing a fantasy novel on her math notebook but lacks confidence of showing it. Junpei discovers the novel and encourages her to become a writer by sharing his dream of becoming a film maker, which he is also afraid to tell anyone.
- The new Ms. Marvel, Kamala Khan, writes Avengers and X-Men fanfics. Specifically, about the Avengers' adventures on "Planet Unicorn", and Wolverine's and Storm's in space (where they fight worm hole farting monsters for example).
- Averted in Loki: Agent Of Asgard where Loki admits they write and publish fan fics online. Slash fan fics. Which are apparently "the most horrible slash upon the Internet". Can you say troll?
- And there is Nancy's best friend of The Unbeatable Squirrel Girl. Her speciality are Cat Thor fics. Yes. Asking Thor if there are cats in Asgard is important research!
- If the 2016 All-New, All-Different Avengers Annual is any indication the trope is averted for the Marvel universe as a whole. This annual has a framing story about two people who write, and (argue about) fanfic, one is the aforementioned Kamala Khan, the other Miles Morales, and also includes 5 in-universe "fics", 3 by male 2 by female real life writers, and at least 2 would probably be written by guys in-universe too (one has a self-insert, the other a "go back to the kitchen" attitude).
- This trope forms part of the main plot in Rainbow Rowell's YA novel Fangirl.
- Due to time-travel shenanigans, the first human novel ever written in the Star Trek universe is a Captain Proton fanfic created by Lieutenant Veronica Fletcher of the starship Colombia, as per Star Trek: Destiny.
- The main character in the AU Sherlock Holmes story "Parallels" by Jenni Hill (written for the AU Sherlock Holmes collection 221 Baker Streets) is a girl called Jane who writes Johnlock slash which is actually about her and her friend Charlotte, and has become "internet famous. A little bit."
- Supernatural has an entire in-universe slash fandom, implied to be primarily female, writing about the show's main characters. The show also features a female slash fic writer as a recurring character, with serious stalking tendency.
- In the season two Psych episode with the case at the school for smart kids, one of the (male) students says to Shawn:
Student: Any leads? I'm adapting this into a Nancy Drew fanfic.
- Inverted with Glee; Sandy openly admits to writing fanfiction.
- It's implied that Willow of Buffy the Vampire Slayer used to write Doogie Howser, M.D. fanfic. (This is probably a Shout-Out to Neil Patrick Harris, the star of that show, who is a friend of Joss Whedon. And funnily enough, her eventual co-star on How I Met Your Mother.)
- In Degrassi: The Next Generation, Clare starts writing Mary Sue fanfic based on a fictional vampire book series called Fortnight.
- In an episode of Monk, Monk's self-proclaimed biggest fan asks him why doesn't he use his gun during a case. When he tells her that he doesn't carry one, she remembers that she made that detail up in a fanfic she wrote about him.
- Inverted in Parks and Recreation. Ben Wyatt is known to write Star Trek fanfic.
- In The Big Bang Theory, Amy is revealed to write Little House on the Prairie fanfiction about a girl named Amelia who falls in love with a time-travelling physicist named Cooper.
- In Dragon Age II, Isabela writes "friend fiction". We only get to hear a bit featuring Aveline and Donnic but according to Isabela, she does it for everyone.
- However, she does it only as a fan of Varric, who is also a professional writer, who uses his friends as thinly veiled "inspirations" all the time.
- In the Fable III side quest, The Pen is Mightier, one of the books is entitled The Pangs of Sunset. It is essentially just an erotic novel featuring the Heros from the previous game. It is implied to contain both Slash and Femslash and is written by a woman named Ilona Pureheart
- Garnet and Gure features this strip, wherein the largely female culture of fanfiction is explained.
- Questionable Content features two female fanfic writers, Tai and Marigold; Tai's stuff is apparently good (she writes a raved about Harry Potter piece), Marigold's, not so much. After a near miss where lesbian Tai hits on straight Marigold, the latter seeks to make up for the awkwardness by posting a story to Tai wherein Hermione and Ginny "go all the way."
- Aeris from VG Cats writes erotic fanfiction about Tidus and Auron, has dreams about Fox and Falco, and likes the idea of a hot molotov throbbing in Bill's hand.
- In Homestuck, the female Cherub Calliope (UranianUmbra) admits to writing fanfic about the Alpha Kids.
- Vriska has a diary from her ancestor who she strives to emulate, the notorious pirate Marquise Spineret Mindfang, who writes in an overwrought, awkwardly sexual, fanfiction-like style. When we meet Mindfang's alternate universe self, she is a fanfic writer. We also discover Meulin, who writes 'friendfic' speculating on ships.
- And then there's "Complacency of the Learned", a wizard fan fiction by Rose Lalonde.
- In Shortpacked!, Amber writes a Web Serial Novel that is a pastiche of Twilight.
- Rinnie from Blaster Nation is a female fanfiction writer, specialized in adult Slash Fics mostly involving heroes from comics. And Batman.
- Belle's dark secret in Pocket Princesses is that she writes Harry Potter fan fics.
- Girl Genius has a short story in which a teenage girl alters the Heterodyne Boys story she's telling to her siblings to include a Mary Sue Author Avatar - her mother apparently did the same when she told the stories as a teenager. It's actually a defense of fanfiction and author inserts, based on the fact that the dreams they inspire in people are more important than accuracy.
- Donut of Red vs. Blue mentions writing some Harry Potter fanfiction but well...
- Tea is made this in Yu-gi-oh Abridged.
- She writes Harry Potter fics, and ships Harry/Hedwig, in case you were curious.
- Similar to the Yu-gi-oh Abridged example, Twilight Sparkle is an avid fanfiction writer in Friendship is Witchcraft... except she writes fanfic about her friends. Creepy.
- The Nostalgia Chick is seen in a Flash Back apparently writing Lord of the Rings slashfic.
- Most of the students at The Official Fanfiction University of Middle-earth are female.
- In an episode of Family Guy, Peter insists that Meg is going to write Ugly Betty fanfiction for a career.
- Tina from Bob's Burgers has shelves dedicated to erotic fanfiction she's written. (In fact, she's written so much of it she's moved onto erotic friendfiction.)
Louise: Ooo! Have you shipped the janitor and the lunch lady, yet? I always thought they'd have cute kids.
Tina: I have, and they don't.
- Candace from Phineas and Ferb writes superhero fanfiction, according to Mission Marvel.
- The "Cabin Fever" episode of Sonic Boom has the male cast (and Sticks) stumbling upon Amy's play script "A Rose Without Thorns", with characters that are thinly-veiled expies of them.
- This Time article on fan fiction mentions this phenomenon.
- An article by the Guardian also mentions this observation, linking directly to this very page.
- Statistically, there are more female doujinshi creators than male, even among the ones who make porn. Comiket has always had a massive female attendance, and even some of the biggest names in eroge writing (ex: Tori, writer of the majority of the Rance series) are women.