TV Tropes pages here, here, here, here, here, here, here, and here.
Plus here, here, here, here,  here, here, here, here, here, here, and here.
And here, here, here, here, and here, for pages I saw but I can’t quite figure out how to make use of.
Note: this may be the most rabbit-hole of a Trope Time post I have written so far.

What is Amnesia:

In case you don’t know, Amnesia is when memory loss has occurred, either partially or completely. In real life, it can be caused by trauma, injury, fatigue, illness, repression, along with a few other causes. In science fiction and fantasy it can be caused by the same things as real life, or any number of other means: magic spells, potions, or powers, gods, authors who need the character to have amnesia no matter what, technology specifically designed for that purpose, amongst anything else imagination can come up with.

There are two main types of amnesia that happen: retrograde amnesia and anterograde amnesia. Retrograde is when the person forgets their past and they lose information pertaining to who they are while leaving other things intact. While this is most common in media, anterograde amnesia is more common in real life. Anterograde amnesia is when the person cannot form new memories, either temporarily or permanently. Amnesia can also be selective, where people willfully overlook or ignore things going on. 

Where do you see Amnesia:

The obvious answer, of course, is you see amnesia in every genre and in every medium. (See my side note at the bottom.)

The less obvious answer is where in the story amnesia occurs. You can often see amnesia occuring at the start or prior to the start of the story, as a foundation for what is to come. It is much rare to see the end of the story result in amnesia, though it does happen. But an interesting observation I have noticed is the longer a series, typically episodic, goes on, the more likely it is to have amnesia in it. See graph below:jokegraph.png

Whether in TV, books, webcomics, or any other medium, the longer the story goes on the more likely an amnesiatic event is going to happen. It can be minor, like the character has to relearn the same thing over and over again. Or it can be major and every single main character forgets who they are because a spell backfired. Not naming any names, Willow.

How Amnesia Appears:

Moral Amnesia AKA Ronald Weasley from Harry Potter

Perhaps the least serious but the most common on this list. This form of amnesia has a character learning something that makes them a better person, and then they forget it to be relearned later. Which is what happens when Ron has to learn not to be jealous of Harry every book, or learn he is good at Quidditch again and again, or any number of other Ron Weasley things.. Or the entirety of Peter Pan, who cannot learn lessons at all or he’ll grow up.

Trauma-Induced Amnesia AKA Jedao from The Machineries of Empire

More rooted in reality, the characters who appear with this form of the trope forget things because of trauma. The trauma can be physical, but it can also be a severe, emotionally traumatic event. This either wipes the episode of trauma from the brain, often with selective bits before/after the trauma, or it can be more widespread and they forget more. Characters often recover these memories over time, through magic, or rarely therapy.

Transformation-Induced Amnesia AKA Dr. Jekyll and Mr. Hyde

This one is very common in the SFF genres. A character transforms, typically involuntarily, and they don’t remember what happens when they have transformed. Most notably, this form of the genre is commonly seen in the averted form, such as with Dr. Jekyll and Mr. Hyde who do actually remember everything from their transformation. This is also something characters tend to grow out of over time, especially when it comes to werewolves because they get a handle on what they are going through over time.

Death-Induced Amnesia AKA Dr. Daniel Jackson from Stargate SG-1

If not exclusive to SFF, it is very uncommon outside of the genres. Someone dies, not naming any names Jackson, and they come back but they don’t remember anything about what happens after death.

See also: Ghost Amnesia, when ghosts don’t realize they are dead.

Identity Amnesia AKA Doctor Who

The most serious form of amnesia on this list. Something happens and the character forgets who they are. This typically leads to them questing to find out who they are. They also commonly become new people. Typically temporary, the memory loss can sometimes be permanent or have gaps. Sometimes it is less severe and they only forget certain specific things. It is also possible that someone deliberately induced amnesia in someone else to try and change some part of who they are or get them out of the way.

How Amnesia is Used:

  • To show character growth in the main characters. They become who they were before the events of the story started, when they were worse people or in a worse situation. Used to show just how far the characters have come.
  • The memories of a certain event or events have been removed, but the characters retain the sense that they have lost something. This often leads to the characters trying to figure out what it is that was lost, for better or worse.
  • Someone, or something, implanted memories. The amnesiac themselves could have done the implanting. The implanted memories could be of something that didn’t happen or something that did happen but an altered version of events. This tends to go supremely poorly.
  • Part of, or entirely, what the character has forgotten is the relationship they have with someone they love. Causes some tension in the relationship and is typically used when someone feels the relationship is going too well and something needs to shake it up and make it go wrong. Also used to invoke that “Awww, they love each other again even though they don’t remember everything!” emotion in the reader or viewer.
  • A bad character gets amnesia, and now they’re a good person. They are innocent and often have childlike qualities. Used to show that the bad person is a good person at heart, something just went wrong to make them act this way.
  • The amnesiac themself is a god, and for whatever reason they lost their memory and are now walking around with us mere mortals (or am I…). They typically either don’t have their god powers at all, or they have limited access. Rarely do they have access to all their powers which goes very poorly. Once these characters remember who and what they are, they get access to their full powers and save the day. Because they are a god.
  • Our typical hero or good guy gets amnesia, but the first people to come across them are our villains who now convince the hero that they are allies. Typically used to make the hero fight against his real allies. Also typically requires a lot of coincidence and the hero to just believe everything they are told.
  • A character is pretending to have amnesia, for any number of reasons.
  • The hero forgets who they are and they go to live a simple life until it their memory is restored again or they are found and told who they are. The most often way I have personally seen this used is to give the hero a much needed break.
  • A Masquerade is in effect, and if a normal person finds out something they shouldn’t have, they end up with their mind wiped. This ensures secrecy or allows the characters to return to normalcy after the damage has been done to them.

As evidenced by the amount of links at the top, this is a trope with a lot of permutations.
What are your favourite examples of amnesia?
Which forms do you hate?
Which do you wish you could see more of?
Do you think stories with more extreme forms of amnesia handle the consequences well?
Does amnesia even look like a word to you anymore after all this?

There is so much to discuss on this topic, have fun with the comments.

Side Note: It is always an amazing sight when you see the section “Pro Wrestling” while researching a trope on TV Tropes.