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"The awakened, the enlightened man says: I am body entirely, and nothing beside; and soul is only a word for something in the body. Your body is a great intelligence... Your little intelligence, brother, which you call "spirit", is also an instrument of your body, a little instrument and toy of your great intelligence."note Original quotation in German: Aber der Erwachte, der Wissende sagt: Leib bin ich ganz und gar, und Nichts ausserdem; und Seele ist nur ein Wort fuer ein Etwas am Leibe. Der Leib ist eine grosse Vernunft... Werkzeug deines Leibes ist auch deine kleine Vernunft, mein Bruder, die du “Geist” nennst, ein kleines Werk−und Spielzeug deiner grossen Vernunft.
The classic Wolf Man fits this trope perfectly: a normal human most of the month but a rabid wolf/wolfman under a full moon.
Anime and Manga
When a Claymore over uses their powers and is transformed into an Awakened being, their personalities are changed, meaning a Claymore that would once thought they would have killed themselves if they transformed will no longer even consider doing that.
Makaku in Battle Angel Alita explicitly references this idea when he becomes quite intelligent after losing his initial large cyborg body and is reduced to an ambulatory head.
Inverted in one chapter of Cat-Eyed Boy. A Mad Scientist attempts brain transplants... but later discovers they can't be successfully performed, as the new mind housed in the body will eventually force its new body to match the one it was originally housed in.
Averted with 007 from Cyborg 009. He's the local Master of Disguise / Rubber Man and a classically trained actor whose powers can literally transform him into someone else... but once he dispells his covers, we see that his mindset hasn't been altered, and he's still good old G.B.
In Darker Than Black, Body Snatcher Mao needs Phlebotinum to maintain a human level of consciousness; without it, his personality is eventually overriden by the instincts of the cat whose body he inhabits.
Happens in Digimon Frontier when Takuya first attains his Vritramon/Burning Greymon form. He flips out and nearly takes Tomoki's head off, until one of Tomoki's tears falls into Takuya's eye, causing him to regain his sanity. While all of them had trouble controlling themselves in their beast transformations at first, only Takuya lost complete control. Except Izumi, for some reason; she attributes her ease of control to being a girl, but says it so obnoxiously that the boys feel ready to throw up. Izumi happens to also have a "beast" transformation that's basically still human in all but the most minor aspects and in fact qualifies as an in-universe case of Ms. Fanservice. Koichi also had no trouble, but he's the group's Sixth Ranger, had previous experience using it while still Brainwashed and Crazy, and by the time he joined up the beast transformations were So Last Season.
Averted in Digimon Tamers: a primary point was that the personalities of the Digimon in this series stayed the same; regardless of the previous Digimon rules. The English version tried to show this by having all the forms have the same voice
Interestingly, as Guilmon goes further up his evolutionary change in the dub, he actually seems more intelligent, even snarky at times. For example, while Guilmon was basically a five year old, and thus not really prone to insults, by comparison in the dub Wargrowlmon made comments like "Hold it hog breath!" and "Whaddaya expect, he's a pig!" when battling the pig Deva. He seems to revert to the more childlike mentality somewhat in the combined Mega form of Dukemon/Gallantmon, but this might be a side effect of merging with Takato. In Guilmon's case, the trope is played utterly straight when he is forced to evolve to the corrupted Mega form called Megidramon, which is a near mindless monster that nearly ripped apart the Digital World simply by existing!
As a general rule, Digimon become more brutish with stronger forms unless they cross the Bishonen Line, while how much their actual personality changes varies by how much personality they have to begin with.
In Dragon Ball, Saiyans who still have their tails turn into gigantic weremonkeys when exposed to the light of the full moon (or a similar enough substitute), becoming savage and violent no matter how docile they are in their humanlike form. Only Saiyans who have had the proper training can stay in control while transformed. Likewise, there's the Super Saiyan transformations, which turn even the nicest Saiyans (like Goku and Gohan) into the ruthless, cold-hearted warriors Saiyans are known as throughout the universe, with no desire other than to exact painful revenge on whoever angered them enough to trigger the transformation.
In a different kind of example, Albireo Imma's Shapeshifter Weapon allows him to become the person he transforms into: all the memories, actions, personality traits and instincts he observes of them are recorded into his many books (he has one for every person he's met). While he's subconsciously aware of his own presence, his body acts completely as the person would, so much that the person themself is essentially speaking. He's able to take over when he needs to, but for the most part the consciousness he imitates is in the driver's seat.
Also is Shiori's persona stealing ability which allows her to do largely the same thing, only for her she almost completely blanks out of consciousness to the point of thinking she's the person. However, she does leap instantly to Negi's defense and gets badly injured, though the only one to notice the lack of magic canceling was the Governor General. Also appears to have a crush on Negi... make of that what you will. Shiori is apparently aware, behind the scenes, of what "Asuna" does. But not the other way around, until she starts having feelings for Negi.
In Ranma ˝, the vast majority of the shapeshifting characters gradually (though not consciously) acclimate to their cursed forms, learning how best to take advantage of their traits (such as size, flight, dexterity, or appearance) and paying heed to their natural instincts.
The most notorious example is with Ranma him/herself: although his personality and sexuality remain untouched, he grows considerably more comfortable with his female self, adopting feminine mannerisms and body language. Halfway through the series, (s)he thinks nothing of dressing up nice and going on dates with men (usually for some ulterior motive, but sometimes solely for the guy's benefit) even though the mere thought of wearing a skirt was anathema to him at the beginning of the story. But is it the body working on the mind, or just gradual desensitizing?
In a Filler episode, Ranma gets knocked on the head while in girl form, making "her" believe she's really a girl (an extremely Girly Girl) who unfortunately changes into a boy. This painfully stereotypical behavior is probably just a reflection of Ranma's own mental image of what "normal girls" are supposed to be like.
An extreme example is Miss Hinako Ninomiya: due to an unspecified illness when she was a little girl and Happosai's chi-tampering, her body was stuck with the appearance of a ten year-old, even though she's somewhere in her late twenties. She acts like an obnoxious, bratty, bossy, hyperactive child while in kid form, but when she absorbs enough Battle Aura to temporarily restore her adult body, she's much more aloof and calculating.
Another good example is Ranma's father Genma, who actually seems to enjoy being a panda more than being a human.
Then there's Rouge, who turns into an Asura - a three-headed, six-armed, fire-breathing demon-goddess. Normally, she's a kind-hearted Chinese Girl, but in her cursed form she's got a case of A God Am I, and her temper disappears. Her irritability is partially explained as a result of the agonizing back-aches caused by having three sets of shoulders. In contrast, Pantyhose Taro turns from bishonen to minotaur-freakshow, without the slightest change in his Jerk Ass personality, although he seems less calculating and more instinctive as a chimera. In his case, he was baptised in the cursed spring soon after he was born. Changing from human to minotaur and back again for all your life is bound to result in a more consistent personality.
Used and subverted in The World God Only Knows. After a gender bending"Freaky Friday" Flip, the boy begins acting more feminine and gains an interest in dating sims for women. However, this is because the evil spirit inhabiting the girl's body is manipulating his emotions as the girl has no such problems while in his body. May also be the case for the goddesses unless Keima really is that much of a chick magnet. Diana has turned tsundere for Keima and Vulcan appears to be following suit.
Inugami Hatsune, a member of Zettai Karen Children Esper team "The Hound," suffers from this. Her powers allow her to technically assume the form of an animal (typically a canine) but the longer she spends in a form the more animal-like she becomes. Generally, the only way to snap her out of it is to calm her down by feeding her. This is the task of her partner, Yadorigi Akira, whose mind-switch powers allow him to throw prey in front of her to catch (and also forces him to experience the feeling of prey being eaten each time).
Once you become a vampire in the world of Kurobara Alice, you retain your old memories - but your mindset starts changing. In Dimitri's case, he becomes less emotional and more regal as the "heir" of the deceased vampire leader, Lord Bradley; when the story sets in the present, he's pretty much an Emotionless Boy.
Snowbird, of Marvel Comics' Alpha Flight, runs the risk of Shapeshifter Mode Lock if she stays in animal form too long—because her mind gets subsumed by the animal. She once had to be talked down by Wolverine while she was an actual wolverine.
A combination of both seems most likely. Since he experiences all of his life simultaneously he becomes distant, but having nigh-limitless power and an indestructible body certainly changed things for him as well.
Jennifer Walters started out as a somewhat demure lawyer - her transformation into She-Hulk initially made her far more aggressive and angry. Over the years though, her continued use of the She-Hulk form has meant that the effect of her powered up form is simply a far more uninhibited version of herself.
Explored in Spider-Man with the Lizard. Made famous by the film; genetic genius Dr. Curt Connors attempts to create a revolutionary regenerative serum from cross species genetics with reptilian DNA and ends up mutating into a large, green, scaly monster.
Also in the Comics; the Six Arms Saga, Peter Parker's attempts at removing Spider-Man's powers using a serum backfires, at first he grows four additional arms, and then mutates into a gigantic spider-human hybrid called Man-Spider. He loses complete control of himself and attacks and overpowers many superheroes attempting to restrain him.
ElfQuest: The explanation for why Timmain eventually abandoned her people, and later, why she remained white-furred even though it was dangerous: She was very deeply subsumed into the wolf-form she originally shapeshifted into for survival.
An unfortunate side-effect of Fauna's powers in DC Nation. Yes, she can do a partial shape-shift into most animals, as well as speak to and persuade animals to ally with her, but the harder she pushes her abilities, or the more wounded/angry she becomes in a battle situation, and the less she thinks like a human and the more she thinks like a beast. Worse, she remembers almost nothing of what happened during one of those "freak outs" afterward. As she told Changeling once, "You keep the mind of a man when you shift; I don't."
George in With Strings Attached. His ring allows him to transform into any animal he can imagine, but becoming that animal also causes him to gain the senses, instincts and tastes of that particular creature. A few times, he has to exert his own will to keep from doing something inappropriate, like eating something (or someone) he's not supposed to.
This is a core theme of Becoming Ponies. The counterparts are all having their personality slowly overridden with that of their associated character, and some of them don't even realize it.
Harry faces this problem in No Longer Human since (as the title indicates) he isn't fully human anymore but a hybrid of a phoenix, basilisk, and human. When Ginny comes to him crying that the attacks were her fault and that she was controlled by Riddle's diary, all Harry feels is annoyance that she's getting his shirt wet and anger that she "allowed" herself to be controlled.
In Walk Through the Valley by Vathara, Hiko transforms uses LEGO Genetics to transform himself into a kiryuu alterant and kiryuu are felinoid aliens from the planet Satoyama that possesses Complete Immortality (anything that doesn't kill them immediately tends not to kill them at all), Innate Night Vision so strong that they very myopic during the day, sensory tendrils in place of hair, EMP-generation and perception which allows to share thoughts with each other, made possible by the sensory tendrils; and they are Knight Templar Parents that will kill any predators that come near the birthing den. Upon becoming half-man, half-kiryuu, he finds himself mentally declaring the cabin and where he woke up and everything within sight of it to be his when he'd previously never stayed anywhere more than a year at a time (except for his stint on Satoyama) and didn't allow himself to get attached and contemplating leaping sixty feet into the air to chase a bushy-tailed rodent on a tree. That was in the first week. Within five years, he finds himself killing in an Unstoppable Rage for the first time just because the victim, a member of the Oniwabanshuu, was under Mind Control like all citizens of The Federation to protect them from The Evils of Free Will (and the Oniwabanshuu are members of the Human Subspecies empowered to do the protecting through Telepathy). After that, his mountain was safe. Later, he finds himself with a dying eight-year-old escaped slave boy (Kenshin) and the only way to save him is an Emergency Transformation into a kiryuu alterant. Previously, Hiko had been a Child Hater, or, at least, wasn't interested in making sacrifices for a child such as free time or career. But after the second time Kenshin's skin shed, he found himself with unable to keep from growling at one of his few friends whenever she made sudden movements towards the cabin. This transformation lasted months. Months he had to spend working with the instincts until they re-categorized her as a "spark-ally" (sparks are what kiryuu packs are called).
Navarone struggles with this in Diaries of a Madman, being particularly vulnerable to pony pheromones whenever he's turned into a pegasus. When gender switched for a prolonged period of time, he also has trouble dealing with certain urges while female.
This is a major theme in The Land Before Time fanfiction The Seven Hunters. Although Littlefoot and the other members of the gang retain their personalities after their transformation into carnivores, they slowly begin to suffer from a personality shift as time goes on. They even begin to enjoy their hunts despite the realities of what that entails.
In the My Favorite Martian movie, the alien polymorph gum can alter one's personality, depending on the alien. This comes up later, where the hero's girlfriend chews one, turns into a giant alien, and kills a couple MIB guards, even eating one. Afterward, she acts as if she had just been in a drunken party.
Subverted in the Day of the Dead (2008) remake. Bub is a vegetarian and thus doesn't want brains, and he follows orders due to being a marine and/or having a crush on Cross.
Subverted in Land of the Dead. The gas station attendant zombie initially remembers that he's supposed to do something with the gas pumps. As the movie progresses, he regains more and more of his humanity, including compassion for his fellow zombies, tool and gun use, and even leadership.
In The Fly (1986), the scientific genius Seth Brundle fuses with a fly, and as his body slowly begins to morph horribly, he realizes that, to his terror, his personality is being overridden by the instincts of the mindless insect. Arguably, the last we see of his original personality is when the crippled wreck that Brundle has become mutely begs for death.
Averted by Seth's son Martin in the sequel The Fly II. Martinfly retains his human mind even after his transformation.
Averted in Captain America: The First Avenger as even after post-procedure Steve Rogers still acts dorky and as Peggy notes, still cannot talk to women. In fact, this was why Erksine picked Steve and tells him as such.
To some degree, it was inverted with Schmidt who developed his ... unusual features in response to the serum because the serum hadn't been perfected yet and he was an evil man at heart. As its creator said, good becomes better, bad becomes worse.
This was a problem for Queen Elinor and the prince in the legend in Brave.
In Piers Anthony's very first Xanth book, Magician Trent can change anyone into any animal. The form comes with instincts built in. Trent said something like (paraphrasing): "If you're a turtle, unless you have the instincts of a turtle, you couldn't survive as a turtle."
Later books introduced a character who could shape-shift but didn't get the instincts, making it a much more limited talent. This was due to her demonic heritage; it wasn't even her personal magic.
Especially nasty in his Soul Rider series, where the revolutionary state of New Eden imposes this on its female population by a binding spell. A transformed "Fluxgirl" is literally ruled by her body, which now craves constant sex, has difficulty with concentration on long-term projects, and cannot read, write, calculate, or figure out complicated machinery.
Chalker also specifically inverts this trope in The Identity Matrix where the body can be made the plaything of the mind by making mental adjustments that influence the endocrine system, affecting things like muscle development and fat distribution.
In The Belgariad, Belgarath pointed out that transforming into an eagle wouldn't necessarily help you travel faster, because you were likely to get sidetracked by every tasty animal below.
However, he did manage to resist Poledra as a wolf.
He also mentioned that his mind was significantly more compatible with that of a wolf than that of an eagle, and thus he was able to think more clearly.
Note also that after Garion shifts for the first time, his thoughts become permanently more wolflike, and wolves consider him one of them. At one point he's challenged by a pack whose territory he has entered, when they would have just avoided humans.
In Robin Hobb's Farseer Trilogy, Fitz fakes his death by abandoning his body and binding his mind to Nighteyes's. When he is brought back into his body it takes him half a year to remember how to be human, and even then he is much more wolfish than before.
Shapeshifting in Ursula K. Le Guin's Earthsea Trilogy also works this way: The longer you stay in a given form, the more your mind gets taken over by that form's instincts. There are allegedly a number of mages who turned themselves permanently into dolphins this way; in the first book, Ged almost loses his mind when he's forced to flee over a long distance as a hawk.
One noteworthy legend in Earthsea is the story of a wizard who so enjoyed turning himself into a bear that he ultimately became the bear and killed his own son.
In HP Lovecraft's Cthulhu Mythos stories, the outer god Nyarlathotep is said to have a thousand forms. Each form is essentially a separate entity (it's implied that he can even manifest several in forms at once). As such, the behaviour and personality of different forms can be completely different.
This is only vaguely implied in the original works, but considering that Nyarlathotep's different forms functions essentially identically to the Avatars of Hindu mythology, this is indeed probably the case, although a central intelligence connects them all in some sense.
In Oath of Gold, the third volume of Elizabeth Moon's The Deed of Paksenarrion, the Kuakgan talks about this with Paks, who was unable to continue her life as a paladin-in-training after being captured, tormented, and rescued, and who is at the brink of suicide when he finds her leaving her worldly goods in his grove's offering-bowl. She also hasn't eaten properly for quite a while, and has a lot of half-healed wounds. The analogy he draws between the mind and the body is that of the snail and the snail-shell; if you poke holes in the snail-shell, will the snail live?
In Garth Nix's Old Kingdom trilogy, Lirael can make skins that allow her to take on the form of an otter, or a bear, or an owl, but each one alters her temperament for a while, even when she takes them off (the otter-skin gives her a great craving for fish, for example).
Terry Pratchett's Discworld series plays it relatively straight. One notable example, and a reference back to it in a later book, likens the mind to water and the body to a jug or vase. The water is only water, it does not become the container, but it does take on the container's shape.
The longer Discworldwerewolves spend in wolf form, the more they start to think like a wolf. The reverse is also true: They become more human the longer they spend in human form. They never entirely become one or the other.
The same is true of Borrowing: The longer you inhabit something else's brain, the more you start thinking like that something else. In Equal Rites, Esk nearly starves to death because she loses track of her human self, but doesn't have the proper instincts to feed herself as a falcon.
The Great God Om points out in Small Gods that the shape of the body influences the shape of the mind, so if you spend too long in the body of, say, a tortoise, you begin to think like a tortoise.
When the Auditors create human bodies for themselves, they eventually start feeling emotions. The end result is a mix of Auditor and human that has serious trouble being either.
The point of the 'water in a jug' analogy is Myria LeJean's realization that it is wrong; for a mind in a body, the 'water' is truly changed by the jug.
And then there's the bit where Granny Weatherwax Borrows the Unseen University, and thinks like a building upon coming out of it.
Addressed in a more technical form in The Science Of Discworld, in which the complicit relationship between physical brain processes, external experience, and minds are discussed.
The History Monks have a Dalai Lama-type figure whose wisdom reincarnates throughout the ages and is present from birth. As his current incarnation is a baby, however, his sage wisdom often loses out to sentiments like "wanna bikkit."
In Animorphs, every morph came with its own set of animal instincts; these were mostly useful but some morphs such as an ant had instincts so strong they risked permanently overpowering the human personality.
Termites caused even greater problems, since they were entirely controlled by pheromones the bodies were hard-wired to obey.
Taxxons were horrifying.
This trope plays both ways in Harry Potter: If a person is involuntarily transfigured into an animal(and this includes werewolves), then they lose their human mind for the duration of the spell. However, with years of study, and at great risk, a wizard can train him/her self to transform into an animal at will and retain their human mind - mostly. (Wormtail suffered no ill effects from being a rat for twelve years straight, but Sirius sometimes turned into a dog to ameliorate the stress caused by Dementors.)
Sirius also once says that James had always joked he had a much sweeter disposition as a dog, implying that there are at least mild changes in personality/attitude which reflect the behavioral patterns of the animal they transform into. Of course this aspect is debatable because of the fact that the wizard doesn't choose which animal they transform into, they find out only once they've completed the training and see what they become (much like the Patronus charm), and implies that the animagus form reveals, in part, the true nature of the character in question. Even so, it is easy to see that a character like Sirius may have very playful and loyal dog-like nature, but frequently behave in harsh or unforgiving ways due to the emotional and psychological trauma of his human life, which might be less at the forefront of his mind because of the less linear nature of the dog's mind. It is also the way he escaped Azkaban without notice — the Dementors could not detect the less complex dog mind, and therefore didn't realize a human was leaving the prison.
Likewise, a Horcrux is Nigh Invulnerable because it's the inverse of a mortal—the spirit it contains will instantly die if it's destroyed, but the physical self regenerates.
The author also noted that animagi tend to suffer a sharp decrease in IQ when in animal form, presumably due to the shrinkage in brain size.
The only note of that type I remember seems to have been about regular transformations, with a statement that it doesn't happen to animagi.
In Sherwood Smith's Wren to the Rescue, the protagonist is turned into a dog, and is explicitly warned that the longer she spends in that form, the more difficulty she will have retaining her human mind; she gets out of it with no more than a few transitory dog habits. In the sequel, her friend Tyron is forced to spend a significantly longer time as a dog and only retains his identity by extreme effort.
There's a several pages long passage in Three Men in a Boat, detailing how a person's mood depends entirely on what they are eating, and what this food is doing to the body.
In the Wild Cards shared world novels, edited by George R.R. Martin, Jeremiah Strauss is a shapechanger who turned himself into King Kong and then spent several years acting like a gorilla and trying to carry blonde women to the top of the Empire State Building. Later transformations into a werewolf from The Howling and the Creature from the Black Lagoon almost turned out badly as well.
In The Immortals by Tamora Pierce, Daine finds that thanks to mind-speaking with and turning into animals, that she can no longer eat wild game. She can still eat beef, pork, chicken, and fish because she's never had an interest in inhabiting the minds or taking on the shapes of barnyard animals or fish for a while, but by Realms of the Gods she can't even do that.
We're told in the Five Hundred Kingdoms series by Mercedes Lackey that only the most powerful Sorcerers or Sorceresses can transform a person into an animal without that person losing his mind to the animal. However, since the books focus on Godmothers, we don't see this in action. In the first book, Elena transforms Alexander into a donkey, but has to give him regular time as a human. In "The Snow Queen", Aleksia takes several animal forms, but even as a polar bear (which has a large enough brain to "fit" a human mind into) the animal instincts get stronger the longer she remains changed.
The skinchangers (who do not physically transform but can telepathically possess certain animals) in A Song of Ice and Fire have trouble remembering their human selves. When Bran Stark is being taught to control his power, he is told to do certain things as a wolf to help keep his mind in control. He keeps forgetting to do these things, and as a result he acts largely on instinct while in wolf form. Some of the older, more experienced skinchangers seem to be able to use their human intelligence more effectively, however, though it is mentioned that certain forms increase the risk of losing touch with humanity. When a skinchanger's human body dies, they can possess their animals to keep on living, but eventually the animal forgets its human life and becomes an ordinary beast.
Taken to an extreme in Dennis L. McKiernan's Mithgar series. A shapeshifter in their animal form is said to have the mind of that animal, albeit one that will occasionally have thoughts similar to those of a human. The flip side of this is that a shapeshifter in human form will occasionally have thoughts similar to those of the animal they transform into.
In a Dragonlance short story, an If I Can't Have You plot winds up with most of the cast shapeshifted. If they do not wear something belonging to their human (or otherwise) selves, the human mind will eventually succumb to the animal form. Taz comes thisclose to being permanently stuck as a squirrel.
The Souls from The Host are strongly affected by natural instincts and sometimes memories of their hosts. If a human host loved someone before the transformation and this feeling was mutual, its very likely their souls will become a couple. It happens even if souls haven't met each other before.
The short story "Lady Into Fox" was this in a nutshell. About a refined elegant woman, and her husband who was a fox hunter. One day she for no explanation turns into a talking fox. They try to live their lives as best as they can, but she gradually starts to lose her mind. Becomes more interested in chasing rabbits, soon enough she slowly loses her humanity, forgetting who her husband his, losing her ability to talk, as well as the once well mannered dainty lady is now eating live rabbits in the living room, blood everywhere. It was never explained why she transformed but it's implied God or some karmic force punished her husband for killing foxes for a living.
Seraphina: While dragons still retain their near-eidetic memories, pedantry, and innate logical processing, they can be affected by the intense human emotions of their saarantras.
This trope is a recurring element in Fablehaven. When you transform (with the exception of the Eternals' guardians) you become yourself if you had been born a member of that species. It's most used with magical creatures that have human avatars, but it was a major plot point in the first book when Grandma had to fight her chicken instincts to get a message through.
Strongly averted in A Wolf In The Soul. Long before he ever becomes a werewolf, Greg demonstrates animalistic behavior. His actions as a wolf can't be so easily blamed on the wolf.
A curious variant in Charlie and the Great Glass Elevator: When Grandma Georgina (who is in her late 70s) is aged to 300+ years old via a Rapid Aging formula, her memories change to reflect the times she would have been born in and lived through had she come to this point naturally. (Her earliest memory is arriving in America on the Mayflower.) When she is returned to her natural age, these memories apparently disappear.
In the Star Trek: The Original Series episode "By Any Other Name", the Kelvans (huge emotionless hundred-armed aliens from the Andromeda galaxy) take human form. They eventually start thinking like humans, including having human emotions.
Similarly, members of Species 8472 suddenly become much more willing to negotiate after taking-on Human disguises in the Star Trek: Voyager episode "In the flesh."
In a reverse example, Ensign Lindsay Ballard is unable to reintegrate back into her life on Voyager after having been transformed into an alien.
Space Cases: In the Personality Swap episode, turning human doesn't grant Radu any new powers like the others, except that he automatically assumes the role of the leader and starts ordering the others, who follow him, just not entirely happily. Apparently, a superiority complex comes with human DNA, though it should come as no surprise since Harlan explained this while he was still human. Rosie's and Bova's personalities are also characteristic of their species and swap accordingly.
Quantum Leap saw a little bit of this at times, usually attributed to the "Swiss-Cheese Memory" side effect of Leaping. In one episode Sam leapt into a retarded man and exhibited some of those same traits in living his day-to-day life.
When Giles is turned into a Fyarl demon in "A New Man", his instincts tell him to smash things, like Fyarl demons are wont to do. The more time passes, the harder it becomes to keep those impulses under control.
Oz's werewolf attraction to Veruca is clearly hard to keep in check, but he manages. That is, until sunset on a full moon night — he loses it after the first stages of transformation.
The result of regeneration. While certain deep traits and values are maintained between every incarnation of a Time Lord, other parts of the personality can become massively different, in addition to being completely loopy and extreme for a day or so as they settle into each new personality. As the Fourth Doctor comments in his initial episode, "A new body is like a new house - it takes some time to settle in.". Every Doctor (with the exception of the Ninth whose initial regeneration wasn't seen) gets an initial storyline about figuring out what their own personality is like, and every Doctor seems to have their own view on how much of a change this even is; One viewed it as a straightforward continuation ("it's not over!") but his successors as 'replacements', Three's was tangled up with Buddhist symbolism concerning change and renewal, Four viewed it as a death although one he was happy to accept by the time it happened, Seven thought it was a change of perspective, Eight described each incarnation as a different and separate life, Ten described it as an actual death where "some new man goes sauntering away", and Eleven's comments suggest that they're like the phases everyone goes through.
In the "Daleks In Manhattan" two parter, Dalek Sec merges himself into a Dalek/Human hybrid for the sake of experimentation and prolonging the Dalek species. Which in turn he became kind, rational, and outright resented what he used to be. The other Daleks didn't take that very well.
The physical avatar that Harper builds for Andromeda is initially just a more solid presence for the ship's AI than the holograms she normally uses, but over time, dealing with humans and aliens on a more intimate and physical level, Rommy develops a more emotional and independent personality.
Dollhouse. In "Instinct" Echo is programmed to believe she is a mother with a baby, not only with her memories but also her glands to induce lactation and parental bonding. Thus even after Echo's memories are wiped, she still instinctively seeks out the baby to protect him.
In The 10th Kingdom, Prince Wendell swaps bodies with a dog and gradually starts thinking like a dog, the dog also slowly starts thinking more like a human (but never quite gets it) although this may simply be training.
A crossover episode of Kamen Rider Gaim feature the story of Ryoma Sengoku who placed his brain into the robotic body of Hakaider which ended up making him behave more and more Ax-Crazy. Something that he found out much to his horror when he returned back to his human body.
Played straight with the spell Polymorph Other in the first and second editions of AD&D — aside from a chance of simply killing the target, its original personality could be subsumed by one more appropriate for the new form either instantly or over time. This is used as a plot point in the Dark Elf novel Exile, written in the second edition era, which features a sympathetic supporting character turned into monstrous form by an evil wizard and fighting a losing battle against the instincts that come with it. He dies in the end before his comrades can find another wizard to undo the spell.
Averted in Genius The Transgression: shapeshifting yourself or others explicitly doesn't affect your mind, although turning someone into something that isn't alive will cause them to lose consciousness until it's reverted. If you really want you can use a powerful mind control device alongside your transforming device.
In GURPS: Magic shapeshifting slowly drains the IQ stat. If it drops to five (animal level) the transformation becomes permanent.
In Mage: The Ascension mages can shapeshift using the sphere of Life. However, if you do not have Mind magic, or the highest rank of Life magic, you eventually take on the mind of whatever form you have shifted into. You can also leave out the Mind magic and force others to change shape causing your enemy's intellect and self image to slowly melt away over the course of weeks to be replaced by an animal's.
In FreeMarket, this is given as the justification for why you have to play some sort of human — albeit potentially a significantly modified one — rather than a catgirl, a total conversion cyborg, or a robot with a human mind uploaded — putting a human mind in an inhuman body causes the mind to become inhuman — or, in more plain terms, the person goes insane.
This trope is used a bit in Warhammer 40k. The pilot of a Titan, called a Princeps, must psychically dominate the much greater and more powerful mind and spirit of the Titan. Even if this works, which is by no means guaranteed, the Princeps instantly becomes addicted to being attached to their Titan, as they become accustomed to being the much larger and more powerful being and find it very hard to return to a human mind frame. The powerful mind of the Titan will eventually consume the Princeps', whereupon the minds fuse and the Princeps would die if detached. To plan for this, the Princeps is sometimes hard-wired to the Titan to start. This will happen automatically if the Titan falls to Chaos, and the Princeps is then merged physically as well as mentally.
This idea is explored again with the Hellbrute, which was originally a mech-like machine that allows for a near-dead Space Marine to keep fighting (called a Dreadnought), but has been possessed by a Daemon and has a Chaos Space Marine in it instead. The machine starts to gain it's own intelligence as the Daemon takes over it, and the Chaos Marine's mind begins to fuse with the Daemon, essentially turning the Chaos Marine into a huge Daemon in it's own right. This is actually a mercy for the Chaos Marine, as the previous concept of the Chaos Dreadnought did not have the Daemon in it, and the Chaos Marine, stuck inside a robotic shell with no sensory input, inevitably went mad.
In the universe of Pokemon Mystery Dungeon, it's implied that this can happen when Pokemon evolve. In particular, a Lombre is unsure whether or not he wants to evolve into a Ludicolo, because Ludicolo are carefree Pokemon, and he isn't sure if he wants to be carefree.
This carried over into the anime as well. Notably Ash's Charmander, reliable and obedient evolved into grumpy and unresponsive Charmeleon, who further evolved into an even more disobedient Charizard. This is supposed to reflect how in the video games, a trainer can't fully control a traded Pokemon whose level is higher than the badge allows. For example, you can have a level 50 Charizard, but unless you get all the badges, it won't always listen to you. Charmander was, technically, a traded Pokemon. Although Charizard never followed his orders until Ash stayed up rubbing his hands raw to defrost Charizard after it was encased in ice.
In the anime, Dawn's Swinub/Piloswine/Mamoswine act exactly like Ash's Charmander/Charmeleon/Charizard, except it retains its obsession with Poffins.
A possession example: In the video game Geist, you can inhabit the body of various mice. While you retain control during those sections, your body is inherently attracted to the cheese in the nearby mousetraps, often to the point of affecting or overriding your control.
Also, the body's own preferences can prevent the player from going places. When the player possesses a woman in the shower, she refuses to exit into the hallway (guarded by a male soldier) until she puts on her clothes. And after the player attacks an engineer with a pair of robotic arms to scare him, he'll never go near them again.
In Metroid Prime 3: Corruption, the cyborg hunter Ghor is quiet and unassuming - until he plugs into his large mecha/Powered Armor hybrid, instantly becoming gruff and hyperaggressive.
In Sanitarium, the main character involuntarily changes forms throughout the game. Each form has a distinct personality, their own unique (fake) memories, and their own voice actor. Near the end of the game, he gains control over these transformations. During his first few voluntary shifts, he frantically reminds himself who he really is...as if afraid one of the other personalities will take over.
In Dragon Age: Origins, the aptly named Mouse is a Mage Apprentice who shapeshifted into a mouse and can't change back in the Mage Origin. He actually talks about how his shape is well suited to hiding and sneaking and that's most of what he does nowadays as the reason why he can't change back. Subverted, as this happens in the Fade (i.e. the dream world) and he's not actually a human, but a disguised Pride demon trying to trick you.
In King's Quest VII, in the third chapter there is a lord who has been turned into a stag, and he says that the longer he is forced to stay in that form, the harder it becomes for him to use rational thought, and implores the protagonist to help remove the curse on the forest before he forgets himself completely.
Also played with in The Kings Quest Companion chapter for King's Quest III. The cat cookie spell eventually will rob the sentience from the person afflicted with it, trapping them in a feline's body forever. Yes, it is a Fate Worse Than Death, but considering Manannan inflicted it on his enemies several times and was going to kill Alexander as soon as his usefulness was up, this makes it closer to Hoist by His Own Petard.
Though someone Jossed and somewhat not... in King's Quest V, which takes place several years later. Manannan is still sentient and capable of human speech, yet then again died by being trapped in a burlap sack, because he wanted to eat a dead fish.
Commonly theorized to be involved in Final Fantasy VII with Sephiroth. After he manages to turn the tables and take control of the Eldritch Abomination that was trying to Grand Theft Me him, he develops a sudden interest in devouring the Lifestream. The theory is, that's just how Jenova's hunger instincts work.
In Portal 2, GLaDOS's body has some built-in functions that affect any artificial intelligence connected to it. The helpful but stupid Wheatley is corrupted by the body to become evil and obsessed with testing and he is quickly overwhelmed by how much power he has. Though later, this is also revealed as a function of the body - to ensure that cores would be convinced to keep testing. He's still rock-stupid though.
In the first Ratchet & Clank game it doesn't matter what enemies are doing prior to being hit with the Morph-O-Ray; after it's turned them into chickens they just go about their chickeny lives.
In Kid Icarus: Uprising, when Pit is temporarily in control of a dog's body, he's drawn to the food garbage in the street. This results in him automatically walking away from where you need to go, and the player has to fight with the controls. When you're in control of a human, however, Pit doesn't take on any aspects of their personality.
In Resident Evil: The Darkside Chronicles, played straight yet averted with Manuela's mother she had mutated into a giant feral vicious fish/lizard monster. But it turned out her love for her daughter was the only spark of humanity left within her.
Attis in King's Quest VII: The Princeless Bride, who was turned into a stag, admits that he's very gradually losing his human mind. And that if the curse isn't reversed in time, he'll permanently be a stag wiped clean of all previous human thoughts.
Somewhat present in Ever17. While possessing Hokuto, Blick Winkel essentially thinks he is Hokuto. Blick Winkel's discovery of his own existence becomes a major plot point.
In Gunnerkrigg Court, Reynardine's behavior seems to match his outward form: when he's a pint-sized plushy, he's snarky and generally unpleasant; when he's a Noble Wolf, he's helpful and seemingly concerned with Antimony's well-being. It's unclear which is the cause and which is the effect.
Kat: You should stay as a wolf more often, dude. You're much nicer this way. Reynardine: Well, the mind is nothing but a plaything of the body, correct?
Wouldn't that make this an example of "The Body Is A Plaything Of The Mind"?
Another possible example is Alistair. Word Of Tom says that Alistair now has a bird's brain, and all that entails. On the other hand, even as a bird he seemed to recognize Kat and retain his feelings for her.
Later information makes it unclear what, exactly "all that entails": Word of God is that City Face, a short story featuring sapient pigeons, is canon. So it's not much of a stretch at all to assume that Ali and his parents retained their sapience when they were turned to birds.
In El Goonish Shive, transformations tend to influence the transformee's behavior somewhat, usually in subtle things like body language (for example, someone turned into a cat-person might find themselves purring without realizing it), and the author has said he enjoys adding goofy little quirks like that: for example he finds it especially funny when Elliot gets genderbent and then acts in a girly way without realizing it.
Ash in Misfile has gone under some relatively minor behavioral changes after being turned into a girl, however, unless s/he's having a period, it's nearly impossible to tell which of the differences are biological and which are due to circumstances. At one time Emily wondered whether differences in male and female brains might be responsible for Ash's poor math skills (it wasn't, as Ash sucked at math both before and after the change). That said, Ash's personality hasn't changed that much, so this is largely subverted.
In Eerie Cuties after a "Freaky Friday" Flip chocolate craving cute-vampire Nina, and standard vampire Layla find their normal cravings and personalities switching to suit their new bodies.
Averted with Ace after a Gender Bender transformation. She just likes to pretend it is this trope, much to the disgust of those around her.
Although the Gender Bender curse seems to affect the victim's mind based on how masculine they were before the curse. Ace found himself attracted in men and interested in dresses. When the curse transferred to Kade, he became a very willing magnet to many a guy.
In the Whateley Universe, most of the main characters underwent Gender benders when their powers manifested; apparently turning a happy boy into a girl doesn't cause gender identity disorder and since the Unfortunate Implications are glossed over (they get therapy, but it's off screen) to the point you'd rarely realise they hadn't been a girl all their life.
Varies from person to person, some change form without any mental effects, some had GID before the change, and its mentioned in passing that some people have developed GID after flipping genders.
Main protagonists Team Kimba right now have 7 characters: two had GID before they became mutants; two have spirits (or something more powerful) riding along helping them acclimatize; one seems to just magically be happy going from girl to boy; and two are massively pissed and unhappy about being shifted.
It's also been known to affect voluntary shape-shifters, to the point they have classes on maintaining an identity.
Its actually specifically mentioned in a few of the original stories (Fey 1 being the one I can find in a pinch) that being an Exemplar (Mutant with Most Common Superpower as an actual power) specifically invokes this trope. That is to say that as a rule, if you are shape-changing due to being an Exemplar, eventually you'll get used to your new body — no matter if it's female, a mutant T-Rex, or a giant piece of sentient coral.
The fact that BITs are generally based on the Exemplar's subconscious idea of an perfect body also helps.
Present in another capacity with regards to the J-Team, though it's possible that it's just Jade's flair for the dramatic showing through. Whenever she creates another instance of her consciousness to possess something, it acts more or less how you would expect a sentient version of that thing to act. The best example here is probably Shroud/Jinn, who acts a lot more mature than Jade, and pulls off a pretty convincing 17-year-old.
You Have Become Your Avatar: Some users fall victim to their Avatars' canon personalities and attempt to act them out. Ichigo Pocky Chama got trapped in the body of Deidara and immediately got the desire to blow things up for the hell of it.
Batman: The Brave and the Bold: deaging to a toddler doesn't reduce Batman's intelligence, but it does alter his behaviour - forcing him to communicate mainly in baby-talk and draw using crayons. At the end, he's able to figure out a cure for his deaging, but has to tell the Marvel family with incoherent baby noises and pointing at things.
In Beast Wars, a Maximal or Predacon that acquires an organic alt mode also acquires the instincts of the organism they scanned; normally, these instincts are separated fro the rest of their memory by mental firewalls, but various disruptions to those firewalls (from damage to the protoform before scanning or staying in alt mode for too long) can cause the instincts to overpower the Transformer's personality to varying degrees, though a 'bot can delete the firewalls purposely and integrate the two together without any lingering effects. Arguably, a Transformer "possessed" by their beast mode is much more dangerous, as they're forced to obey their unpredictable animal instincts instead of following typical Maximal or Predacon stereotypes (for instance, the Predacon Inferno - who had his personality almost entirely enveloped by his fire ant alt mode - became fanatically loyal to his "queen" Megatron and suicidally and recklessly effective as cannon fodder, all in line with his role as an ant drone).
Tigatron however, managed to solve this problem on his own, simply by removing the firewalls and allowing his two halves to mesh. He later passed on the trick to the other Maximals after they were forced to remain in Beast Mode after losing their shielding system.
Also, happens whenever Ben turns into Rath. He becomes quick to anger in that form.
And also his 'Big Chill' alien gave birth to alien babies after it took over Ben's mind several times in order to build a nest. Word of God stated this was due to asexual reproduction.
In Omniverse when Ben transforms into his Inkursian form Bullfrag he suddenly finds Princess Attea very attractive.
Subverted and played in a single episode of Extreme Ghostbusters. The Monster of the Week's payload is to occupy human bodies by driving the original mind out. In the case of our Paraplegic Badass Garrett, after his body was occupied by the ghost and his mind driven out, his mind was still drawn in a (wheelchair-)sitting position, while the gang found something was amiss when the corporeal Garrett began walking...
A minor version in Ultimate Spiderman, for laughs: after Mesmero causes Spiderman and Wolverine to switch bodies, Spiderman (in Wolverine) uses the "I'm the best there is at what I do" line (followed by "...and I have no idea why I said that"), whereas Wolverine (in Spiderman) realises that he talks as much about inane things as Spidey does, even when there's no-one to hear him.
Wolverine (in Spiderman):Now I'm talking about how much I'm talking!
Lobotomies, anti-depressant and other medications, all sorts of odd quirks and personality changes caused by brain disorders that go away if those disorders are ameliorated, a nice person becoming a cranky one under the influence of stress (or other) hormones, a cheerfulGenki Girl who endured tons of Break the Cutie becoming paranoid and bedridden when they get old enough for clinical depression to kick in, Alzheimer's disease: it's very well demonstrated that the personality is a creation of the physical structure of the brain, and even minor physical and chemical changes can turn someone into a very different person, forget turning into a different species. There's also puberty, PMS, or menopause, all of which tend to induce mood swings ranging from inconvenient to debilitating.
There's also the story of Phineas Gage, a famous person one will no doubt learn about in any Psychology 101 class, who took an iron rod to the head◊ (warning: pic might be Nightmare Fuel). He had a significant personality change as a result, and his friends remarked that he was "no longer Gage." This case began the idea that changes to the brain's physiology will have an impact on behavior and personality. Though how much of the changed behaviour was attributable to brain injury and how much to post-traumatic stress disorder, the chronic pain he suffered afterwards and the heavy drinking Gage resorted to as a coping mechanism for both are a matter for conjecture.
There's endless medical conditions that could cause altered mental status or psychiatric changes, but here's a small sample of those due to altered morphology (regardless of the cause): there are mental disturbances secondary to hyper-or-hypothyroidism. Errors in sex hormone metabolism can change the development of the body and brain, as can production of stress hormones. Speaking of sex hormones, testosterone causes what can only be called "stereotypical maleness." Cancers can produce hormones that cause electrolyte or other imbalances that mess with the mind. Tissue changes in Diabetes I and II can cause altered mental status via hypo-or-hyper blood glucose. A stroke, injury, embolism, hemorrhage in the brain can cause all manner of terrifying neural deficits depending on the part of the brain affected. A mass effect from an tumor in the head can press on brain structures. Failures of the kidneys or livers can cause all sorts of toxic product build-up or electrolyte imbalances.
The Proteus Effect - peoples' avatars in virtual worlds affect how they interact.
The physical transition period from childhood to adulthood is not an easy or pleasant one for most people.