Robert: Take it from me: A very good sign that you're crazy is an inability to ask the question, "Am I crazy?"Some people laugh as a sign of mental instability. Others speak in a Creepy Monotone or openly show signs of being violent. Then there's the character who's revealed to be insane and shows almost no signs of being crazy until that point... at least not publicly. A character that feigns sanity, quite simply is the kind of person that can maintain their facade in most social situations but is completely insane on the inside. So naturally when the audience figures this out it can sometimes be a major Plot Twist. Other times the character's craziness could be apparent to the audience from the start, and not the other characters. Either way beware of spoilers. The Trope Namer comes from a book called The Mask of Sanity, which describes how people that aren't stable can blend in with society. Yandere and Cute and Psycho are Sub Tropes, when people may act perfectly normal until that certain trigger is pressed, then they quickly turn into murderous psychopaths. Taking off the mask can be seen as a Villainous Breakdown. Compare and contrast with Stepford Smiler (Unstable). Often overlaps with Reluctant Psycho. Contrast with Obfuscating Insanity. See also Beneath the Mask and Hyde Plays Jekyll.
Catherine: Even if the answer is "yes"?
Robert: Crazy people don't ask, you see?
Catherine: Even if the answer is "yes"?
Robert: Crazy people don't ask, you see?
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Anime and Manga
- Solf J. Kimblee of Fullmetal Alchemist maintains his polite and empathic facade despite being a sociopath that is heavily implied to have pretended to be sane in order to join the military.
- Seemingly played straight with Itachi from Naruto, who appears as stoic individual and is seemingly composed. That is until Sasuke sees his inner-self in their final battle, in which he reveals himself as an Ax-Crazy power hungry sociopath ranting to surpass Madara Uchiha. Then subverted later on after it's revealed that Itachi was intending to die by his brother's hand all along, and was playing the part of an unrepentant psychopath in order to provide Sasuke with further motivation to kill him.
- Death Note:
- Light Yagami goes insane by the second episode yet still manages to blend in with society. By the end of the series, he's defeated and everyone figures out just how crazy he was.
- Mikami also counts. There are quite a few scenes in which you see him completely calm, and controlled... until you see just how crazy he is in the warehouse scene.
- Yuno from Future Diary only appears to be an innocent girl, but reveals that she's an Ax-Crazy sociopath early in the series.
- Ryouko Asakura from Haruhi Suzumiya seems like a calm, kind student, but follows a brand of Blue and Orange Morality that from a human perspective is utterly insane.
- Attack on Titan has Reiner Braun. Considered by the others to be stable and reliable, he's actually the least mentally stable of The Moles.
- Mami Tomoe from Puella Magi Madoka Magica is a tragic example. Her selfish wish to save her own life, but forgetting her parents made her incredibly guilty. As an atoner she decided to fight the witches and clear off her sin by saving as many lives as possible. Of course, as a teenager who risks her life everyday, she's often afraid, depressed and lonely but has created the image of a flawless, elegant and matured Cool Big Sis. She eventually becomes The Mentor and Cool Big Sis to the other Magical Girls. However, in the third known timeline, when she found out the truth of Magical Girls eventually turning into witches like Sayaka had done before, she went completely berserk, Mercy Killing the other girls with the intent to commit suicide. She succeeds with Kyoko, but Homura is saved by Madoka killing Mami. After this event, Homura discovered that Mami's mental stability is probably the worst, and from then on dissociates herself from her.
- Mobile Suit Gundam SEED has Rau le Creuset, who throughout the series seems to be a calm, efficient Reasonable Authority Figure who just happens to be working for the bad guys. This made the revelation of how seriously twisted he really is all the more shocking when he started ranting and raving in front of Kira and Mu. And then he goes right back to being calm and collected in public after said confrontation. Bonus points for actually wearing a mask all the time, and it only comes off when he's at the height of his crazy episode.
- Gendo Ikari from Neon Genesis Evangelion is ostensibly the leader of the heroic organization fighting to prevent the end of the world. The truth of the matter is far more complicated as he's plotting to cause it instead in a mad quest to be reunited with his long-dead wife.
- Dragon Ball:
- Freeza. While he's able to hide beneath a polite and suave act and function well enough to run a business, beneath it all, he's a brutally sadistic, Ax-Crazy psychopath who will kill anyone, even his own minions, For the Evulz, and the mask falls off completely when Goku defeats him. In Resurrection 'F' and Dragon Ball Super, he suffers from Sanity Slippage from his time in Hell, and his humiliating defeat to Goku and his death to Future Trunks, and while he tries to hide it under his usual polite facade, the mask is considerably more transparent.
- When we first meet Black in Dragon Ball Super, he didn't talk and just killed everything in sight. When we finally see his form, he grins and tells Future Trunks how he was finally going to kill him, while still remaining calm and polite. It's in the second episode that we get hints that Black may not be completely sane, given how he laughs when Trunks punch him in the stomach. In Episode 51 when Black tells Trunks why he's killing humans, it's confirmed that he isn't really sane, especially with all his mood-swings.
- While Future Zamasu mostly acts calm and confident, it's obvious by his Slasher Smiles and Motive Rants that he fell off the deep end a long time ago.
- Saiou/Sartorius in Yu-Gi-Oh! GX. For the most part, he remains stoic in his behaviour, without anyone possibly suspecting he is possessed by the Light of Destruction, until the Light comes straight forward with its plan and duels Edo/Aster or Judai/Jaden, behaving like a maniac in the process.
- Kano from Texhnolyze. Any time he speaks, his words are ever cool and quiet, but it's made emphatically clear that saying Kano is fucked in the head is akin to saying Lux isn't a holiday resort.
- Tokyo Ghoul has quite a few characters that turn out to be masking a deranged mind.
- Ken Kaneki masks his enormous psychological issues behind a meek, shy personality. It isn't until he's forced to confront his own damaged mind that it begins to become clear just how unstable he really is. He's prone to hallucinating conversations with people alive or dead, suffers Trauma-Induced Amnesia, and is deeply suicidal at various points. His insanity is at it's worst during his tenure as the "Black Reaper", masking his fracturing psyche beneath an intimidating and cold mask.
- Dr. Akihiro Kanou seems to be a very polite, fatherly surgeon with a kind bedside manner. He maintains this same fatherly behavior when he's explaining how he is Above Good and Evil, leaving his "daughters" to die, or taking an electric saw to a test subject's insides. He isn't insane or a sadist, his genius is merely not appreciated or understood by society.
- In a shocking twist, Tooru Mutsuki is discovered to be repressing memories from several horrifyingly violent incidents. After being forced to confront their own damaged mind, they have a complete psychotic breakdown. In the following month, they begin hiding their newly-awakened sadistic urges by pretending that everything is still normal. A few others pick up on something being wrong, but can't quite figure it out.
- Sugou Nobuyuki of Sword Art Online pretends to be a friendly, stable guy, and the act is convincing enough to completely fool Asuna's parents, who set him up in an Arranged Marriage with their daughter. Beneath it all, he's a totally sadistic, Ax-Crazy Jerkass prone to Evil Laughs and Slasher Smiles, especially in his role as Oberon in ALO. Once Kirito defeats him, the mask comes off, and he's arrested and incarcerated.
- While Darkseid is well-known as one of the most cunning and stoic villains in The DCU, he really is just as Ax-Crazy as all of his minions. He just manages to hide it all due to extreme self-control. His son, Orion, who was raised to be heroic isn't quite as good about his Dad at hiding it and it has the effect of just making him an asshole.
- Anyone who takes the Goblin formula is driven completely insane, but they can still act perfectly sane when they need to.
- Rosario Vampire: Brightest Darkness Acts III and IV have a perfect example in Hokuto Kaneshiro. While he constantly acts polite and sane, and even manages to win the trust of Tsukune's group, it's revealed that he himself personally arranged for Kuyou to attack the academy so he could steal an Artifact of Doom while the gang was distracted fighting him, needing said artifact to revive Alucard and destroy the world out of the nihilistic view that all live is evil and meaningless. Also, when his Co-Dragons, Jovian and Jacqueline Kikion, are resummoned in Act VI, they state that their morality is shaped by their master; when one considers that Jovian and Jacqueline were Ax-Crazy Psychopathic Manchildren who committed such atrocities as rape, Cold-Blooded Torture, and wanton mass murder and destruction as a matter of course, it's even more proof of just how off his rocker Hokuto really was.
- As The Rise of Darth Vulcan goes on, it becomes increasingly apparent that the titular Villain Protagonist is wearing one. Ted/Darth Vulcan spent years suppressing his true nature under a Jerkass Façade in order to avoid bullying, to the point that even he forgot what he was really like, leaving him deeply disturbed. This is why he's at first seemingly immune to the Sanity Slippage-inducing effects of the Alicorn Amulet — it can't drive him insane, because he was never sane to begin with.
- In Digimon 02 The Story We Never Told, Oikawa spends his first few appearances as a coldly calm and slightly sinister Mysterious Watcher. Once he reveals himself as The Man Behind the Man, however, it quickly becomes apparent that he is a highly unstable Psychopathic Man Child, prone to bouts of sudden shouting and raving about his megalomaniacal world view.
- Who Framed Roger Rabbit. Throughout the film Judge Doom appears somewhat malevolent but not blatantly so. Near the end he's revealed to be a Toon who's even more crazy than most Toons.
- Battle Royale: Kiriyama sits quietly in the background, but starts killing students once the game starts because he signed up for The Program.
- Scream: The Ghostface killer ( well, killers) appeared perfectly sane, if odd, until the big reveal at the end. Billy is better at keeping the mask on than Stu is, which makes him all the more creepy when The Reveal comes.
- In Apartment Zero Jack appears to be perfectly normal. Even when he is implied to have killed someone, he remains perfectly calm. And then Adrian walks in on him with a body, and we see just how crazy he truly is.
- Subverted in Battle Royale Kiriyama, who was always The Quiet One, kills students mercilessly, but it turns out he's not crazy, he just has brain damage.
- Parodied in American Psycho. Patrick Bateman is an Ax-Crazy Serial Killer working on Wall Street. He constantly tries to confess the murders he has committed but no one takes him seriously. Finally he flips out completely and does everything in his power to get caught... and still no one cares.
- The murderer of And Then There Were None is a calm rational person in public, but an Ax-Crazy Large Ham once he arrives to kill the last victim.
- This is one interpretation of the title character's actions in Hamlet.
- In Breakfast of Champions, after Dwayne Hoover goes on his rampage, people say afterwards they should have seen the danger signals in his behavior, but the narrator then points out that this "cry for help" was only obvious in retrospect: Dwayne's behavior prior to meeting Kilgore Trout had been within the bounds of normally acceptable behavior, and his closest companion and mistress at the time, Francine Pefko, had thought that he had been getting happier.
- Roose Bolton from A Song of Ice and Fire first appears to be merely cold and ruthless. However, he is actually almost as insane as his torture-loving bastard son, Ramsay.
- Petyr "Littlefinger" Baelish, as well. In spite of being a seemingly-charming cheeky chap, he's ruthless homicidal sociopath underneath it.
- In The Stormlight Archive, Nale, who goes around killing prospective Radiants under the belief that this would save the world. As Edgedancer shows, most of the time he seems perfectly normal, if a tad obsessed with formalities and documentation, but whenever a crime is committed in his vicinity, he turns into a murderous monster you can't reason with.
Live Action Television
- Game of Thrones:
- Roose shares his son's sadism. He's just a lot better at hiding it.
- Ramsay is rather calm, polite, and approachable when pretending to be Theon's savior, though there is something subtly strange about his forced mannerisms and speech pattern.
- Kamen Rider Wizard: While it's no surprise that all of the phantoms are insane, special props goes to Sora/Gremlin. Before the Reveal we see that he is a giggling little troll. He's obviously dangerous and even man handles Wizad and Beast a couple times, but that was barely the surface. He then reveals that he is the only phantom to retain his human persona, that's when the real reveal begins. The people who knew him before the Sabath said he was a nice, if eccentric, guy. Turns out he's a serial killer who continues this as a hobby!!!!
- Post-snap Dukat in Star Trek: Deep Space Nine manages to hold it together fairly well for about half of "Waltz", only reacting to the hallucinations when he's safely away from Sisko. This does not last.
- Arrow: Slade Wilson spends most of his post-Face–Heel Turn appearances in Season 2 as a seemingly coldly detached and stoic Magnificent Bastard, who keeps his calm even when fighting Oliver. However, near the end of the season, the mask comes off, and he reveals how deranged the Mirakuru has left him, in all his ranting, hallucinating glory.
- Hannibal Lecter is one of the most famous psychopaths in fiction and is almost always portrayed as getting away with his crimes for many years because of his almost flawless mask, but Hannibal shows several occasions when the mask slips a bit and his real nature shines through. Usually only the audience sees this, but on one occasion his psychologist Bedelia comments on this and describes Hannibal as wearing "a person suit" that fools most people but not her.
- Children of Bodom's "Mask of Sanity" is a song about a person who's at least trying to "keep his mask on":
Daylight's sin inside, day's growing closer, wait for painI cannot wait another night to be alone
- Kanye West's first verse from the song "Monster" references this:
Bought the chain that always give me back painFucking up my money so yeah I had to act sane
- Call of Cthulhu. Cthulhu Mythos cultists with a Sanity of zero usually act in a completely insane or otherwise irrational manner. However, powerful cultists (such as the leaders) are usually able to act in a sane manner when dealing with sane (normal) people in order to conceal their true nature.
- The related game, Trail of Cthulhu explains this by separating Stability from Sanity. People with little Stability are prone of panicking and fainting and such, but may still retain a good grip of their Sanity. People with high Stability and low Sanity, on the other hand, embody this trope.
- Hunter: The Vigil supplement book Slashers, which focuses on serial killers and similar psychopaths, gives us Charmers, a category of slashers who specialize in killing people by winning their trust, meaning they specialize in giving other people the illusion they are sane, perfectly nice people. This is only a mask however, and anyone who manages to see through their acts and refuses to trust them is very likely to cause them to lose their cool, breaking their mask.
- Geralf in Magic: The Gathering is, in his own way, just as childish, petty, short-tempered, obsessive, and eccentric as his sister Gisa, but he tries to act more intellectual and rational about their endless, pointless pissing match over how to best build zombies.
- The Butcher from Evil Genius qualifies- a cannibalistic, cleaver-wielding psychotic ex-medical student turned henchman can temporarily fool Investigators with his first skill called 'Facade of Normality', which renders him less suspicious and even able to pass for a sane person, as opposed to instantly alerting them due to his violent insanity.
- Hazama of BlazBlue is an odd example, considering how often he laughs maniacally you'd be surprised how calm and stable he can appear.
- A Malkavian (the incurably-mad vampire type) character in Vampire: The Masquerade – Bloodlines ingeniously hides their split personality by pretending to be twins, namely, Theresa and Jeanette Voermann.
- Zant from The Legend of Zelda: Twilight Princess. He seems like a calm, cold, mildly angry villain throughout the whole game. But before you fight him he takes off his mask as well as his controlled demeanor. Cue an enraged verbal exchange with Midna, and one of the most spastically moving and sounding bosses in the franchise.
- Throughout Luigi's Mansion, King Boo was calm, collected, and in complete control. After, however, he was overtaken by sheer insanity and, while he was outwardly almost the same throughout the sequel, you could tell that behind his face was a sociopath out for revenge, no matter the cost.
- In Shin Megami Tensei: Strange Journey, in the Law Path, after Zelenin has brainwashed all Red Sprite personnel, Dent assumes the role of the Only Sane Man and keeps issuing EX Missions. Finish all missions, and Dent sighs contently - everyone is happy - happier? He then proceeds to rip off his Mask of Sanity and stomp on it by groveling and addressing you as "Master".
- Dr. Eggman Nega of the Sonic the Hedgehog series comes off as a calm, well-mannered, gentlemanly individual, but when push comes to shove proves to be a complete psychopath who delights in destruction and the suffering of others.
- This happens quite often in the Ace Attorney series. Most of the culprits will start off faking innocence or serenity, only to have an inevitable breakdown after the player pokes enough holes in their testimony.
- In Justice For All, Matt Engarde is able to fake being stable enough to be a famous actor and trick Phoenix into thinking he's an innocent, lovable ditz. It turns out that he really lacks any empathy, being perfectly fine with hiring an assassin to kill his rival actor and letting said assassin blackmail Phoenix into defending him via kidnapping Maya. He also seems to completely believe he's innocent, since he hired an assasin and didn't commit the murder himself.
- In Trials and Tribulations, Dahlia Hawthorne is heavily implied to be a sociopath, stating that she has never done a single thing for the benefit of anyone besides herself. She's easily able to pass herself off as a vulnerable victim in need of protection and even is able to disguise herself for some time as her much-saner twin sister.
- In Dual Destinies, the phantom is an utterly remorseless killer who claims to have trained himself not to feel emotion at all. He seems eerily detached from the many killings he does. He is also able to fake having emotions perfectly as it's revealed when Phoenix discovers that he'd been perfectly masquerading as the incredibly emotional Detective Bobby Fulbright.
- Nagito Komaeda from Super Dangan Ronpa 2 appears to be sane, until halfway through the first trial, where it's revealed that he's completely insane about hope, going so far as to attempt to murder somebody to preserve it.
- Yandere Simulator: Word of God says that the "Sanity" mechanic is somewhat of a misnomer, as Ayano is always insane. Instead, it tracks how well she manages to keep the mask on. If you keep it high, she's able to interact normally with her classmates with minimal suspicion. However, if it's low, she loses her composure, making it obvious to anyone looking at her that she's not quite right.
- Snadhya'rune, the effective Big Bad of Drowtales, masterfully demonstrates this trope over the course of chapter 46 when her previous behavior where she presents herself as an enlightened peacemaker and the best hope for her kind's future shatters when her plans start going awry, culminating in a scene where after she's surprised by a literal stab in the back she screams in utter fury and starts tearing the place down, not caring who she takes out in the process. You pretty much see the mask start to fall away and then crack completely in the course of three panels, and then several pages later she attempts to put it back on with limited success.
- Downplayed in The Order of the Stick with Belkar, the Ax-Crazy Token Evil Teammate of the Order - the kind to butcher a random bystander who looks at him funny. During the course of the story, after getting in serious trouble and no longer getting away with his erratic and criminal behaviour, he is advised by a mysterious source that if he doesn't want to become a better person, he can at least fake it for his own good. He then keeps a low profile for a time, becoming more of a Blood Knight and focusing his murderous urges on actual threats to the Order. Later subverted when it is revealed (at least to the reader) that he actually grew as a character and is no longer focused on killing as he used to be.
- Teenage Mutant Ninja Turtles (2012): Slash, at least in his debut episode, appears to be a smart, loyal, hardworking ninja who understands Raphael's position and his problems with his brothers. But underneath, his mind is too twisted to understand things like how no matter how frustrating families can be, that doesn't mean things would be better if they weren't around, that there are more important things in life than being strong, or that caring for others isn't a weakness. Later episodes reveal that he straightened up — from all appearances, for real.
- In Wakfu, Qilby the Traitor's mask is so good that he spends most of season 2 as a Hidden Villain. In episode 20 "The Zinit", he finally ditches the mask and reveals his true self: an Ax-Crazy nihilistic Omnicidal Maniac who is even less mature than Yugo despite being far older.
- Toffee of Star vs the Forces of Evil seems to keep extremely calm, composed, and even professional at all times. All times, that is, save for one brief moment in the Season 1 finale where he reveals his true nature and completely loses his marbles for all of about a second and a half, putting on a huge, toothy Slasher Smile while gleefully yelling "SURPRISE" after revealing his plans. He then... immediately goes back to calm, collected and professional like nothing happened.
- Steven Universe:
- Eyeball. At first, she seems to be the Only Sane Man of her squad of Rubies. But then the Season 3 finale "Bubbled" reveals that she's been wearing this since her introduction and when Steven heals her gem and reveals that he is the reincarnated Rose Quartz, the mask comes off and out comes a dagger as she attempts to essentially disembowl Steven by removing his gem.
- Navy. At first, Navy seems to be as friendly and ditzy as the rest of the Ruby Squad. But then the Season 4 episode "Room For Ruby" reveals that, like Eyeball, she's been wearing this since her introduction and is revealed to be a sadistic and manipulative Bitch in Sheep's Clothing who pretended to want to live on Earth just so she could steal back her ship and only kept up the facade just to see the look on Steven's face when he realized he'd been betrayed by his "Favorite Ruby". What unsettling is that she still keeps up her cheery demeanor even after The Reveal.
- Ben 10 Has Kevin Ethan Levin. In his very first appearance, he seems to be a normal, albeit slightly more mischievous foil of Ben. As the episode progresses, it becomes clear that this isn't the case. He talks about robbing a mint in a way that would've resulted in the deaths of countless people and seems genuinely shocked when Ben refuses to help.
- The Marvelous Misadventures of Flapjack has an example. In one episode Captain K'nuckles meets a charismatic man who is cleaner and more articulate than many of Stormalong's other residents. Flapjack discovers that the man is an escaped mental patient who believes wholeheartedly that the mole on his face gives him immortality. The episode ends with the crazed man jumping into a volcano and being burned alive, all the while managing to say his catchphrase one last time.
- Milo Murphy's Law has Jackie, who seems like nothing but a big risk taker at first, but by the end of the episode everyone believes her to be insane.
- Batman Beyond features this In an episode entitled "Sentries of the Last Cosmos." Three youths are able to assume the roles of sentries when they meet the owner of the franchise (who is capable of making technology that gives the wearer the abilities of a sentry). He is able to convince them that the characters and events from the titular films are not fictional, but part of an ongoing galactic war that they can help win. They obey him blindly and commit several crimes. Despite being brainwashed, they draw the line when the owner of the franchise demands that they kill an obese man they'd abducted. It turns out that the man is the true creator of the franchise and he'd filed a lawsuit against the fake for taking all the credit. The fake goes toe to toe with Batman and the dialogue that follows suggests that he actually believed what he told the youths about a galactic war.
- The psychology text The Mask of Sanity offers several insights on this point. Genuine psychopaths aren't like the Hollywood trope of a psychopath. Rather, they seem utterly incapable of empathizing with other human beings. Subsequent psychology research on imprisoned psychopaths with neuroelectric instruments has actually measured that their brains seem structurally deficient in the areas responsible for emotion. In normal people, when shown violent or offensive words such as "murder", "rape", etc., neural activity spikes in the parts of the brain responsible for emotion, i.e. the amygdala. In psychopaths, there is no similar activity spike: their amygdalas seem stunted and dead. The great unanswered question is the Nature vs Nurture debate: whether they were born with non-functioning amygdalas, or if years of systemic childhood abuse stunted their mental development. The end result is that psychopaths have all of their higher brain functions intact such as speech, reading, and higher logic - but human emotions are an alien sensation to them. They have the raw intellect of an adult, but their level of emotional understanding and empathy for others is below that of a selfish toddler, who does not yet understand that other peoples' lives don't revolve around them. The argument made in The Mask of Sanity is that because psychopaths still have their raw intelligence, can through careful analysis over the course of their lives gradually learn to very accurately mimic the normal emotional responses that other people expect of them: if they cut ahead of someone in line they understand that they're supposed to say the words "I'm sorry", but the words have no meaning to them. They learned it as a rote response - but as the book points out, mimicry - no matter how careful - is still not comprehension. So sort of like a human Chat Bot, psychopaths have learned a lot of stock phrases for "small talk", but it's just by rote: chit-chat about the weather, complimenting someone on losing weight, etc. This can also make them come off as very obsequious, when they're trying too hard to convincingly act like a caring, normal person. The book advises that to discover a psychopath all you really need to do is engage them beyond surface-level conversation for any length of time, i.e ten minutes of talking about "What are your views on violent crime?" and they'll eventually say something bizarre, revealing that they're just attempting to appear normal as best they can grasp it. It's very much like the Voight-Kampf test from Blade Runner: ask them emotionally provocative questions, and they won't know how to properly respond as a normal person would.
- For example, consider the case of infamous convicted child molester Jerry Sandusky. When asked short yes or no questions he could keep up the act, such as "did you molest boys when you were a coach?" ("No") - but any interview of length that he gave, he would start rambling off on bizarre and incriminating tangents. Particularly the bizarre telephone interview on live TV when asked the more involved question, "Are you sexually attracted to young boys?", instead of immediately and flatly saying "No", he started pondering the question aloud because he wasn't sure how a "normal" person would respond to such a question, leaving the interviewer to roll his eyes at his absurdity.
- Serial killer John Wayne Gacy is often pointed to as a prime example of a psychopath who was able to put on the facade of a seemingly "normal" domestic life, but it was all just an act. He seemed to have a normal house in the suburbs (despite his rocky marriages) but he was storing dozens of bodies of men he had killed in his crawlspace.
- "High functioning" when used in the context of mental illness and in particular sociopathy refers to someone who's able to do this, and consequently they're generally harder to spot and get away with their crimes for longer.
- The concept of neuro-plasticity complicates this trope further. Essentially, it is now known that one can actually change the way ones' brain works through experience, beliefs, or other habits or methods, deliberate or otherwise (though, probably only around 50% of our brain at most is capable of this, and it becomes harder as we age as our brain settles into patterns- barring brain damage, that is). In other words, it is possible that normal people can become psychopaths (or at least, psychopathic) through committing terrible deeds or perhaps even just researching or dwelling on them long enough to lose their natural aversion or disgust to them, and perhaps even the memories of that disgust. Conversely of course, this means that just because someone becomes desensitized to acts or conceptions of violence or other behaviour doesn't mean they are actually likely to commit it, nor that they are incapable of emotions or empathy, so it may be harder to tell apart genuine functioning psychopaths from ordinary people who are just "used to it", as it were.