A form of psychological warfare in which you do things to convince someone they are going crazy. Named after the 1940 British film Gaslight, in which a man tries to do just this to the heroine of the story.

An example would be to change a person's furniture around and tell them it's always been that way, or that their name is a different name, and then they check their ID card and it says the different name, and everyone who's in on the gaslighting must call her by this new name, etc. In the film this is possible because the woman's mother died and she is ill in the house, confined inside. But however it is achieved, the aim of gaslighting is to reinforce doubt on the person's own sense of judgement, to the point where they no longer trust their own mind, and resign themselves to the idea that they are insane.

Gas Light was a 1938 stage play that was adapted into movies in 1940 and 1944; the plot centers on a man who tries to drive his wife insane by changing the environment of their house in small but perceptible ways while insisting to her that everything is the same. The titular tactic he employs is dimming the gas lights of the house; she notices it, but he insists it's all her imagination.

Gaslighting is a form of psychological and emotional abuse. Most times it doesn't actually involve the physical environment manipulations portrayed in the play and movies. Typically Person A uses various types of emotional manipulation to convince Person B that his or her authentic emotional reactions to an injustice or injury caused or abetted by Person A are crazy or unreasonable, or that the injury never even happened. This is done so that Person A doesn't have to change (or even feel remorseful about) his or her behavior, because a successful gaslighting results in Person B doubting and second-guessing his or her own emotions, perceptions and judgements and ultimately going silent about whatever bad thing that Person A keeps doing. 

There are also instances in which one person gaslights another largely for the sadistic amusement of seeing another person distressed and confused; these situations often do involve physical environment manipulations, which the instigator will trivialize as harmless pranks if he or she is discovered staging them. Imagine, for instance, the wife of an elderly man with mild dementia who repeatedly hides his beloved pocketwatch and then tells him he lost it, sending him on frantic, fruitless hunts through their house while his wife quietly plants the watch someplace he already searched thoroughly.

Children suffering from sexual abuse are frequently subjected to gaslighting by their abusers, as are rape victims and domestic violence victims.

"(E)ven today the word gaslight is used to describe an attempt to destroy another's perception of reality"
- Florence Rush, The best-kept secret: sexual abuse of children 

Here are a couple of typical gaslighting exchanges:

Amy: "I wish you would stop flirting with other women at parties."
Joe: "What? What are you even talking about?"
Amy: "Bella. You were flirting with her."
Joe: "Christ, Amy, no I wasn't. God. You are so jealous; it's just ridiculous."
Amy: "I saw you kiss her!"
Joe: "No you didn't. All I did was lean in to tell her something about our boss that I didn't want Judy hearing."
Amy: "Judy was on the other side of the room --"
Joe: "No she wasn't. She was right there. You should get your eyes checked. Or maybe it was all that tequila you put away, huh? Christ, you are so unreasonable."
Amy: "But --"
Joe: "I'm so sick of your jealousy. Can't you just relax and let me enjoy a party without trying to control every move I make? Jesus. Why do you have to try to ruin everything?"
Amy: "I'm sorry. I just ... nevermind." 

In the above exchange, Amy starts off very reasonably expressing her desire that Joe stop doing something hurtful to her. Joe comes back at her with a "the best defense is a good offense" style gaslighting, and he is certainly offensive. If Joe were merely trying to cover his ass about the incident, he might go with a standard apologetic lie: "I'm sorry, dear, I didn't know what I was doing and I'd never intentionally embarrass you like that, honest!" But here he completely denies what he did and abusively pretends it's all a figment of her crazy, jealous mind. He paints himself as the injured party and browbeats Amy into apologizing to him. Joe doesn't care if Amy really believes him or not; he's satisfied thinking he's punished her so much for bringing up his infidelity that she won't protest the next time he does it.

Sarah: "I ... I need to talk to you about Bob, that new guy in your gaming group."
Mike: "Sure, what's up?"
Sarah: "We went to college together, and once we were at a party, and ...."
Mike: "And what?"
Sarah: "And I had too much to drink, and I went to lie down in a back room, and when I woke up Bob was on top of me, and he'd gotten my jeans off --"
Mike: "No way! Bob's such a nice guy; he'd never do something like that."
Sarah: "Yes, I know he seems nice, but he did it, and ... I'm not comfortable with him in the house."
Mike: "That's crazy. Why would Bob need to pull that kind of creeper stuff? He's good-looking, he's got a kick-ass job, and the jokes that guy tells are hilarious."
Sarah: "I know, but --"
Mike: "Come on. You were drunk, right? It probably wasn't even Bob that did it. And besides, that was what, five years ago? Aren't you over it yet?"
Sarah: "I ...."
Mike: "Aw, why are you crying? There's nothing to cry about!"
Sarah: "He tried to rape me. He would have done it if Mark hadn't come in and interrupted him." 
Mike: "You're so emotional. Come on. Bob's not some scary rapist; he's a good guy. He's the best GM we've ever had! I'm sure that even if all that happened, he's totally changed."
Sarah: "But --"
Mike: "But nothing. He's fine. You're fine. You just need to get over this weird emotional thing you've been harboring against him all these years." 
Sarah: "He --"
Mike: "Enough, already! He's coming to the house with the guys next Tuesday. Just make us some sandwiches and be cool, okay?"
Sarah: "..."
Mike: "Come on, Sarah. You better not cause a bunch of drama and embarrass me in front of my friends."
Sarah: "Okay." 

Mike never wants Sarah to suffer emotionally; the problem is that he just doesn't really care if she suffers as long as he gets what he wants. Defending her right to feel safe in her own home would involve disrupting his own social activities and saying "No" to his buddies, and Mike's too selfish and cowardly to do the right thing. Mike's also emotionally invested in the idea of Bob as a Really Cool Guy, because otherwise he'd have to admit he's been thoroughly tricked by a predator. From his perspective, it's easier to pretend that his wife is overly emotional and irrational than to address the problem that he's friends with a rapist. This exchange, unfortunately, is a pretty classic example of how rapists get other men in their social circles to help keep their victims quiet. 

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