Gaslight was the play that made its writer Patrick Hamilton a very rich man. It opened in London in 1938 to exceptional reviews. Noël Coward was a fan. King George VI took his wife to see it. In 1940, it became a British film, followed four years later by the Hollywood version starring Ingrid Bergman. When domestic abuse was barely whispered, Hamilton shone a light on coercive control and marital manipulation. He caught it exactly.

The play is set in the upper-class house of Jack and Bella. She tiptoes around him. He’s kind, then cold. He flirts with women, but when Bella objects, she’s told she “reads meanings into everything”. He hides her things so she questions her sanity. At night, he secretly visits the top floor of the house, turning up the lights, causing the downstairs lights to dim (hence the title).

As a study in psychological abuse, it’s a devastatingly accurate picture. Eight decades on, gaslighting is the go-to term for a special sort of torture – the kind designed to discredit and disorient its victims, make them doubt what they know, distrust and turn against themselves.

Psychotherapist Stephanie Sarkis, author of Gaslighting, began to suspect that many of her patients were victims. She posted an article online – 11 Warning Signs of Gaslighting – which went viral. Gaslighting was published in the US last October and Sarkis still receives multiple calls and emails each day from grateful readers. “People tell me the book saved their life,” says Sarkis. “The more we know about it, the less vulnerable we are.”

In 2016, “gaslight” was declared the “most useful word” by the American Dialect Society and, in 2018, it was one of Oxford Dictionaries’ “words of the year”. In the UK, gaslighting within intimate relationships has become a crime under coercive control legislation, as well as a recurring plot point in popular culture. We see it in thrillers, like Girl on the Train, the heroine manipulated by her murderous ex. We see it in soaps – Helen Archer so tormented by her abusive partner, she consults her GP who prescribes medication. It’s even made reality TV – last year’s Love Island contestant Adam Collard was accused of gaslighting by Women’s Aid.

Gaslighting is the deepest kind of moral wrong

In the US, President Trump’s blend of lying, denying and intimidation has sparked cries of gaslighting from NBC to USA Today to Teen Vogue. Harvey Weinstein has been held up as another high-profile perpetrator.

So what gives gaslighting its dark power? Kate Abramson, philosophy professor at the University of Indiana, calls it the “deepest kind of moral wrong”.

“Imagine you’re going through the worst experience you’ve ever had,” she says, “and, at the same time, you’re being told it’s not happening.” So perhaps that’s some executive emerging from a hotel bathroom naked. At the same time, he’s saying: “We haven’t done anything!” When you’ve escaped, he bombards you with gifts while insisting “nothing happened”. He assures you that he’s done this with lots of women – he names many – they always end up “throwing themselves at me”.

“There aren’t many ways of interacting that manage to be simultaneously wrong in so many dimensions,” says Abramson. It’s not just the abuse, but the erasure of abuse as it happens. It’s the obliteration of another person’s perspective, insistence that it’s not the action that’s wrong, but their reaction. “If your judgment is ‘irrational’, you can no longer be a source of challenge,” says Abramson.

“We all question whether we’re right about something. Gaslighting takes that necessary quality for human interaction and uses it to undermine our ability to interact at all. And that’s dark.”

It’s now recognised as a common component of domestic abuse. “Freya”, an artist, was gaslighted by her ex-husband just as Bella was. He didn’t “hit” her to establish control – he isolated her and broke her. He sabotaged her work. “If I sketched in the day, he told me I was neglecting the children,” says Freya. “If I sketched in the evening, I was neglecting our marriage.” He froze out her friends, convincing her that they made passes at him (she discovered it was the other way round). She didn’t know who to trust. He repeated that she was “naive”, “too innocent” and “stupid”. “He’d tell our children that the only safe place was ‘Daddy’s arms as Mummy wasn’t doing a very good job’.”

A colleague whispers abuse or ridicules you in front of others

At the same time, he hid things. “I was a nervous wreck and had lost a lot of weight, so my wedding rings kept slipping. I took them off to wash up,” Freya remembers. “One day, they disappeared from the microwave top and I was frantic. I knew I’d be in for it if I didn’t find them. He looked so calm and happy that weekend. I kept trying to hide my hands but on Sunday night, he kept asking. ‘Are you OK, you’ve looked a bit preoccupied? Have you lost anything?’ I denied it, then he dragged me downstairs and took me to a cupboard of champagne glasses we never use. The rings were inside a glass and he shouted that I was a liar and failure.”

Gaslighting also happens in the workplace. “A gaslighting colleague might whisper abuse when they walk by your desk, sabotage your work or take credit for it, give wrong times for meetings, ridicule you in front of others,” says Sarkis. And when it comes to political leaders, there can be no better example than Trump. When challenged, he viciously denigrates the challenger. (Words like “wacky”, “crazy” and “dopey” feature heavily in his Twitter feed.) His obsession with how things are perceived is standard gaslighting – his claims that “what you’re seeing and what you’re reading is not what’s happening”. Straight from the gaslighter’s handbook.

Except there isn’t one. So how do gaslighters learn their craft? Do they know they’re doing it? There’s no clear answer. It’s common among psychopaths and narcissists, but it could be that someone learned it from parents, or stumbled upon it as a strategy to thwart a challenge. Dr Robin Stern, whose 2007 book The Gaslight Effect was updated last year, says it’s not always sinister or conscious. “It might be that when you’re feeling wobbly, you’ve learned that destroying someone’s alternative perspective is a way of centring yourself in certainty.”

It’s also hard to make statements about gender. Stern has found that most of her patients and friends encountering it have been women – and UK studies of coercive control show it to be practised overwhelmingly by men. However, Sarkis has treated many male victims of female gaslighters – and Stern points out that teenage girls can be prime perpetrators. She gives the example of Odd Girl Out, the book by Rachel Simmons about bullying. A victim is blanked by former friends, but when she asks why, she’s told, “What are you talking about? You’re so sensitive!” Hopefully those girls grew out of it. A person might gaslight once or twice but when it’s repeated patterned behaviour, be very afraid.

“A gaslighter is someone who can’t bear other viewpoints,” says Abramson. “They need the way they see the world to be placed beyond dispute, and set out to destroy not just differing perspectives, but the source as well.” If you have one in your life, advises Sarkis, “the best thing you can do is get as far away as possible.”

How to spot a gaslighter

Their apologies are always conditional When someone says, “I’m sorry you feel that way,” that’s not an apology; the other person is not taking responsibility for their behaviour, they’re simply manipulating you. Gaslighters will only apologise if they are trying to get something out of you.

They use splitting Gaslighters love to pit people against each other. This is known as splitting. An example would be lying to one friend about another, saying a mutual friend had said something unflattering about them.

Gaslighters are the ultimate agitators and instigators The gaslighters will then watch comfortably from the sidelines, the very fight that they caused.

They’ll do anything to get in with you Gaslighters are good at buttering people up. As soon as you fulfil their needs, they drop their mask of niceness. Trust your gut. If the friendliness seems phoney, beware.

Extract from Gaslighting by Stephanie Sarkis (£14.99, Orion). Buy it for £13.19 at guardianbookshop.com