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Johann Hari: The dark seduction of horror films

I was drawn to these movies by my granny, one of the sweetest people I've known

Show me what scares you, and I will show you your subconscious leeching out into the world. Every culture – every person – imagines there are terrors waiting for us in the dark: the shape of the monsters changes from year to year, but the fear remains. Man, it seems, needs dread and circuses.

I was drawn into the world of horror movies by my grandmother, one of the sweetest and gentlest people I have ever met. Yet she has always loved terrifying movies – the more violent and savage, the better. Even though she is nearly 90, she loves to be wheeled into the Saw movies and to gasp at how the inventive serial killer Jigsaw has managed to rip out another person's ribs. "Dinnae be such a weakling! Look!" she will josh at me as another pile of guts splatters on the screen and I discreetly vomit into my popcorn.

Thanks to her, I can debate for hours which is the best Mutant Insect movie (it's a toss-up between Them! and Slugs, with an honourable mention for Ants), or whether Sewage Baby is a better evil-foetus film than The Unborn II. (It is.) I can recite whole sections of I Spit on Your Grave, and deliver monologues from Nightmares in A Damaged Brain.

And I know what some of you are thinking: how sick. Why would anybody want to watch blood and cruelty and sadism? Once a generation, there is a horror-panic against Victorian penny dreadfuls or 1950s horror comics or 1980s video-nasties: we are overdue for another flurry.

Yet our monsters are only our own hidden anxieties, sweated on to the page or screen. Look, for example, at the early Gothic horror novels from the 18th century. The villains are depraved aristocrats, who (often literally) live off and feed on the masses. This was a resentment that was not yet politically sayable in England – yet it emerged in nightmare-tales.

The fears that have rippled through our minds since have all taken their purest form in horror. The reactionary, romantic fear of science was distilled into Mary Shelley's Frankenstein in 1818 – a metaphor that is used today against stem-cells, IVF or any other spurt of progress. The first great horror film was The Cabinet of Dr Caligari, made in Germany in 1921. It shows how a charismatic, crazed hypnotist and fraud enraptures the German crowds and leads them to madness. "You will become Caligari", the posters warned – and the German people did.

The growing independence of teenagers in the 1950s spurred a string of teenage monsters, beginning with I Was a Teenage Werewolf: they spout hair, get a gruffer voice, and become obnoxious. (Sound familiar?) The freedom of women met a repellent backlash in the slasher movies of the 1970s, where women flirt with sex and get stabbed. The formula was simple: get your tits out, and get them cut off. Forget Star Wars; this was scar wars, in misogynistic technicolour.

Yet by the 1990s, horror looked like an exhausted genre that had finally had its own ribs ripped out. The films became knowing, camp comedies – the antithesis of fear. In Scream, the characters comment glibly on their chances of survival: "There are certain rules that one must abide by in order to survive a horror movie. First, you can never drink or do drugs. Second, you can never have sex. Big no-no." When they finally pull the Scream mask off the killer, there is nothing but a blank space: the perfect symbol of the emptiness of 1990s horror – and culture.

But something strange has happened in Noughties Britain: straight-faced horror has risen again. In the past year, a string of superb low-budget horror films have been released: Eden Lake, Donkey Punch, Hush, The Children and Mum & Dad.

They all have a similar aesthetic: they are stripped-down and ultra-naturalistic. There are no supernatural shenanigans, or incidental music, or self-conscious winks at the audience. There is just a steady camera and sweaty fear.

These films are the purest form of cinema. Movies aren't a cerebral medium; they are visceral. The first people to see a film screamed in terror and ran from the room – and these movies bring us back to that first, primal response.

And what do they show Britain to be frightened of today? Our country's prejudices against the young and the poor are the beating soon-to-be-ripped-out heart of the films. Eden Lake is the perfect example, distilling some of our ugliest fears into an hour and a half of torment. A glossy middle-class couple drive out of London to Eden Lake, a development in the countryside. There, they anger a gang of teenage "chavs" – who torture them to death.

It is superbly made – and expresses our vilest social anxieties. As we have become a grossly unequal country, we demonise and dehumanise the poor to salve our soiled consciences. They aren't human! They don't have feelings! They want us dead! Where early horror feared the rich, our horror fears the poor, and tells us to run from them, fast.

All the years I have loved horror films, I have asked myself: why? Why do I like watching this stuff, when what it represents is so foul? Is horror morally corrupting after all?

This is the question at the heart of the best horror film – and quite possibly the best British film – ever made: Peeping Tom, released in 1960. It follows a serial killer who films his own murders of young women and obsessively rewatches them. The film constantly forces the viewer to ask: why is he watching this? What pleasure can he get? Are we like him? It was such a disturbing question that the film was pulled from the cinemas in a great national retch, and the career of the genius director Michael Powell ended overnight.

Aristotle believed that by experiencing the terrifying vicariously through monster-stories, we are purged of our fears and hatreds – and our desire to act on them in the real world. It is a safety valve. In Greek mythology, Perseus has a polished shield that enables him to look indirectly at the Gorgon – and thus slay it. Are horror films our shield of Perseus, enabling us to see our monsters, and wipe them out? Is my grandmother so gentle precisely because she can exorcise her natural sadistic impulses before a screen – and is she an exception among horror fans?

As I gaze out over my small mountain of horror DVDs and contemplate watching The Evil Dead for a 10th time (highlight: a woman is raped by a demonic tree), I wonder: when we peer into the dark, do we become darker – or do we leave lighter?