Horror has been a form of entertainment since humans first looked up to the sky and thought there were gods. The rites of Dionysus - which over time became changed into the earliest of the bloodstained Greek tragedies - saw sacrificial victims torn apart in woodland. The rites spoke to the worshippers about the dark, primitive nature of humankind - but were also form of drunken participatory theatre as well. I've often wondered if, in the slasher film, when the last survivor is racing through the forest to escape the wild, deranged killer with the chainsaw, whether or not something very ancient is being tapped into within us all. Might it rouse the ancestral memory of old religions in which we killed or were killed?

Shakespeare earned his stripes with Titus Andronicus. Cannibalism, rape, mutilation - the Daily Mail would ban this filth today, no question, even though the play tapped into the fears of a Europe riven by the brutality of religious war, despotism and terror. Universal Studios took horror to the bank with Dracula, Frankenstein and The Wolf Man - and traded on the easiest of horror's tropes: the supernatural, the unknown, the bogeyman; irrational terrors which scare all of us from the cradle to the grave. Though, of course, there is symbolism aplenty for the modern fears which even these well-produced movies exploited: the dangerous foreigner, science gone mad, our inner beast.

Horror also allows us to transgress, at least vicariously, because it is a transgressive medium. Students of theatre will know that drama can be split in two: Apollonian theatre which is well-made, conformist, and ends with order restored; and Dionysian (there's that dark god again) which breaks rules, challenges wisdom, and ends in chaos.

Now, many folk think horror is Apollonian - Dracula is always killed at the end of the movie, after all, isn't he? Yes, but Dracula often gets up again, and for every dead supernatural monster, there is an undying earthly Michael Myers or Jason Voorhees, and a catalogue of films where the end isn't just chaos but absolute nihilism - I'm thinking of A Serbian Film, Martyrs, and Funny Games, here. These are films where evil doesn't just win, evil has always reigned and always will reign. Evil is there to toy with humans.

And to add transgression to transgression, horror is ultimately voyeuristic. After all, isn't there a bit of the voyeur in us all. Horror knows that and plays with it - just watch Psycho or Peeping Tom, or even last year's BBC series The Fall: all good barometers for the voyeur inside every one of us.

Horror isn't just cathartic in that it lets us deal with our everyday fears - spiders, blood, darkness, ghosts - it also allows us to deal with metaphysical fears: that the world is empty, alien, blind, deadly, cruel, pointless. It's a release valve for humans trapped in a world filled with very real horror and violence. We may live in the west with our first world problems - like where to get a good flat white - and Heat magazine inspired life goals, but we also inhabit a planet with Islamic State and ebola. Perhaps, only dark art can offer a release from such existential threats - better to deal with the fear inspired by terrorism and disease in a movie house in front of a blood spattered screen than work through the real world horror with a rational mind to the awful, inevitable, unthinkable conclusions.

While it is not one of my favourite movies, by any means, The Purge is a pretty good examplar of all of the ideas above: it is a film about a civic ceremony which takes place once a year in an America in the near future. On one night, anything goes - murder, rape, whatever you want to do is legal. Blacks kill whites, whites kill blacks, rich kill poor for fun, poor rise up against the rich, sadism as sport, the good and decent hopelessly lost and at risk. Latent social fears, transgression, chaos, terror - it's basically a university lecturer's go-to film to explain the theory of horror movies in society. It's also a neat little dissection of class. It's little wonder in the new Victorian world which international finance has forged for us in the west, of food banks and the super rich, that class has crept into modern horror movies in a way it has never done before. Eden Lake, one of the best British horror films of the last four decades, is a brutal descent into the violence of underclass life - though it fails by pitting a nice middle class couple against wicked feral kids and ends up peddling a quite reactionary and patrician view of the world. The Americans tend to deal with class better - with Cheap Thrills (what would you do for money?), and In Their Skin (the politics of envy) the best of the recent crop of depression-era horrors.

This year's Frightfest at the Glasgow Film Festival looks as if it does a pretty good job of sticking its bloody finger into the open wound of modern woes and fears. We've got The Atticus Institute back in the territory of science gone wrong, gone mad. The Hoarder, with shades of Fritzl and slavery, and that deep dread we have that the people we know and love may not be the kind souls we think they are. Then there is Mario Bava's 1964 classic Blood and Black Lace - a precursor to the video nasties of the late 70s and early 80s. The title really says it all: it's a study in sexual sadism. There's the Belgian movie The Treatment, which looks as if it will become this year's The Vanishing (the gruelling Dutch original, not the vanilla American remake). It picks apart our very modern monster, the paedophile. Finally, we have There Are Monsters, a cult classic before the negative was even dry. It does a double-take on consumerist society - are we all devils, all wicked, all cruel, all irredeemable?

One film stands out above - Blood and Black Lace - it's the only movie that isn't a new release. So why is it there? Well, for a reason that all horror fans will immediately get. It was the first film that Alan Jones - the co-director of Fright Fest in London and Glasgow - ever saw. As I said above, all horror fans yearn for the high of that first film scare - and Jones was swept up by Bava's colour, music, style, and the terror he felt as a 16-year-old kid watching his first X rated movie.

"The first horror you see remains with you for the rest of your life - marks you - and you chase that," he says. "We keep looking for more and more."

There's nothing in made-up horror to match the reality of IS, he says, but "horror films subliminate our fears" - so they serve some modern function by letting us work through the things we dread. He's picked movies for Glasgow's Fright Fest this year which, he believes, will flick our modern psychic switches. "Horror fans live vicariously," he says. "That's why I'd trust them with my life. They've seen the horror - they'd never do it. They are good people."