Nowadays vampires are heart-throbs, monsters are neurotics, zombies are comic turns, serial killers are bores and aliens are cutie pies. So what's left to scare us?

If the remarkable success of Paranormal Activity is anything to go by, the answer's to be found in the spirit world. We're not talking about ghosts, who seem to have become almost as endearing as those other now threadbare bogeymen. This film makes it clear that its own baleful spectre is no mere unquiet revenant, but a fully-fledged fiend.

Demons have an impressive record of delivering the spine-chilling goods. Lots of people consider The Exorcist to be the most frightening film ever made. However, like many other supernatural scarers, The Exorcist was able to tap into the residue of terror instilled in us over the centuries by the priestly guardians of our souls. Paranormal Activity, on the other hand, has no truck with our Satanic heritage.

You might have expected its tormented protagonists to call in the Catholic church, what with that outfit's unmatched expertise in onscreen exorcism. This never occurs to them. They, and the only qualified mentor they manage to summon up, maintain a ruthlessly secular attitude towards the unearthly peril confronting them.

So we're dealing simply with a dislocated, disembodied entity defined and motivated solely by its own malevolence. It doesn't even try to be original. Believe it or not, what seems universally considered to be the most terrifying big-screen apparition since 1973 relies on moving bunches of keys around work surfaces, opening doors which ought to be shut, switching lights on and off and even going bump in the night – literally.

Which raises a question: why is this thing so scary when so many of our other one-time hair-raisers have lost their moxie? The evidence points to an unsettling possibility – that we actually believe in poltergeists, in a way that we don't believe in more physically tangible bogeymen.

About half of us seem prepared to tell pollsters that spirits do indeed exist, or at least might do. But just try asking people who dismiss the idea as nonsense if they fancy playing with a Ouija board or spending the night in a supposedly haunted house. Even those for whom alien abduction and mutant crustaceans hold no terrors can turn surprisingly chary.

Just why this should be the case in our supposedly sceptical age is far from clear. Evolutionary theorists suggest that hypersensitivity to indeterminate threat may have helped our ancestors evade swift, guileful and deceptive predators. All the same, we've had plenty of time to get used to what are now readily available explanations for apparently paranormal experiences.

Perceptions don't just reflect relays of environmental stimuli to networks of active brain cells. Memory and emotion pollute the flow from an early stage. Raw sensation is thus fighting a constant battle with internal inputs to shape cognitive awareness.

But many things can disrupt the balance between these two which prevails most of the time. Sensory deprivation or overload, poisons, oxygen deprivation, hyperventilation, hypoglycemia, fever, pain, fasting, dehydration and social isolation can all trigger hallucinations. So too, however, can less obvious influences. A relationship has been established between the incidence of bereavement apparitions and global geomagnetic activity. In the laboratory, the application of magnetic fields to the right hemisphere of the brain can evoke the sense that another being is present.

The whole point of the videotaping project in which Paranormal Activity's hero engages is to get round such spoilsport information. Sadly, convincing footage of ectoplasmic entities seems to turn up more often on cinema screens than on actual camcorder viewfinders.

Doubtless we'll be seeing more of it. Home video mocumentary spookery has indeed lighted upon a mysterious phenomenon. The Blair Witch Project laid it bare, while Paranormal Activity was but an infant phantasm in a software programmer's mind. It's that in this strange corner of movie-making, a tiny budget can generate wildly disproportionate box office revenues. The big screen, we must therefore assume, will be further addling both hemispheres of a great many brains for a long while to come.