Kids, huh? They say the darnedest things, they look at the world with fresh eyes and then, once every decade or so, you remember they can be murderers.

The fictional terrors are more theatrically demonic than the real-life ones: the smiling, faintly Talented Mr Ripley-esque Sam in Channel 4’s Born to Kill; the fabulously mythic teens in All4’s The End of the F***ing World, who raise the ghosts of fiction’s most magnetic villains, with their caustic detachment like a water chute into evil. Or there’s suspense in the ambiguity of their dire purposes – embodied in the middle child in Toni Collette’s Hereditary, which was billed after Sundance as the scariest horror film ever; seen-it-all audiences screaming out loud in theatres, not out of Blair Witchy hysteria, more at the atmospheric inevitability of looming disaster. Kids are quite binary in cinema: if you get a whiff that they’re not purely good, odds are they are fathomlessly bad.

In some cases, they are simply more knowing and sly. Viewers of the ITV crime-noir excursion Marcella were appalled to see Anna Friel’s son crushing a mouse in his hands. This, from a show punctuated by corpses, graphic murders and a horrifying range of decomposition, was the act that had viewers (granted, according to Twitter, via the Daily Mail) threatening a boycott. But perhaps the revulsion is less excessive than it sounds: cruelty to animals is a well-known flag of psychopathy.

In real life, child murderers are aberrations, too rare to draw into a pattern. In fiction, and in a more general cultural fascination, wrong ’uns come in gangs: Jude the Obscure and Turn of the Screw appeared within three years of each other; The Midwich Cuckoos (later filmed as The Village of the Damned), Lord of the Flies and the film The Bad Seed likewise. It is a satisfying coincidence that the last trio arrived in the middle of a baby boom (in 1957, 1954 and 1956 respectively) and the babies wouldn’t stop coming – there were more every year until they finally tapered off in the 60s. These fictions felt like reasonable imaginative responses: all these children everywhere … what if they were evil? What if they were aliens? What if they turned on us?

Heather Shore, professor of history at Leeds Beckett University, has this more detailed analysis. “In the 1950s, you’ve got the arrival of a whole discourse around child psychology. The DSM [the diagnostic handbook of the American psychiatric profession] was first produced then, and a lot of ideas were coming through, following the second world war and children’s experience of the Holocaust. This was an important phase in the developing notion of the disturbed child.”

Quite what the reasoning was in the 1890s – or the 1990s for that matter, with Macaulay Culkin’s devilish The Good Son, JG Ballard’s Running Wild having arrived five years prior in 1988 (a double whammy of kid-phobia in which the children are both homicidal and working together) – is less plain. Children seem to become a locus of anxiety during times of technological upheaval.

“Children represent the future,” historian Eleanor Betts, author of Understanding Children Who Killed, points out. “And when there’s new technology, fear of the unknown, attention turns to how it affects children. So in the 90s you’ve got video games; in the 19th century, they had the same fears about cheap literature, Penny Dreadfuls, and the idea that children might read it and try to relive these violent adventures in real life.” Childhood is a convenient site on which to play out anxieties about a changing world.

“There’s this idea that, because they’re children, they’re extremely receptive,” Shore observes. “So if they read impure literature or watch films, they’re very easily corrupted.”

There’s also the fear of one’s own obsolescence as technological advance accelerates and only the children understand it, their world becoming mysterious and their power amplified. Like in the Virgin advert where only a seven-year-old knows which remote controls which box.

“Meanings imputed to childhood turn on these two poles,” says Ellie Lee, professor of sociology at the University of Kent. “The trope of the child as innocent and pure and lovely, on this road to perfection until some adult comes along and messes it all up; and the trope of the child as wilful harbourer of malevolence and evil, which you’ve got to knock out of them.” Culturally, we don’t move cyclically between these two ideas, but rather flip between them in response to anxieties of our own. “Childhood becomes a cipher for what adults should be like and what our moral purpose should be. Are we inherently good or are we inherently evil? But you can never start with children.”

Childhood, rather than being a mitigating factor, actually makes every crime worse, defiling not just its victim but all of us, since it has destroyed our faith in innocence. All the normal concessions made to youth – that it may be a bit delinquent, and at the very least lacks impulse control; those shoulder-shrugging kids-will-be-kids truisms that suck oxygen from outrage and make everything a little bit boring – are suspended. If a child is purely bad, then the more strongly we react against it, the more purely good we become.

Hereditary is out in cinemas on 15 June