Films are our fairytales, but few of them are as explicit in their allegiance to this form as the kinky comedy Cyrus [2], which opens on Friday. It belongs to a comparatively recent cinematic sub-genre which takes as its subject the tensions that arise from the fluidity of modern domestic life; let's call it "step-family entertainment".

When this particular fairytale begins, you're not quite sure who the ogre is. The likeliest candidate appears to be John (John C Reilly), who is first seen with pants-down and buttocks out. He even likens himself to Shrek when he meets Molly (Marisa Tomei). Could she be the beautiful princess who falls for the ogre? Well, sort of.

But that ship has sailed. She is in love, unconsummated love, with another ogre -- her obese adult son, Cyrus (Jonah Hill), who doesn't take kindly to her having male friends, or being away from the house. (That house, on a featureless Los Angeles street, demonstrates the importance of good location scouting: mounted on a hillside, it seems to float above pavement level in what must be a nod to the tower in which Rapunzel is imprisoned.)

On those occasions when Molly sleeps with John at his apartment, she makes sure to steal away in order to get home before Cyrus wakes in the morning and, presumably, something or someone gets turned into a pumpkin. There's another allusion to Cinderella in some important monkey business involving shoes. When John stays over at Molly's house, his trainers are gone in the morning, and he has to pad to work in his stocking feet; their disappearance goes gradually from sticking-point to crisis-point, an escalation not untypical of family life.

What's fascinating about Cyrus is how it shifts cruelty and treachery away from the step-parent (as seen in Snow White or Hansel and Gretel) and onto the potential step-child. The original purpose of the wicked step-mother figure, as anyone who knows their Bruno Bettelheim [3] will recall, was to help the child rationalise and segregate its mother's pleasure-denying or disciplinarian tendencies: by dividing the mother in this way, the child is able to keep intact the mother's loving, nurturing side. With time and emotional maturity, we come to realise that it's all the same: it's all mother. If we don't, we may turn into that victim who clings to the attractive side of his or her partner by insisting that "(s)he only hits me when (s)he is drunk/depressed/skint."

For all the creepy and inappropriate comments made by young Cyrus, perhaps the one which terrifies John the most is when the lad says "It's great to finally have a new Dad", mere hours after their first meeting. Jonah Hill's age and girth makes the mental image conjured by this line even more unsettling.

I think that's part of what lies beneath the dread in Cyrus. Fear of commitment is one thing. But what if someone wants more than anything to commit, as John does, only for there to be another adult who makes that impossible? Like any decent analysis of family, the picture is a study of the forces of control floundering in the face of chaos.

It's worth noting too that Reilly starred in another recent example of step-family entertainment, the curious 2008 comedy Step Brothers, in which he and Will Ferrell played grown-up but infantilised children whose single parents marry, making resentful step-siblings of these middle-aged morons. Like Cyrus, it shows the family unit in free-fall: if you can acquire siblings in your forties (or, in Cyrus, a son who is to all intents and purposes a walking, Godzilla-sized Oedipus complex) then anything might feasibly happen. All bets are off.

My own favourite "step"-movie remains the impossibly tense thriller The Stepfather (the 1987 original [4], not the recent remake, dummy). It's a highly intelligent study of the fissures caused not just by the jigsaw-puzzle of step-family life, but by the entire concept of family. The unattainable ideal of domestic bliss drives Jerry Blake (Terry O'Quinn) to first insinuate himself with a widow and her children, and then to slaughter them when they fall short of his impossible expectations.

Perhaps the filmmakers' choicest decision was to open with Jerry calmly vacating the scene of one massacre before moving on to the next unsuspecting, fatherless clan. The movie is a nasty, clever little cracker that won't lose its pertinence until we outlaw family life.

Ryan Gilbey is the New Statesman's film critic [1]