The slyly generic title of Jean-Pierre and Luc Dardenne’s L’Enfant (The Child) suggests ample room for doubt about who’s the real kid in their new film, a domestic drama set in Seraing, the down-at-heel Belgian steel town that’s been home to every one of the brothers’ intimate examinations of lives eked out on the lumpen rump of working-class. It’s not the luckless newborn baby, hauled from pillar to post, a formless bundle with its tiny arms sticking helplessly out of a powder-blue snowsuit. Nor, after a rocky start, is it the baby’s teenage welfare mother, Sonia (Déborah François). Like the Dardennes’ previous film Le Fils, L’Enfant is a movie named for a child that’s really about the struggle to become a dad. It’s a tale of agonizing transformation for the baby’s hopelessly ill-equipped father, Bruno (Jérémie Renier), a strictly small-potatoes young hood who subsists from street begging and petty theft through the schoolboy gang he runs from a primitive dugout in a field — the same one where he and Sonia also sleep after he casually sublets her apartment while she’s in the hospital giving birth. With equal nonchalance, Bruno ups and sells his son to child traffickers, barely pausing to ask if the adoptive parents are “a good family.” By which he means, Do they have money?

As Dardenne films go, with their slow, minutely observed journeys from despair to faint hope, L’Enfant is a horror movie of sorts, and for a few minutes at least, a kind of thriller. There’s even a car chase, which, if less were at stake, might seem an entertaining digression from an aesthetic shaped by documentary hyperrealism and a novelistic accretion of mundane detail. L’Enfant ebbs and flows with frantic scrambles for recovery and survival, yet its eventfulness, far from being pressed into service to drag out audience suspense — the return of the baby turns out to be a sideshow — instead brings us to feel our jaggedly uneven way along with Bruno and Sonia in something approaching real time. L’Enfant was worked up from a simple act of noticing — the Dardennes saw a young woman endlessly, aimlessly wheeling a pram with a small baby in it while they were shooting Le Fils — and at first the movie seems to focus on this anarchic, but oddly affectionate, couple, gamboling like frisky tiger cubs as they attend to and ignore the needs of their baby with unnerving arbitrariness. Then comes the sale, and a kind of angry regrouping for Sonia. For all François’ fragile prettiness — she has the undefined evanescence of a young Julie Delpy — she brings to Sonia an indomitable focus, a readiness for self-definition that rapidly outstrips the comprehension of her chronically feckless boyfriend. Yet it’s Bruno, with his coarse, yet vulnerable, features (Renier made his screen debut playing the more morally evolved son of Olivier Gourmet’s slumlord in La Promesse), who gets the lion’s share of the Dardennes’ fabled close-ups, as he haltingly absorbs the import of what he’s done to the baby, to Sonia and to himself.

Aside from a glancing hint at the chaotic parenting Bruno himself may have received, the Dardennes refuse to prop him up with a back story of the kind that other working-class champion, Mike Leigh, spends months stolidly developing before he lays hand on a camera. Instead, the movie revs up an intense, at times almost unbearable, audience anxiety for Bruno and Sonia, as well as for their baby. By almost any measure, Bruno’s impulsive betrayal is sociopathic, yet he’s no sociopath. It’s not just that the Dardennes recoil from such judgments, as they do from deifying their subjects. They have always grasped what most other middle-class keepers of the proletarian flame have not — namely that brutalized lives often produce brutish behavior. L’Enfant doesn’t make everything better for Bruno or his family; it’s a movie about the perils of the unstructured, unguided life. In Europe, where some vestigial societal feeling for the dispossessed remains even as they’re exiled to crime-infested exurbs, the Dardennes’ fiercely unsentimental loyalty to such lives is sufficiently admired to make them regular stars at Cannes: L’Enfant won the Palme d’Or last year, just as Rosetta did in 1999. For the same reason, they’ll never become household names outside critical circles in this country, as Leigh has done with his increasingly schmaltzed-up lionization (a saintly backstreet abortionist, for Christ’s sake!) of the working stiff. To call L’Enfant a movie about growing up would be to trivialize its intention, as in every other movie by this indispensable duo, to bestow on its bruised souls what the writer Grace Paley sublimely calls “the open destiny of life.”

L’ENFANT (THE CHILD) | Written and directed by JEAN-PIERRE and LUC DARDENNE | Produced by the DARDENNES and DENIS FREYD | Released by Sony Pictures Classics | At selected theaters

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