If you’ve ever wondered what “The Omen” might have been like had the story been shorn of its Satanic elements, George Ratcliff’s “Joshua” provides a pretty good idea: the title character is a manipulative young boy, the apple of his family’s eye, who reacts to the birth of a baby sister by methodically ridding himself of parental control so that he can have the life he wants with a doting uncle. It’s not the first tale of such a sociopathic child, of course—“The Bad Seed” paved the way back in 1956. But little Joshua Cairn (Jacob Kogan) isn’t portrayed as a genetically-disposed killer, like Rhoda Penmark. He’s simply a calculating, amoral sort, more like Henry Evans, the angelic-seeming kid that Macaulay Culkin plated, in the “change of pace” role that initiated his career slide, in 1993’s “The Good Son.” But unlike that movie’s rustic setting, this one is located in the tony atmosphere of upper-class New York City.

It’s there that Brad Cairn (Sam Rockwell), a financial funds manager, and his wife Abby (Vera Farmiga) keep an apartment overlooking Central Park with their gifted son Joshua, a brilliant student at a posh private school and a budding piano virtuoso, whose talent is nurtured by Abby’s brother Ned (Dallas Roberts), a composer of Broadway musicals. Their blissful life changes, however, when Abby gives birth to a little girl, who quickly proves unmanageable, colicky and crying uncontrollably. The stress of caring for the newborn sends Abby into a severe downward spiral—and Brad is eventually forced to take a leave from work to attend to the household. The situation isn’t helped by the supposedly helpful presence of Brad’s mother Hazel (Celia Weston), whose habit of offering unwelcome advice, especially about the fundamentalist form of Christianity she thinks the children should be introduced to, worsens Abby’s condition.

Watching—and perhaps causing—it all is precocious little Joshua—almost preternaturally quiet and inconspicuous, except for his keyboard practicing, his odd interest in ancient Egyptian burial practices, and his strange emotional breakdown at the talent show for which he’s been preparing so long. There are also some strange deaths in his vicinity—of the beloved family dog he frequently walks, of the class hamster at school, and ultimately…well, that would be telling.

There’s not a great deal of plausibility to “Joshua,” not because it’s impossible to conceive of an inherently malicious child (far from it), but mostly because the attitude of the adults sometimes strains credulity, and the actors playing them don’t succeed in making them entirely believable. Rockwell is an excellent actor in many roles, especially rebels and societal fringe-dwellers, but he frankly doesn’t quite convince as a Wall Street trader, his quirks and fussiness being more likely to scare off clients than attract them (or gain the confidence of his boss, played by an equally casting choice, Michael McKean). And Farmiga plays Abby so high-pitched from the very start that she doesn’t have much room to increase the character’s trauma. (It also seems to take a very long time for those around her to realize that she must be suffering from post-partum depression, an ailment that’s been in the news so much recently that her husband seems positively obtuse for not taking quicker action.) Weston and Roberts, meanwhile, accentuate the stereotypical aspects of their characters—the fluttery mother-in-law, the swishy uncle—just a bit too much.

“Joshua” nevertheless works reasonably well because Ratcliff, working closely with cinematographer Benoit Debie and composer Nico Muhly, is successful in building an atmosphere of increasing dread, and because young Kogan manages to be both impassive and subtly dangerous. He persuades you, as the miscast Culkin signally failed to do, that the malignancy of the boy is a real possibility.

The result is a genuinely unsettling film about a sociopathic child that avoids both the staginess of “The Bad Seed” and the excessive slickness of “The Good Son” without resorting to the supernatural mumbo-jumbo of “The Omen.” “Joshua” is an enjoyably decorous creepshow.