Starring Will Ferrell and John C. Reilly. Rated 14A. Opens Friday, July 25, at the Cinemark Tinseltown

It’s a given that there’s a 12-year-old boy lurking inside the bodies of all men. Hell, the marketing of movies, music, cars, and pretty much everything else is based on that notion. But Step Brothers goes the extra mile by bringing us a couple of middle-agers with their 12-year-old selves still clinging to the outside.

This latest effort from the writing-directing team of Will Ferrell and Adam McKay (Anchorman) plays on the natural, if nervous, chemistry between Ferrell and John C. Reilly, who are here Brennan Huff and Dale Doback, two fortyish dweebs whose comfortable suburban dotages are disturbed when Brennan’s mother (Mary Steenburgen) marries and moves in with Dale’s dad (Richard Jenkins).

Most of the extremely foul-mouthed film’s early sections are devoted to the men’s intense rivalry. It’s unclear, especially to them (since they look and sound almost identical), if calling each other “a curly-headed fuck” is more than an act of pure projection. In any case, things get even worse for the parents when the boys pull an about-face and become best friends.

They particularly bond over Brennan’s hidden singing talents. (Dale calls his stepbrother’s voice as “a combination of Fergie and Jesus”.) They also develop a seething antipathy toward Brennan’s younger brother (Adam Scott), whose success at everything irritates many, including his sex-starved wife (Kathryn Hahn).

It’s unclear what Jenkins and Steenburgen—who largely look bewildered, if game—are doing in this movie. More problematic is a draggy climactic sequence at the “Catalina Wine Mixer”, which comes across as underfunded, underpopulated, and woefully underimagined. Still, the movie is ruthlessly hilarious in dirty spurts, depending on your appetite for nut-sack humour, beating up schoolchildren, and, as always, curly-headed fucks.