Comedy. Will Ferrell, John C Reilly, Richard Jenkins, Mary Steenburgen, Adam Scott, Kathryn Hahn. Director: Adam McKay.

THE biggest laugh in Adam McKay's comedy of escalating sibling rivalry comes before the opening credits roll.

We are treated to one of George W Bush's infamous turns of phrase, taken from a campaign speech in 2000, when he proudly declared, "Families is where our nation finds hope, where wings take dream."

This unintentional verbal faux pas surpasses anything in McKay and Will Ferrell's script, which is as gleefully mean-spirited as it is unnecessarily foul-mouthed.

The flimsy premise reunites Ferrell and John C Reilly, who co-starred in Talladega Nights: The Ballad Of Ricky Bobby, as 40-something men who possess the mental age of adolescents. Forced to live together, these two idiot savants - with the emphasis on idiot - bicker incessantly, threatening physical violence ("As soon as your eyes shut, I'm gonna punch you square in the face!") before realising the error of their ways.

Brennan (Ferrell) is 40-years-old going on 14.

He still lives at home with his mother Nancy (Steenburgen) and openly nurtures resentment for his boorish younger brother Derek (Scott), whose perfect family includes a blonde, beautiful wife (Hahn) and two adorable children.

During a medical convention, Nancy meets Robert (Jenkins), who has his own adult son at home - Dale (Reilly) - and the lonely 50-somethings embark on a whirlwind romance.

Wedding bells ring and Nancy moves in with her new husband, with disgruntled Brennan in tow. The new living arrangements pose a dilemma because for the first time ever, Dale must share his bedroom.

Sibling resentment boils over with Dale's cherished drum kit taking the brunt of the frustrations, but the stepbrothers unexpectedly discover common ground: favourite dinosaurs and their shared hatred of Derek.

"Did we just become best friends? Do you wanna go and do karate in the garage?"

When Dale subsequently discovers his stepbrother's gift for singing ("You have the voice of an angel...it's like Fergie meets Jesus!"), they suddenly find their direction in life as a musical double-act.

However, Derek mocks their efforts, waiting smugly for the pair to fail. Step Brothers is just as scatological, messy and puerile as Anchorman and Talladega Nights, and will therefore delight audiences who lapped up Ferrell's buffoonery in those pictures.

There's undeniably a spark between the leads and their energy, riffing off one another, powers the film through its many lulls to its shameless feelgood resolution.

Laughs are in desperately short supply though. If the lead characters were likeable, or their childish behaviour in any way comprehensible, then we might feel compelled to chuckle as Brennan gets his revenge by wiping his sweaty lower portions on Dale's beloved drum kit.

Instead, when a gang of pint-sized playground bullies beats the gruesome duo to a pulp, we strongly resist the urge to cheer.