One lump or two?

In "Step Brothers," you get a pair: Will Ferrell and John C. Reilly, both 40-year-old adolescents whose development is not just arrested but convicted. Their hapless, mega-enabler single parents have never quite noticed that the boys are still living at home, doing exactly what they want to do -- which is absolutely nothing.

Brennan (Ferrell) wasn't "fired," he was laid off from PetSmart. There's a difference. Dale (Reilly) never even pretended to have or want a job. His affluent doctor-father leaves him pizza money each morning before leaving for the hospital.

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'Step Brothers'

3 stars = Good
Ratings explained

• Starring: Will Ferrell, John C. Reilly, Mary Steenburgen.
• Rating: R for crude and sexual content and pervasive language.

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But when Brennan's divorcee-mom (Mary Steenburgen) meets Dale's widower-dad (Richard Jenkins), it's a match made in purgatory. They wed, to form one big unhappy family in which the overindulged stepbrothers must now share a room. The resulting mutual hatred escalates from sulking to open warfare -- until Brennan's "perfect" brother and lifelong tormentor Derek (Adam Scott) shows up. With a common enemy, the step-fratricidal rivalry develops into blood-brother bonding, while Derek's repressed wife (Kathryn Hahn) develops a nymphomaniacal passion for Dale.

Reunited (along with director-writer Adam McKay) from "Talladega Nights," Ferrell and Reilly truly enshrine 13-year-old minds in 40-year-old bodies. There's no denying their comic chemistry and timing -- those fast liftoffs from stupor to ballistic rage. Everybody with siblings will identify with the "if you touch anything in my room, I'll kill you" motif. Brennan's ultimate desecration of Dale's drum set has to be (but shouldn't have been) seen to be believed.

Their hostilities are interspersed with bursts of lavish affection and pubescent profundity. When Brennan, who fancies himself a crooner, is finally coaxed to sing (read, butcher) the great Bonnie Raitt anthem "Something to Talk About," Dale is transfixed: "Your voice is like a combination of Fergie and Jesus!"

Reilly is a fine comic, as only a "serious" actor can be (he was Oscar-nominated for "Chicago," critically acclaimed for "Streetcar Named Desire" on Broadway). Steenburgen in her matched suits is all the funnier for being so sweetly, imperturbably genteel. Jenkins ("Six Feet Under") is a likewise delightful straight man, unaware of his own absurdity.

Ferrell and his crudites, of course, are an acquired taste -- more like pork rinds than caviar. His film oeuvre is amazingly uneven: the bad "Anchorman" vs. the good "Melinda & Melinda"; the good "Blades of Glory" vs. the bad "Bewitched"; the hit "Talladega" vs. the bomb "Semi-Pro." His vehicle at hand is peppered with gross-outs (extended flatulence, doggie-do gags) and contains more F words than the collected works of David Mamet. But stick around for the martial-arts massacre in the post-credits.

"Step Brothers," in sum, adheres to the grand tradition of Apatow-"Animal House"-"American Pie" tastelessness. I tried not to laugh -- but gave in to the guilty juvenile pleasures.