If a new American comedy starts out with curses that would have made your great-grandmother blush and an obsession with poop and penises just this side of X-rated, you can be sure that it will end as warm and fuzzy as an old Andy Hardy movie. Raunch, scatology and four-letter words are nothing new in Hollywood comedies. They may have begun as underground outrages ("Pink Flamingos"), but by the time of "Porky's," "American Pie" and the Farrelly brothers they were as mainstream as, well, apple pie. What is new is the shotgun wedding of obscenity and sentimentality. If the bad boy-man hero (it's always a guy) seems stuck in the eternal pigpen of adolescence, you can be sure that by the end he'll have learned his lessons, shouldered responsibility and earned the love of the gorgeous, competent woman he pines for.

How did this formula become the new comic orthodoxy? We can all thank Judd Apatow, whose "40-Year-Old Virgin," "Knocked Up," "Superbad" and "Step Brothers" ushered in the new Hollywood math: scrotums + swear words + third-act saccharin = success. The spirit of Apatow looms heavily over two new comedies, though he had nothing to do with either. Kevin Smith's "Zack and Miri Make a Porno" and David Wain's "Role Models" are both cast with Apatow rep company players, and both share the striking but suddenly ubiquitous conjunction of lowdown raunch and huggable humanism. If this combo came as a sweet surprise in a movie like "Superbad," it's now threatening to become a cliché.

This must all seem very strange to a veteran outré filmmaker like Smith. When his low-budget, trash-talking "Clerks" came out in 1994, it seemed shaggily revolutionary. Its verbose, scruffy store clerks, who could wax eloquent on the fine points of oral sex, paved the way for the slob-prince heroes Seth Rogen now plays. And now Rogen is starring, alongside Elizabeth Banks, in Smith's own "Zack and Miri Make a Porno," a sex-obsessed love story that looks a lot more conventional than its creator may have imagined. The title characters (Rogen and Banks), friends since childhood, platonically share a run-down Pittsburgh apartment made worse when their water and electricity are shut off. Unable to pay the bills, Zack turns to porno for financial rescue. This means convincing his old pal, Miri, to have on-camera sex with him. Guess what? Their coupling is an earth-shaking event that leads each to realize How They Truly Feel About Each Other. As good as Rogen and Banks are, their love story feels born of design, not desire. Despite some scabrously funny dialogue, "Zack and Miri" can't compete with the best of the Apatow comedies, which have wrung finer variations on the moves Smith pioneered. Which puts him in the odd position of looking like a shadow of his shadow.

"Role Models" is even more formulaic, but also a whole lot funnier. The cynical, burnt-out Danny (Paul Rudd) and the swaggering horndog Wheeler (Seann William Scott), energy-drink salesmen, are arrested after a property-destroying freakout and given an option: jail or 150 hours of service as big brothers to troubled kids in the Sturdy Wings program. The sour Danny, who's just been dumped by his lawyer girlfriend (Elizabeth Banks again), is saddled with the superdork Augie (Christopher Mintz-Plasse, a.k.a. "McLovin"), who's totally obsessed with acting out his medieval sword-and-sorcery fantasies. Wheeler gets the incorrigible Ronnie (Bobb'e J. Thompson), a foulmouthed fifth grader who has scared off all previous mentors.

Predictable? Yes, yet within the all-too-familiar uplifting arc of "Role Models" there's real wit and lots of opportunities for the beguiling cast to show off its comic chops. The fun is in the details: Rudd's snarky rant in a coffee shop where the large is called "vente" will resonate with anybody who's ever wondered why Starbucks calls a small a "tall." Scott, who's gone from being a hormone-revved adolescent to a hormone-revved overaged adolescent, gets better at it with every attempt: he and Rudd, at each other's throats, create delightful comic friction. Wain ("Wet Hot American Summer") does his best not to bog down in the puddles of redemption that accumulate in the final reel. Comedies by definition have happy endings, but "Role Models" would have left a deeper mark had it found a way to leave us smiling and surprised. This new comic formula needs shaking up; when the rules get this entrenched, it's time for a new rule book.