Early in Due Date the odd-couple main characters, Peter and Ethan, visit a pot dealer. As Ethan (Zach Galifianakis) checks out the bud, his reluctant companion, Peter (Robert Downey Jr), waits in the living room with the dealer’s two children. Peter has been kicked off a plane and forced into a car with Ethan, and he isn’t happy. He swats away the children’s questions and mocks the girl. Her brother, who looks about nine, bounces a toy off Peter’s head in seeming retaliation. One thing leads to another, and Peter punches the boy in the stomach so hard that the child doubles over, which certainly gives the expression “belly laugh” new meaning.

Of course the boy (Jakob Ulrich) is acting, and it’s all meant to be good, nasty fun. The director Todd Phillips, who scored big last year with The Hangover, isn’t saying in Due Date that it’s ok for an adult to strike a child. Phillips likes to push studio comedies to the edge of mainstream acceptability, retrofitting old ideas (here, Planes, Trains and Automobiles) with new-school aggression and obscuring his social conservatism under the fig leaf of calculating comedy. But he’s too canny to make a movie in which an adult hurts any ol’ child. Rather, in Due Date, he tells us it is perfectly acceptable to laugh as a movie star pretends to whale on a pesky child who ostensibly had it coming.

The (faked) child abuse theme is something of a continuation of the baby-in-jeopardy stunt that Phillips pulled off, if barely, in The Hangover, a funnier, more intricately plotted and ambitious movie. In that comedy three amigos (including one played by Galifianakis) wake up in a wrecked Vegas suite after a collective blackout during a bachelor party and find themselves the surprised custodians of a baby. Strapped to Galifianakis’s chest like a ticking bomb, the baby is the story’s savviest touch. It’s an easy prop, good for audience sighs and gasps, but it also works like an amulet because its very presence protects the men, signifying that all vulgar and violent evidence to the contrary, they are father material.

The sucker-punched boy in Due Date, by contrast, is the son of a hippie-ish space case (Juliette Lewis), whose profession, low-rent digs and seemingly altered state both set the scene and effectively justify Peter’s brutality. Maternal neglect sets up the joke, and a wallop from a daddy-disciplinarian, who’s every bit as ill behaved and finally uncontrolled as the child he hurts, brings the funny home. It’s an unfair fight, but that’s also part of the joke. As is the self-interest that drives Peter’s fist into that tiny belly.

In Due Date the missus (Michelle Monaghan, purely decorative), who’s married to Peter and pregnant, is understandably eager for him to return for the Caesarean delivery of their first child. Like The Hangover, Due Date creates two oppositional spaces for the sexes — the giant playground in which men run riotously amok, and the domestic sphere of waiting women — you know, kind of like The Odyssey, but with masturbation jokes and vomit.

Regurgitation and a self-pleasuring dog define the limits of the new film’s ambitions. Most road movies have a sense of the life rushing past that the guys in Due Date scarcely notice, though they do stop to stock up on product placements. This narrow field of vision is mirrored in the main relationship, with Peter, an architect with a sleek wife and house, forced to suffer Ethan’s ignorance and lower-class affectations. Ethan is a rube with a perm and delusions of acting grandeur, as well as a fount of mayhem, but as a surrogate child he has lessons to teach the expectant father. Equal parts appealing and appalling innocence, with a spark of anarchic menace, Galifianakis is good enough to make you almost forget the movie.

Downey either can’t, or doesn’t, do much with Peter, who, as written by Phillips and three others, remains an off-putting, hollow man — a presumably unintentional lack of character development that represents the movie’s sole honest moment. Peter suffers a litany of abuse during his travels with Ethan. He’s roughed up in a test run for what lies ahead. Yet he’s as obnoxious at the end as at the start. For Phillips, a hero’s journey doesn’t mean betterment for all; it’s about giggling into the wind while riding shotgun in a world of trouble.