Anyone who hoped that a new comedy from James L. Brooks (“Terms of Endearment,” “Broadcast News”) would bring some much-needed joy to the holiday season is in for grave disappointment with “How Do You Know.” The writer-director’s first film in six years turns out to be inferior to its predecessor, 2004’s already mediocre “Spanglish.”

And even that film, though mostly a mess, made a stab at humor with some real humanity to it, and boasted a few—though very few—keen observations on relationships. By contrast this effort is pure sitcom fluff, a silly story about a romantic triangle that actually gets worse when it tries for a touch of seriousness. It makes you wonder whether Brooks remembers what real people are like.

The character who serves as the lynchpin of the plot is Lisa (Reese Witherspoon), a spunky Olympic softball player who’s unexpectedly booted off the team by a sour new coach. Looking as zonked out over being dumped as when she’s conked with a wayward ball (which, of course, happens early on), Lisa doesn’t seem inclined to look for work of any sort; instead she’s persuaded by her boyfriend, star Washington Nationals pitcher Matty (Owen Wilson), to move in with him. The genially oblivious fellow is a hound dog who goes through women like Kleenex, but nonetheless constantly engages in goofy introspection about how to become a more sensitive guy, and regularly points out to Lisa when he does or says something she should appreciate as self-improvement on his part.

This relationship seems totally artificial, but Brooks adds another sitcom layer to it by introducing George (Paul Rudd), an amiable doofus who’s head of the investment firm started by his explosively irascible father Charles (Jack Nicholson)—until he suddenly finds himself the target of a federal investigation for security fraud. Cut loose by the firm and simultaneously dumped by his girlfriend, George, through an improbable set of circumstances, falls for Lisa (with whom he’s had a single “pity” date).

So the big question in “How Do You Know” is which guy Lisa will wind up with—and we all know the answer to that without thinking about it for an instant. As to the supposedly cute gyrations that Lisa goes through on the way to her decision, one can only say that they require all three of the principals to act in a thoroughly infantile fashion. You can accept that happily with Matty, who’s supposed to be a big kid. But Lisa and George are really no better; they’re nothing more than comic stereotypes, too, and their blundering stupidity is far more irritating because they’re supposed to be more than that.

Brooks folds into this triangle a subplot involving the machinations that led to George falling under federal scrutiny—a turn that involves his relationship with Charles—and another concerning his highly pregnant secretary Annie (Kathryn Hahn), whose boyfriend (Lenny Venito) has put off marriage because, as we’re told in a teary-funny hospital scene, he doesn’t think he’s good enough for her. Both of these attempts to add emotional weight to the proceedings fall thunderously flat.

The proof of how synthetic all this is lies in the desperation with which most of the cast respond to Brooks’s curiously flaccid direction. Witherspoon bugs out her eyes in an effort to look darling and sympathetic, Rudd out-flusters every fluttery character he’s ever played, and Nicholson bellows and puffs out his cheeks furiously until calming down to embarrassed seediness toward the close. Hahn is equally over-the-top. It’s as though all of them were trying to amp up material that they realize is seriously lacking in humor and insight. The only person who appears relatively at ease is Wilson, who merely recycles the blissfully unaware slacker-dude persona he’s played so often before and can now do on auto-pilot.

On the technical side the picture comes up short as well. Janusz Kaminski’s cinematography is overly bright, the editing by Richard Marks and Tracey Wadmore-Smith lackadaisical, and Hans Zimmer’s score annoyingly bubbly.

“How Do You Know” should have a question mark at the end of the title, of course. But that’s hardly the only thing missing here, or the most important. Wit, charm, style and comic timing are all absent, too. So the real question isn’t “How Do You Know?” but “Why Did They Bother?”