American Reunion; R, 113 minutes; Carmike Cinema 6

It’s a little unnerving how the American Pie movies are starting to add up to some kind of national sociology. With the latest, American Reunion, exuding a relaxed sense of duty, the whole franchise seems like the adolescent sex-farce equivalent of Michael Apted’s Up series of documentaries, in which, since age 7, the same group of kids has been visited by a film crew every seven years. Presumably in both cases this will continue until they’re all dead, as well as the rest of us.

So, not that you asked, but here’s what’s going on with the class of ’99. The hapless Jim (Jason Biggs) and horny Michelle (Alyson Hannigan), parents to a toddler now, are cordially married but sexually disconnected. Sweet beefcake Oz (Chris Klein) settled into sub-ESPN sportscasting after losing a TV dance contest. Non-entity Kevin (Thomas Ian Nichols) compensates for “Real Housewives” date nights with a gently manicured beard. Pseudo-sophisticate Finch (Eddie Kaye Thomas) has fashioned himself as a bohemian drifter, and rude-boy Stifler (Seann William Scott), now with the shit-eating grin wiped from face, grumbles his way through corporate-temp subservience.

Collectively they yearn for regression, and so, on the occasion of a 13th high school reunion, they return to their shared adolescent idyll of suburban Michigan, where life was so raunchy and so sweet. The premise of this fourth piece of Pie (not counting however many slices have gone straight to DVD) suits its franchise-wide proclivity: getting sentimental about the shame-based comedy of crude bodily functions. Magnanimously not possessive of the market they’d once cornered, the lads indulge another round of untoward yet healing high jinks —penile indignity here, beer-cooler bowel movement there—plus the occasional telegraph flash of This Is a Reference to the Previous Films. At least some water under the bridge makes their friendship more plausible.

As ensemble vehicles, reunion movies can be hard to steer. Writer-directors Jon Hurwitz and Hayden Schlossberg labor over a lot of set-up, a lot of wrap-up, and some perfunctory in-between bits to tie it all (very loosely) together. It takes too long to set the table for tasty bonbons like Stifler’s rattling discovery that half his lacrosse team was gay (“I thought they were wrestling,” he whimpers), or the latest awkward talk between Jim and his dad (the ever-game Eugene Levy). Ali Cobrin is necessarily appealing as the grown-up girl next door whom Jim used to babysit, now suddenly a naked drunken flirt, but overall the movie could be kinder to her. As it could to Mena Suvari and Tara Reid, who appear merely blonde and hard to tell apart.

Being so focused on shoehorning sequel-mandated shtick into the latest life-stage anxiety, American Reunion seems at times strangely humorless. Audience willing, it becomes funny almost in spite of itself, and therefore at least gives off a minor cathartic charge. So it goes with millennial nostalgia: Sometimes it’s easier said than done to really keep in touch.