AMERICA'S nuclear families are splitting apart and breeding new single-parent households like atoms in a socio-economic cyclotron. "Firstborn," a highly-charged, energetic film, looks at the family behind the label with insight, warmth and precision.

It features a tender Teri Garr, fragile and versatile in her first major dramatic role. She gives a meritorious interpretation of a lonely divorcee's conflict between her lover and her sons, but graciously gives up center stage to child costars Christopher Collet and Corey Haim, the focus of Ron Koslow's excellent screenplay.

Like "The Karate Kid," the story is seen through an adolescent's eyes.. Both the Kid and Collet, as 15-year-old Jake Livingston, pass from childhood to manhood by facing a violent crisis without cowardice. Bravery remains the criteria for coming of age, more than ever in the '80s.

Jake, with Haim as his little brother Brian, are funny, likable kids, the kind you saw in "E.T." (still another household headed by a divorcee). Here the stranger, an ominous drifter, also seems to have come from another dimension, a blue collar invader in a once white-collar home. Peter Weller, playing a multi-dimensional villain, is all the more frightening for his tendency to disarm the kids with sudden kindnesses, after slamming them into the wall. It's a turn about for Weller who played a good lover in "Shoot the Moon," another domestic drama with an abusive theme, its family torn to shreds by the non-adult male at the center of the maelstrom.

Michael Apted (who was due for a hit film) directed this firey film, brillantly layered scene-on-scene without a wasted frame. The odd camera angles presage the evil that will infect the happy home and put us on an eye-level with the boys whose spats gradually disappear as the two come to rely on each other.

It's a gradual film that moves continuously and logically from the normal day-to- day difficulties of getting off to school on time to the impossible nightmare of saving mom from Sam the psychopath.

Mother disintegrates slowly, but relentlessly, all the way from herb tea to coffee to cigarettes to cocaine. Apted builds it up with nuances in the film's color and action. The sexual tension between the mother and son in a glance or the hazy golden dreamy tones that turn to murky blues. There are tender scenes, when Jake comforts her or his little brother, or chides him for fighting at school, or killing a blackbird. Brian's violence is a secondary theme that presages Sam's coming and perhaps his own future.

It's a serious film, but not a heavy one, with its crucial chase scene, a precursor to the climax, in which the first born draws last blood.

FIRSTBORN -- At area theaters.