Toni Collette's Tara in the new Showtime series "The United States of Tara" is the latest in TV's modern-day line of desperate housewives, but she comes with a twist: She has at least three alternative personalities. At times of extreme stress, Tara switches into a brash, sexually promiscuous teen; a condescending '50s-style homemaker; or a tough-talking biker dude.

This is the kind of stuff that actors — and Emmy voters — love, and Collette is impressive in all roles, using her elastic face and fluid voice to slip into several different skins. But this comedy about a woman with dissociative identity disorder — aka multiple personalities — wouldn't work if Collette didn't find Tara's heart.

Tara is worn down by the difficulties of motherhood, and Collette is willing to let herself look haggard and (for an actress, anyway) a little flabby; she makes it touching when Tara takes her most awkward stabs at doing the right thing.

Tara has a loving and incredibly patient husband (John Corbett, the only actor easy-going enough to make Matthew McConaughey look like Al Pacino); a rebellious teenage daughter (Brie Larson) whose shock-value sex references sound like more bravado than action; and a younger teen son (Keir Gilcrhist) with a thing for classic jazz and old movies.

The show pushes the teens' quirks, as well as some of the altered-personality antics, a little too hard, something that's quickly become a trademark of creator Diablo Cody, the quirk-loving stripper-turned-screenwriter who gave the world Juno. As in "Juno," the characters' dialogue doesn't always ring true in the more whimsical sequences, but when the show is at its most grounded — when it's in its Tara moments and the family's problems are more or less normal — it's funny and affecting.

For all of Cody's love of eccentrics, she's so good at writing "ordinary" characters that it's easy to wonder just how strong one of her projects could be if she'd just drop all the irony for once. Still, she does manage to come up with a first-episode conclusion that features one of the altered personalities, and is as good an "us against the world" slice of family bonding as anything from "Malcolm in the Middle" or "The Simpsons."

MORE THAN POLYGAMY

In a way, Corbett's character in Tara is married to four people, but they're all in one person's body. In HBO's "Big Love" Bill Henrickson (Bill Paxton) is married to three women — and as season three begins, he's starting to court a fourth. That core plot has earned "Big Love" the tag "polygamy drama," but at its best (i.e., season two), it's so much more than that: a meditation on family; parenthood; the ways some people mold their religions to suit their beliefs; consumerism; and modern America. This is one of the few TV shows, after all, that has shown a character getting way over her head in credit-card debt, which happens all too often in real life.

Having seen the new seasons' first three episodes, I'm not sure it's operating at peak form. Bill's first wife Barb (Jeanne Tripplehorn) has a cancer scare; second wife Nikki (Chloe Sevigny) is secretly helping her father, the fundamentalist polygamy-sect leader Roman Grant (Harry Dean Stanton) avoid prison; and third wife Margene (Ginnifer Goodwin) is ... well, she doesn't have all that much to do yet, after Goodwin had such a strong second season.

This all adds up to the drama becoming more intimate, more about the Henricksons' extremely large extended family, but also more insular, not touching on as many themes as season two which had a wider scope. Still, there are some strong moments — Sevigny gets a great scene where she literally shouts from a rooftop — and it looks like Bill's daughter Sarah (Amanda Seyfried) is going to get a meaty plot line.

So much goes on in "Big Love" that it may take the whole season for the show to shake itself out, but for now it's a minor step down from season two — still good, but not great.