In our highly mobile and divorce-ridden society, says writer Karen Lindsey, the old saw -- "You can choose your friends, but you can't choose your relatives" -- is rapidly being rendered obsolete.

"Families today are something more than your spouse or your blood kin," contends the 37-year-old Bostonian, who sees a "revolutionary" trend toward friends as family across the country. "More and more people are bonding with one another to create familial relationships with people who are not blood relatives."

For proof of this "families as friends" phenomenon, she says: "Look at your holiday celebrations. The holidays are traditionally a family time, and many people celebrate with close friends -- another couple, coworkers, neighbors, single people -- who are 'like family' to them.

"The 'Father Knows Best' family is dead. Today friends have often lived through as many experiences together as relatives, and have created -- perhaps unconsciously -- equally strong bonds."

These "chosen kin" are most often close colleagues (the "office family"), a tight circle of friends or "honorary relatives" -- like the unrelated older woman you've always called "Aunt Martha" or the boy-next-door you love like a brother.

"If the relationship includes love, commitment and continuity, well, that's the stuff of which kinship is made. These are the people you celebrate with, confide in, mourn with and call at 3 a.m. if you need someone to talk to."

While "familial friends" have always existed, says Lindsey, "in the past they just happened. You invited a single person to share holiday dinners and, over time, he or she became part of the family."

But today "people are actively looking around to create family." Much of this trend is the result, she says, "of all the cliches about the break-up of the traditional family."

The statistics:

* Half of the couples married since 1970 are divorced.

* One out of every six children under 18 lives with only one parent.

* Only 11 percent of the elderly live with relatives.

* The average American moves once every three years.

* The number of Americans who live with people to whom they are not related has jumped from about 3 million in 1973 to more than 5 million in 1981.

* Over the last decade the number of males living alone has doubled, the number of females living alone has increased by 60 percent and the number of unmarried couples living together has more than tripled.

"A good barometer of the change is television," notes Lindsey. "In the '50s we had 'Ozzie and Harriet' and 'I Love Lucy' for our model of family. Today we have on-the-job families on 'M*A*S*H,' 'Love Boat' and 'WKRP in Cincinnati.' And for the first time there's a program about a gay man living with a heterosexual woman and her daughter, 'Love Sidney.' "

Lindsey, a published poet and teacher who proudly calls herself "a spinster who lives with two cats," became intrigued by the idea of friends as family after she suffered "a minor breakdown" in 1977. "The only good thing that came out of that awful experience is that so many people came by to help me out of it.

"When you're feeling so down, the best thing to get you up is having people you respect call you and visit you and really care about you. I wrote a piece about the experience for Ms. magazine, which they used as part of their 'family' series. An editor who saw the piece called and asked if I'd be interested in turning it into a book."

Lindsey started by interviewing her own "chosen kin": Mark, an ex-boyfriend; Kathy, a close friend; John Paul, an 82-year-old "honorary uncle." Through her students, friends and acquaintances she tracked down and interviewed about 40 networks of familial friends in a broad range of ages, races, backgrounds and economic situations.

The result is Friends as Family (Beacon Press, 282 pages, $14.50) a thoughtful look at the new family forms. A major virtue of "the chosen family," Lindsey writes, is that it's "a custom-made, not off-the-rack, structure. As such, it is the most revolutionary, and hence most demanding form of family."

In the forefront of the movement, she says, are single people and homosexuals.

"Often the people who set out to create a family are past 30, unmarried and childless, coming to terms with living as single people in a coupled society. Not surprisingly, some were lesbians or gay men, who felt that their sexual preference not only precluded their living in a traditional family structure, but also weakened, and in some cases severed, their ties with parents and siblings."

Economics also plays a part, particularly for singles and the elderly. "Since many single people can no longer afford to live alone, there's a trend toward communal, familial living among many different ages and types of people."

For people who grew up in large families, geography may be the reason.

"Your family may be on the West Coast, and you may live in the East," she says. "So you seek out older people, or children or folks your own age to recreate that extended family you miss." For those people who choose not to have children, "this is a way to become related to a child."

Friends who are family usually fit at least one of two criteria. "One is history," she says. "These are people with whom you've shared intense experiences in your lives."

The second she calls "crisis assumption. They are the people you'd call at 4 in the morning for an emergency, and you know they'd be there for you. These are people you feel bonded to. Even if you haven't seen them in 20 years, you get together and the connection is still there."

While a chosen family provides many of the same benefits of a biological family -- love, belonging, caring -- Lindsey claims it carries less pressure. "If you do something your parents disapprove of they may feel responsible and wonder what they did wrong. With friends your actions don't have that same impact.

"It's more a relationship among equals, possibly because you do have the option to leave. If you're my friend and you're getting too bossy, I may decide you're not my friend after all."

Familial friends also "share different parts of your history. Your biological family may always consider you the baby. But to your chosen family you may be a wise older friend."

No name has evolved for the nonbiological family, claims Lindsey, "because to some people it threatens the supremacy of the nuclear family . . . a kind of unconscious and unacknowledged resistance."

"We have metaphors -- 'She's like a second mother' or 'He's the brother I never had' -- but no single word to describe the relationship.

"Our culture says you have family or loneliness . . . It's a good way of blackmailing people into traditional families."

Written and unwritten laws reinforce this "blood-is-thicker-than-water myth," she says. "You may not be able to get off work if your friend has a heart attack, but you probably could if it was your uncle. Many towns and cities have zoning ordinances limiting the number of unrelated individuals who can share a household."

Despite these obstacles, "People will continue to create alternatives to the traditional family," Lindsey predicts.

"Personally I don't think it will destroy the structure of the heterosexual, monogamous couple raising their kids. But I do think the trend says you don't have to be married or related to people to love them at that deep, intimate level."