'WHEN HOUSING is cheap and available, two snot-nosed brats can go out and leave home and get an apartment together. When housing is expensive, you don't [move]. You stay in and double up with your parents . . . Welcome to the 1940s."

That's what housing expert George Sternlieb of New Jersey's Rutgers University has said. It is also what's suggested by others who have been participating in or watching a developing trend -- the increasing return to the parents' home of grown children faced with at tightening job market, soaring housing prices or just expensive habits they can no longer afford on their own.

In short, those who yearn for the days of the Waltons, of two-generation and three-generation families under one roof again, evidently are starting to get their way, and observers expect the trend to grow as the economic crunch worsens. It is not all clear, though, that all parents of today (any more than those of yesteryear) view children who are looking homeward again as angels, or that all their offspring like the situation any better than did children of the past.

"After I had been away for a number of years, my parents were thrilled to have me home again," says one 24-year-old Washington-area woman who had spent about six years away from home. "They were really trying to help by not letting me pay for room, board and utilities so that I could save some money." But whe adds bitterly: "I couldn't take it anymore. They treated me like a teenager, watched every move I made and who I was with. I had zero privacy and I just had to leave again."

Others, however, adjust rather well. Another 24-year-old Washington woman tells of being away eight years at boarding school and college and then working for a U.S. corporation overseas. But after two years out in the "real world" on her own, she says, she likes it back home.

"At 21," she remarks, "I would have found the situation intolerable. But at 24, I am old enough enough to consider my parents friends rather than authority figures. And I have so much extra cash to indulge in all the extras I couldn't afford when I lived alone."

Some parents, though, are not so thrilled by their returning progeny. "Quite frankly," says one guilt-ridden father in Boston, "my wife and I had just begun to really enjoy each other in a very special way -- a real second honeymoon: travel, dinners, theater, etc. Suddenly our oldest son 'drops in' with an unspeakable-looking girl from the West Coast, dead broke from a hidden marijuana habit, and pleads for shelter.

"After trying to reason with him for several weeks, we tried a counselor (he insisted that a full-time job would interfere with his Yoga schedule). Then, one night, I had too much to drink. I almost killed him. He got the message and left."

But whether parents or adult children adjust well or not, the return to the nest seems to be growing, even if researchers have not yet gotten around to nailing it down to percentages.

What one finds at this point, for example, is Albert Sidlinger, chairman of the research and forecasting firm that bears his name, reporting: "Big bills and lost jobs are forcing households to double up. Married couples are moving in with parents; singles are giving up apartments, and elderly parents are moving in with children."

Sometimes you get a scholar who is living through the trend himself. Anthony Downs of the Brookings Institution, for example, says that when his daughter graduated from college "she couldn't find a job and came back to live with us. She has a job now, as the assistant to the president of a consulting firm, but she is still living at home. It's a lot cheaper. We have a big house."

He adds: "Moving back with parents is typically a part of the doubling-up" trend in housing that's been widely reported in recent years, though most attention has gone to groups of young people sharing houses and apartments with each other.

Joyce Payne, who was a staff member of last year's White House Conference on Families, says the return of the offspring also "was evident from conversations conference representatives had with parents around the country, particularly in rural areas." Often the children were going home "to build savings which will enable them to move out again," she adds.

Still others, however, question whether economics is the chief cause of the development. "I don't believe that this trend, is so related to the ailing economy," says John Pitkin of the Harvard-MIT Joint Center for Urban Studies. "The most significant trend is that young adults are not marrying," with more of the singles therefore living longer in the parents' homes or returning there later.

Whatever the precise cause, some observers are concerned about the effects. "The on-again, off-again relationship can be extremely trying for all concerned," remarks Payne.

Dr. Paul Kingsley, a California psychiatrist who has noticed the same trend in Marin County where he practices, suggests that living with parents too long impedes the children's development of their own identities. In the long run, he says, living at home doesn't benefit the "child" psychologically.

More broadly, the back-to-the-nesters may also end up altering the face of American housing, according to Allen Cymrot, president of Robert A. McNeil Corp., a California real estate company. If housing prices continue their upward climb, he says, the result may be the spread of homes built especially for the newly extended family.

"After World War II," Cymrot notes, "two-family facilities were built in this country, and a landlord-tenant relationship did not exist as the two families shared the same mortgage. I feel strongly that this situation will happen again."

"There also will be definite interior changes" in such homes, he adds. "The living room will give way to a large recreation room to be shared by the two families. Kitchen facilities may also be shared."

As Rutgers' Sternlieb says, welcome to the 1940s. That past for which so many Americans have yearned appears to be turning into something more than faded photographs in the family album. It remains to be seen how many prefer living through those days rather than just remembering them.