There is a senseless notion that children grow up and leave home when they're 18, and the truth is far from flint, says sociologist Larry Bumpass of the University of Wisconsin. Today, unexpected numbers of young adults are living with their parents, 'There is a major shift in the middle class,' declares sociologist Allan Schnaiberg of Northwestern University, whose son, 19, moved back in after an absence of eight months. Analysts cite a variety of reasons for this return to the nest. The marriage age is rising, a condition that makes home and its pleasantness particularly attractive to young people. A high divorce rate and a declining rate are sending economically pressed and emotionally hurt survivors back to parental shelters. For some, the ex pense of an away-from-home college education has become so excessively great that many students now attend local schools. Even after graduation, young people find their wings dipped by skyrocketing housing costs. Living at home, says Knighton, a school teacher, continues to give her security and moral support. Her mother agreed, 'It's ridiculous for the kids to pay all that money for rent. It makes sense for kids to stay at home.' But sharing the family home requires adjustments for all. There are the hassles over 15athrooms, telephones and privacy. Some families, however, manage the delicate balancing act. But for others, it proves too difficult. Michelle Del Tureo, 24, has been home three times--and left three limes. 'What I considered a social drink, my dad considered an alcohol problem,' she explains. 'He never liked anyone I dated, so I either had to hide away or meet them at friends' house.' Just how long should adult children live with their parents before moving on? Most psychologists feel lengthy homecomings are a mistake. Children, struggling to establish separate identities, can end up with 'a sense of inadequacy, defeat and failure'. And aging parents, who should be enjoying some financial and personal freedom, find themselves stuck with responsibilities. Many agree that brief visits, however, can work beneficially. According to the author, there was once a trend in the U. S. ______.