There was a time when that attitude would have scandalized the neighbors. But living together has become so commonplace that it hardly raises an eyebrow. More than 50% of opposite-sex couples tying the knot lived together first, up from 10% in 1965, the latest statistics show. The 2000 Census will provide a new snapshot of live- ins. Experts hope folks fill out their Census forms accurately and return them promptly to provide fodder for their research.

The dramatically increasing numbers of live-ins and the swelling numbers of kids involved are transforming family life, with "legal marriage losing its primacy as the manifest center of family ties," says University of Michigan sociologist Pamela Smock. She is a co- author of a report from the University of Michigan Institute for Social Research to be published in the Annual Review of Sociology in August. Smock crunches the numbers and analyzes the latest studies.

In general, living together is not good for cohabitors' well being, says Linda Waite, a University of Chicago sociologist. Her book The Case for Marriage, arriving this fall, will detail research showing that living together can undermine marriage. "Cohabiting changes attitudes to a more individualistic, less relationship- oriented viewpoint," she says. Live-ins become less committed to marriage and that affects the quality of their married life later.