Most couples also didn't consciously decide to live together, the federally funded University of Denver dating and cohabitation study found.
In fact, two-thirds of cohabitors say they either "slid into it" or "talked about it, but then it just sort of happened," said the study, presented Thursday at a Smart Marriages/Happy Families conference of marriage and family experts in Orlando, Fla.
"People who are engaged think of (living together) as the next step before they get married, but in many couples, it's part of the dating relationship -- pretty serious, but still well shy of the marriage part," researcher Scott Stanley, co-director of the University of Denver's Center for Marital and Family Studies, told USA Today.
Almost half of cohabitors of both sexes said spending more time together was a reason they moved in together, with only 9 percent of men and 5 percent of women citing "to test the relationship before marriage," the study of 1,294 unmarried Americans ages 18 to 34 indicated.
People following their understandings of religious dictums were less likely to cohabit than others, with 49 percent of dating couples and 30 percent of cohabitors agreeing that "my religious beliefs suggest that it is wrong for people to live together without being married," the study found.
The U.S. Census Bureau reported 13.6 million unmarried, heterosexual couples living together in 2008.
Researchers say 50 percent to 70 percent of couples who marry today lived together first. Most couples who live together either marry or break up within two years, researchers say.