Dr. Susan Brown, co-director of Bowling Green State University's National Center for Family and Marriage Research, said 63 percent of women cohabited versus 57 percent of men.
"Today, most marriages are preceded by cohabitation," Brown said in a statement. "It's really become a stage in the courtship process. It's unusual for couples to marry without first cohabiting."
Using data from the National Longitudinal Survey of Youth 1997, the study found 61 percent of U.S. adults have formed a family by age 25.
Family formation in early adulthood was most prevalent among young adults with a GED diploma, at 81 percent, while those with at least a bachelor's degree were least likely to form a family before age 25 at 44 percent, the study said.
More than one-third of men followed a "traditional" pathway into marriage, meaning they did not cohabit or have a child before getting married, and it was more prevalent among Hispanics and less so among African-Americans. Twenty-six percent of African-Americans who married by age 25 did not live with their partner or have a child before getting married.
Fifty-five percent of young adults with at least a bachelor's degree were most likely to follow a traditional marriage path, the study said.