"Marriage is no longer compulsory," Susan Brown, co-director of the National Center for Family and Marriage Research at Bowling Green State University, said in a statement.
"It's just one of an array of options. Increasingly, many couples choose to co-habit and still others prefer to remain single."
The U.S. marriage rate is 31.1, the lowest it's been in more than a century. This equals roughly 31 marriages per 1,000 unmarried women. In 1920, the marriage rate was 92.3 marriages per 1,000 unmarried women.
In addition, a woman's average age at first marriage is the highest it's been in more than a century, at nearly 27.
"The age at first marriage for women and men is at a historic high point and has been increasing at a steady pace," said Wendy Manning, co-director of the center.
There has also been a dramatic increase in the proportion of women who are separated or divorced. In 1920, less than 1 percent of women held that distinction. Today, 15 percent are separated or divorced.
In the last 50 years there have been only modest changes in the percentage of women married among the college educated and the greatest declines among women without a high school diploma, the study said.