The study found those who were engaged and living together before the wedding were about as likely to have marriages that lasted 15 years as couples who hadn't lived together.
But what about the couples who were living together but weren't engaged? The new study found marriage was less likely to survive to the 10- and 15-year mark among couples who weren't engaged when they lived together – findings similar to earlier research.
For example, for women, there was about a 60 percent likelihood a marriage would survive 15 years if the couple either hadn't lived together before the wedding or were engaged while they were sharing the same living space. But if no firm marriage commitment was made while she and her boyfriend were living together, the likelihood the marriage would last 15 years fell to 53 percent. The numbers were similar for men.
Potential explanations include more lax attitudes about commitment, lower education levels or family histories that made these couples more pessimistic about marriage, Copen said.
Some young people put off marriage because they're pursuing a college education and starting a career. For them,"cohabitation is a trial marriage, usually without kids, that often ends in marriage," Settersten said.
Others – in many cases, people not on a college track – move from one living-together relationship to another, some of them producing children, he said.
Commitment has made a difference. In interviews with some women who have been married 20 years or more after living with their spouse first, firm belief in a future together was a common theme.