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Best predictor of divorce? Age when couples cohabit, study says

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For years, social scientists have tried to explain why living together before marriage seemed to increase the likelihood of a couple divorcing. Now, new research released by the nonpartisan Council on Contemporary Families gives an answer:

It doesn't. And it probably never has.

This is despite two decades of warnings from academics and social commentators who pointed to studies that claimed a correlation between "shacking up" and splitting up—warnings that increased as the number of couples living together before marriage skyrocketed.

As it turns out, those studies that linked premarital cohabitation and divorce were measuring the wrong variable, says Arielle Kuperburg, a professor at the University of North Carolina, Greensboro, who produced much of the research released Monday. The biggest predictor of divorce, she says, is actually the age at which a couple begins living together, whether before the wedding vows or after.

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"Up until now, we've had this mysterious finding that co-habitation causes divorce," she says. "Nobody's been able to explain it. And now we have—it was that people were measuring it the wrong way."

Couples who begin living together without being married tend to be younger than those who move in after the wedding ceremony – that's why cohabitation seemed to predict divorce, Professor Kuperburg explains. But once researchers control for that age variable, it turns out that premarital cohabitation by itself has little impact on a relationship's longevity. Those who began living together, unmarried or married, before the age of 23 were the most likely to later split.

"Part of it is maturity, part of it is picking the right partner, part of it is that you're really not set up in the world yet," she says. "And age has to do with economics."

Indeed, other research released by the nonpartisan academic group Monday suggests that the longer couples wait to start living together, the better their chances for long-term relationship success.

(Read more: Getting divorced? It pays to get organized first)

This makes sense to historian Stephanie Coontz, director of research and public education at the Council on Contemporary Families.

"Marriages require much more maturity than they once did," she says. In the 1950s, husband and wives stepped into well-defined gender rolls. "Nowadays, people come to marriage with independent aspirations and much greater ideas of equality. Maturity is so important, and negotiating skills are so much more important."

Over the past 50 years, the number of couples who live together before marriage has increased some 900 percent.

There has also been some softening of the perceived link between these living arrangements and divorce: In 2012, the Centers for Disease Control released a report saying that couples who moved in together while engaged had no greater risk for divorce than those who moved in after they were married; other researchers said the difference between premarital cohabitating couples and married couples seemed to be lowering, at least for those who moved in together in the 1990s.

But the hand-wringing continued.

"People, even well read people, end up having a lot of misinformation about what makes divorce more or less likely," says Virginia Rutter, professor of sociology at Framingham State University in Massachusetts. "The big message [of the new reports] for me is thank goodness there is now really good, clear, unambiguous research that can help us get rid of the 'cohabitation is the issue' approach. It goes from the easy answer that life's problems are about character to the more challenging answer, that life's problems are about context - [about] what are the resources that you are empowered to pull together to create a good life."

(Read more: Pre-nups: Why it may no longer pay to divorce here)

Indeed, Sharon Sassler, a professor at Cornell University in Ithaca, N.Y., argues in a different academic paper made public Monday that the length of time a couple has been romantically involved before moving in together is also crucial to whether they end up divorcing. That, she says, has connections to a host of educational and financial factors.

Those with higher education levels tend to take longer to move in with their partners, she found. Half of college-educated women moved in with their partners after at least a year; one-third were romantically involved for two years before joining house. Data from the most recent National Survey of Family Growth show that more than half of women with only a high school degree in a cohabitating relationship moved in with their partner in less than six months.

(Read more: More spouses are cheating (financially, anyway))

Professor Sassler found in her research that many couples with lower incomes and less education decided to move in together because of financial pressures. She argues that it is the type of premarital cohabitation that predicts divorce, not necessarily cohabitation in itself.

This jibes with another one of Kuperburg's findings: that there are two general types of cohabitation, one that the couples view as the first step toward marriage, engagement ring or not, and one that is done without the same commitment, or out of perceived convenience.

"People who think they are going to get married act the same as people who are married," she says. "People who are not sure, they act very differently. They have different work habits, health habits. But living together to 'test drive' the marriage? According to my research, that shouldn't hurt marriage at all."

—By Stephanie Hanes of The Christian Science Monitor

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