Couples who live together before marriage, particularly without a clear commitment such as engagement, are more likely to divorce than couples who didn't cohabit, says Meg Jay, a psychologist and author of "The Defining Decade: Why Your Twenties Matter — and How to Make the Most of Them Now."

Most 20-somethings will live with a romantic partner at least once, and more than half of all marriages will be preceded by cohabitation, she writes in an opinion piece in the New York Times. It's a trend that has increased more than 1,500 percent in the last half century and, for many of its practitioners, cohabiting is intended as a test run to make sure the two doing it will be successful as a couple before they take the step of marrying. It's seen as a way to prevent divorce.

"But that belief is contradicted by experience," she writes. "Couples who cohabit before marriage and especially before an engagement or an otherwise clear commitment tend to be less satisfied with their marriages — and more likely to divorce — than couples who do not. These negative outcomes are called the cohabitation effect."

Researchers once thought the ensuing breakups were more a mark of the fact that those who lived together before marrying were less conventional about marriage and more open to divorce. But further research, she says, "suggests that at least some of the risk may lie in cohabitation itself."

Often, she notes, couples "slide" into living together without a clear sense of where they want the relationship to go and from there they may slide into marriage. Women are more likely to see it as a stepping stone toward marriage, while men are more likely to consider it a trial run that can be stopped if things don't work out.

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