The Downside of Cohabiting Before Marriage AT 32, one of my clients (I'll call her Jennifer) had a lavish wine-country wedding. By then, Jennifer and her boyfriend had lived together for more than four years. The event was attended by the couple's friends, families and two dogs. When Jennifer started therapy with me less than a year later, she was looking for a divorce lawyer. "I spent more time planning my wedding than I spent happily married," she sobbed. Most disheartening to Jennifer was that she'd tried to do everything right. My parents got married young so, of course, they got divorced. We lived together! How did this happen? Cohabitation in the United States has increased by more than 1,500 percent in the past half century. In 1960, about 450,000 unmarried couples lived together. Now the number is more than 7.5 million. The majority of young adults in their 20s will live with a romantic partner at least once, and more than half of all marriages will be preceded by cohabitation. This shift has been attributed to the sexual revolution and the availability of birth control, and in our current economy, sharing the bills makes cohabiting appealing. But when you talk to people in their 20s, you also hear about something else: cohabitation as prophylaxis. In a nationwide survey conducted in 2001 by the National Marriage Project, then at Rutgers and now at the University of Virginia, nearly half of 20-somethings agreed with the statement, "You would only marry someone if he or she agreed to live together with you first, so that you could find out whether you really get along." About two-thirds said they believed that moving in together before marriage was a good way to avoid divorce. But that belief is contradicted by experience. 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 originally attributed the cohabitation effect to selection, or the idea that cohabitors were less conventional about marriage and thus more open to divorce. As cohabitation has become a norm, however, studies have shown that the effect is not entirely explained by individual characteristics like religion, education or politics. Research suggests that at least some of the risks may lie in cohabitation itself. As Jennifer and I worked to answer her question, "How did this happen?" we talked about how she and her boyfriend went from dating to cohabiting. Her response was consistent with studies reporting that most couples say it "just happened." "We were sleeping over at each other's places all the time," she said. We liked to be together, so it was cheaper and more convenient. It was a quick decision but if it didn't work out there was a quick exit. She was talking about what researchers call "sliding, not deciding." Moving from dating to sleeping over to sleeping over a lot to cohabitation can be a gradual slope, one not marked by rings or ceremonies or sometimes even a conversation. Couples bypass talking about why they want to live together and what it will mean. WHEN researchers ask cohabitors these questions, partners often have different, unspoken - even unconscious - agendas. Women are more likely to view cohabitation as a step toward marriage, while men are more likely to see it as a way to test a relationship or postpone commitment, and this gender asymmetry is associated with negative interactions and lower levels of commitment even after the relationship progresses to marriage. One thing men and women do agree on, however, is that their standards for a live-in partner are lower than they are for a spouse. Sliding into cohabitation wouldn't be a problem if sliding out were as easy. But it isn't. Too often, young adults enter into what they imagine will be low-cost, low-risk living situations only to find themselves unable to get out months, even years, later. It's like signing up for a credit card with 0 percent interest. At the end of 12 months when the interest goes up to 23 percent you feel stuck because your balance is too high to pay off. In fact, cohabitation can be exactly like that. In behavioral economics, it's called consumer lock-in. Meg Jay is a clinical psychologist at the University of Virginia and author of "The Defining Decade: Why Your Twenties Matter - and How to Make the Most of Them Now."