According to Markman, studies have shown that couples who clearly view cohabitation as a step toward marriage have a much better shot at lasting happiness than those who move in while still in the "testing" phase.

In an ideal world, a couple who has yet to move in together would switch effortlessly between their two spaces, feeling equally at home in both places. But it rarely works that way. Many successful couples who live separately circumvent conflicts by identifying one place as home base.

"Chris knows I don't like his apartment as much as mine, because my dog isn't there, he has a roommate, and the bathroom is not that clean," says my friend Jessica. "As a result, we tend to spend more time at my place. But to make him feel at home, I stock the fridge with cheese and beer he likes." All healthy relationships rely on showing your affection and appreciation for each other in significant ways; for partners who live apart, this means taking steps to make the other person feel welcome in your home.

A Home of One's Own
People who have weathered a failed cohabitation seem better able to appreciate the perks of separate abodes. Although they had broken up two months earlier, Alex, 36, continued to live with his ex. "When we moved in, we were young and unconcerned about the future," he recalls. "Splitting up was the worst, because we had to break up a home, too. I would never move in with someone again unless I had grander plans."

Alex chose to do it a bit differently this time around: His current girlfriend lives a few blocks away—and he loves that he can retreat to his own apartment if he’s in a foul mood or needs to have a private phone call with his folks.

"You can present a slightly polished version of yourself," he says. He also appreciates that he and his girlfriend can indulge their respective decorating whims, including a cardboard cutout of a pro athlete (for him) and Viggo Mortensen photos (for her). "When you share a space, you share an identity. For now, the objects in our apartments are a part of our personal histories and create an environment where we can get to know each other for real."

Jessie, who also lived with an ex unsuccessfully prior to her current relationship, echoes that sentiment. She believes she has found The One, but still relishes having her own place. "I'm not married yet, so why not enjoy the little pleasures of living alone—like doing exactly what I want to do after work and making exactly what I want for dinner?" she asks. "There will be plenty of shared space in the future."