It seems to many like the sensible thing to do: Move in with your boyfriend or girlfriend, spend more time together, save money by splitting the rent and see if you can share a bathroom every morning without wanting to kill each other.
But if you were Prof Scott Stanley's child, he would beg you not to.
Prof Stanley, a University of Denver psychologist, has spent the past 15 years trying to figure out why premarital cohabitation is associated with lower levels of satisfaction in marriage and a greater potential for divorce.
At a conference last month, Prof Stanley and his colleagues presented the latest findings of a five-year study being sponsored by the National Institute of Child Health and Human Development.
He estimates that between 60% and 70% of couples will live together before marriage, and that for two-thirds of them, cohabitation is something that they slid into or "just sort of happened".
And a study he co-authored in February found that of the 1050 married people surveyed, almost 19% of those who lived together before getting engaged had at some point suggested divorce, compared with 10% for those who waited until marriage to live together.
Those findings mimic the reports from the mid-1990s that first attracted Prof Stanley's interest, showing that men who cohabitated before marriage were, on average, less dedicated to their relationships than those who did not.
"It was one of those kind of findings that I would not have suspected," Prof Stanley (53), recalled.
But he immediately had a theory: "The basic idea was, `OK, there's a group of males there that married someone they wouldn't have married if they hadn't moved in with them'."
The problem is one of inertia, he said.
Living together, mingling finances and completely intertwining your lives makes it harder to break up than if you had stayed at separate addresses.
"Some people get trapped by that and they end up hanging around," he explained.
Even if a couple does not eventually marry, they might prolong the relationship and "miss other opportunities with a person who's a better fit."
But not all cohabitations are created equal.
Prof Stanley's studies have shown there is almost no difference in marital satisfaction between couples who moved in together after they got engaged and those who did it after their wedding day.
He attributes this to varying levels of deliberateness; engaged and married couples have committed to a future together, while some couples who cohabit before engagement are ambiguous about where their relationship is headed.
It's often the case that one partner sees cohabitation as a step towards marriage, he said, while the other is thinking more loosely about the arrangement.
Prof Stanley said couples could slide into living together and then slide further into having kids and getting married without openly discussing the transitions and decision-making about them.
"Commitment is fundamentally about making a decision . . . making the choice to give up other choices," he said.
"It can't be a commitment if it's not a decision.
"But people, on average, don't seem to be talking about what [cohabitation] means for them as a couple.
"They just find themselves doing it."
It is not that the act of cohabitation weakens relationships, however.
Couples who live together after making thoughtful decisions to commit their lives to one another have no higher risk of marital dissatisfaction, his research has found.
It is less stable couples who decide to move in together who might have trouble down the road - especially if a child becomes involved or they marry because of societal pressure.
"Cohabitation may not be making some relationships more risky," he said.
"What it may be doing is making some risky relationships more likely to continue."
For Prof Stanley, the bottom line is that people should "not assume that living together is such a harmless, easy thing to do." - Ellen McCarthy
