Recently, a college classmate told me that her 25-year-old son has a girlfriend about whom he can’t make up his mind. The couple have been going together for two years. He is serious about the girl, thinks she’d make a good wife, but tells his mother that he’s not sure about taking the plunge. Should he propose? Should he take more time to think about it? Maybe date other girls to gain perspective? The son is at a loss.

My friend is generally a sensible person, and she gave her son what she thinks is good advice: why not move in with her and see how it goes? In other words, do a trial marriage. If the sharing of bed and board goes smoothly, then tie the knot. If it doesn’t, you can go your separate ways and will have been spared what nobody wants: a broken marriage.

My friend unfortunately was steering her son into what might be called the “co-habitation trap.” It so happens – contrary to widespread belief – that the divorce rate among couples who live together before marriage is notably higher than the normal divorce rate – up to 40 percent higher, depending on which study you look at.

There are variations within this disquieting statistic. Couples who get engaged before moving in together do better than couples who don’t. If it’s the woman’s first and only live-in situation, the divorce rate is lower. Brief cohabitators are more likely to stay married than longer-term ones. Whatever the nuances, however, all these categories produce higher divorce rates than that for couples who don’t live together before marriage.

Why is this? It seems counter-intuitive. A young woman might say to her friends, “I wouldn’t dream of marrying a man until first living with him for a couple of years.” And her friends would nod sagely. It makes a certain kind of sense: take a test drive before committing to a model.

But this scenario doesn’t always work in real life. Years ago, a friend of mine moved in with his girlfriend. They shared a loft in SoHo and seemed to have a marvelous time being a young Manhattan couple. After two years, they married. A year later, the marriage cracked up bitterly. I said to him one day, “What happened? The two of you seemed great together.” “I don’t know,” he replied. “It’s as though all of a sudden all the wrong buttons were pushed.”

The point is that there’s no such thing as a trial marriage. As Barbara Dafoe Whitehead puts it, “Living together is not to marriage as spring training is to the baseball season.”

Here are some problems with cohabitation:

• When a couple move in together, they seldom ask the sort of questions one ought to ask about a partner with whom one is going to spend the rest of one’s life. Do I really share this person’s values? Do I want my children to have this person’s values? The worst scenario is sliding into cohabitation and then sliding into marriage. Decide, don’t slide, as the saying goes.

• It has been well observed that in a good marriage, whenever a wife or husband uses the pronoun “I” he or she also means “we.” But when a cohabiting person uses the pronoun “I”, he or she often means “I.” The couple have separate names, separate bank accounts; there’s an implicit agreement that either can back out of the relationship. In brief, they are rehearsing a low-intensity commitment. But marriage involves a high-intensity commitment.

• Sex can get in the way of the prudential judgments one should make about the person one is going to marry. Sex and lucid judgment don’t always go together, to say the least. Sex releases hormones like oxytocin, which, among other things, act like a bonding agent, even when the couple in reality may not be suited to one another. It is much harder to break up a bad relationship when sex is going on. Abstaining couples, on the other hand, tend to look at one another with greater clarity. The emotional growth of their relationship is not short-circuited by an act that presumes more commitment than is the case.

• Men and women go into cohabitation with very different assumptions and expectations. A woman will tend to regard living together as a dress rehearsal for marriage, while her partner has much looser ideas about the arrangement. She will typically take less time than he does deciding in favor of marriage. In fact, he’s happy to postpone the decision for as long as possible. This can lead to scenes. She doesn’t even have to utter the word “marriage” to make him defensive. All she has to say is something like, “I don’t see where this relationship is going,” to set him off. “You’re putting pressure on me!” “Things are fine the way they are!” “I don’t want to be pushed into anything!” And so forth.

“We suspect, rightly,” James Q. Wilson writes in The Marriage Problem, “that marriage differs from cohabitation. Cohabitation means that two people agree to live together, sharing rooms, meals, and sex. Marriage means that two people promise to live together until they die, sharing rooms, meals, sex, and a permanent obligation to care for one’s spouse. The promise is at the heart of the matter.”

It’s not easy to abstain from sex prior to marriage. Especially when a couple are already engaged. But to reserve sex for marriage is to affirm its meaning and ultimately strengthen the bond of marriage. Sex is the consummation of a solemn promise; it doesn’t work so well without it.

Besides, no happily married couple have ever looked at one another, slapped their foreheads, and exclaimed, “If only we had started having sex six months earlier!” Instead, they can share a fond memory of waiting for the starting gun to go off.