Nearly 4 million couples were living together outside marriage in 1990, eight times as many as in 1970, according to the U.S. Census Bureau. Linda Waite, a professor of sociology at the University of Chicago, has done several studies on cohabitation and marriage. She spoke of modern myths about the lifestyle recently with Register correspondent Jay Copp. Her book, The Case for Marriage, will be published in the fall.

Copp: What is a major drawback of cohabitation?

Waite: People who cohabit are more likely to get a divorce. We know that's true in every society where it's been studied — Sweden, Canada, the United States.

People who cohabit say they do so to find out more about the person before they marry them. It doesn't work that way. If it worked that way and if people who lived together got more information to make a better marital choice, then people who cohabit should be less likely to get a divorce than people who married. In fact, it works just the opposite.

Why is divorce more common in cohabitation?

It's pretty clear that cohabitation changes people's attitudes toward marriage and divorce. It makes them more accepting of divorce and less positive about marriage as an institution. People who think divorce is an option invest less in their marriage. Because they think marriage is something they can get out of, they keep their exit doors open.

I think a lot of people who cohabit don't want marriage. What they want is something with lower costs, and it may be the benefits are commensurate with the costs.

There's a little bit of evidence that cohabiting is really bad for women with children. The uncertainty about the future of the relationship has big costs in emotional well-being for the children — depression and so on.

What about the rates of domestic violence?

Seventeen percent of cohabiting couples who had no plans to marry said their arguments became physical over the last year. It was 14% for cohabiting couples who plan to marry and 6% for married couples.

How about faithfulness?

Both men and women who are cohabiting are substantially more likely to report they had a second sex partner in the last year, although almost all married partners and cohabiting couples expected the relationship to be monogamous. You have to be careful about cause and effect. But a study at Brigham Young followed women over time and found that when women got married, the chances they had a second partner went way down. So their argument is that marriage causes a change in people's faithfulness.

So what exactly are the benefits of marriage, according to studies?

Longer life, better physical health, better emotional health, more satisfying sex life, better options for kids, improved career prospects for men. Marriage delivers benefits in different ways for men and women.

How's that?

Women tend to manage the family's emotional life. Women also manage health care and health behaviors in families. So married men have someone who has a big investment in what they're doing and watches out for their health. They also have somebody who is watching out for their emotional well-being.

In what way is men's health better?

Married men are less likely to drink, less likely to drink and drive. Drug usage goes down. Emotional health seems to get better.

So wives nagging their husbands is a good thing?

I think it's true. The reason that men's health improves when they get married is they sort of clean up their act. They reduce drinking and smoking. They live more orderly, healthy lives. One of the reasons they do so is because their wives won't put up with them coming home at midnight from the bar with their buddies. They have a home, a family, someone who matters to them. They do things they wouldn't otherwise do. They sit down for dinner with their wives instead of grabbing a cheeseburger and a six-pack.

Tell me about the financial benefits of marriage?

They're pretty striking. Any two people living together and working will have a better standard of living. This benefit is available to both cohabits and married people. But marriage improves men's career prospects. So they earn more. And something about marriage encourages people to save and invest. All these differences mean by the time you reach retirement-age the difference between married people and single people in terms of assets and wealth is huge.

An influential study by sociologist Jessie Bernard in 1972 basically concluded that marriage is good for husbands but bad for wives. True or false?

False. Recent studies don't support her conclusions. She did no original research. Most of her results were from the mid-1960s. She reinterpreted people's results and said, “Oh, look at this.” She focused only on psychological well-being. And she didn't look at changes in psychological well-being when marital status changed.

She found that married women had higher levels of psychological symptoms than single women, which studies now do not support. She mostly compared married men to married women, and women always report more psychological symptoms and distress than men do. She didn't look enough at married women compared to single women.

Is marriage as an institution declining?

USA Today does a poll every year and asks young adults what their life goals are. Having a happy marriage is the No. 1 goal. Young adults may be more worried about achieving this goal but they still hold it.

Jay Copp writes from Chicago.