For the first time in 150 years, households headed by single adults and unmarried couples now outnumber married-couple families. In 1960, married-couple households represented more than 78 percent of American households. As late as 2000, married couples were 52 percent of all households. But in 2005, according to the recently released American Community Survey, households with a married couple at their core made up less than 50 percent of all households.

That’s a psychologically significant number, of course. Headlines trumpet that “Married people are now a minority.” But there are some things that this news means and others that it doesn’t. It doesn’t mean marriage is doomed. It does mean we have to start thinking differently about the way we design our social policies. They are founded on 60-year-old assumptions about marriage that are now definitively outdated.

Contrary to what many people believe, this turning point was not created by a continuing rise in divorce rates or a widespread revolt against marriage. Divorce rates do remain near historic highs, but they have actually declined slightly in the last 15 years. And demographers predict that 90 percent of Americans will eventually marry, down from 95 percent in the 1950s and 1960s, but not substantially different from the marriage rate at the beginning of the 20^th century.

The biggest reason married-couple households are now outnumbered by unmarried ones is the rising age at which people first marry. In 1960, the median age of marriage (with half of all individuals marrying before this age and half after) was 23 for men and 20 for women. Half of all women married before they left their teens! Today the median age of marriage is 27 for men and almost 26 for women. Among women who pursue graduate degrees, it is about 30.

Furthermore, many people marry far past the median - indeed, the age of first marriages past the average extends far later than in the past. In the 1950s, a woman still single four to five years after the average age at which her peers marrried was unlikely ever to marry. She was what the Japanese call “Christmas cake” - unlikely to be taken off the shelf after the 25^th. Today, however, many women marry for the first time in their 30s, 40s and even 50s, meaning that even people who eventually do marry live for long periods in other relationships. And many aren’t marrying. Thirty million unmarried men and women live alone, in their own household rather than with roommates or parents. The number of heterosexual couples who live together outside of marriage has soared from 500,000 in 1970 to more than five million today.

Another contributor to the number of households headed by unmarried individuals is that we’re living longer. A person who reaches 65 today can expect to live another two decades. If a spouse dies at 60, because of illness or an accident, the survivor is far more likely then in the past to live for another 20 to 30 years - though not necessarily alone. Elders are the fastest growing segment of the cohabiting population.

When we factor divorce in, we find that, on average, Americans spend half their adult lives outside marriage!

Marriage no longer regulates social life the way it used to (or was once supposed to). It is no longer the main way people establish a separate residence on leaving home or school. It’s no longer the first step people take toward settling into a permanent job, buying a home, and embarking on adult life. More often, it’s what they do a considerable time after they have achieved those goals. Forty percent of homebuyers in America are single, the majority of them women.

Marriage is certainly no longer the way that most men and women begin their sexual lives. Increasingly, it is not the only way that individuals or couples beget and rear children. Forty percent of cohabiting couples, compared to 45 percent of married couple households, have children in the home. Nor is marriage any longer the main institution in which people live out the final decades of their lives.

Having said all that, let me make one thing clear: This reduction in the social dominance of married-couple households is not a threat to the existence or the quality of marriage. Most men and women eventually marry, and women who marry at an older age are less likely to divorce than women who marry at age 25 or younger, according to a research paper to be released next month by the Council on Contemporary Families.

Furthermore, people who postpone or forgo marriage do so not because they don’t value that commitment. In fact, contrary to the assumptions of the early policy-makers who designed programs to promote marriage, many young couples postpone marriage precisely because they value it so highly. They want to enter marriage on a more solid economic and emotional base than when people simply married because it was socially expected - and when women had to settle for a bad marriage because they had no other way to support themselves.

Still, these changes do pose a huge challenge to the way we traditionally have organized work life, designed homes and communities, financed health care, and constructed social welfare policy.

The 1950s was a period of early, nearly universal marriage. Unlike most Western nations in the era after World War II, the United States chose to channel many of its social benefits through marriage. Gambling that this historically exceptional “golden age” of marriage would always exist, American policymakers - instead of developing universal health care or old-age pensions for individuals - assumed that wives and children would normally get health insurance through a male “breadwinner.”

Social Security also privileged married couples. While widowed spouses of covered workers got significant survivors’ insurance benefits, single workers got fewer returns for their investment, and the unmarried elderly had to depend on far less generous welfare payments. Postwar transportation, work and housing policies were designed on the assumption that most workers would have a wife at home to raise the children, run life’s daily errands, and take care of the home.

Giving special privileges to married-couple households and denying them to the unmarried made some sense in the days when marriage played the central role in regulating sexuality and childbirth, redistributing resources to children, organizing the exchange of men’s wages for wives’ household services, supporting youth until they could move into jobs that provided health-care insurance, and caring for the ill and the old.

But marriage is not the only way people organize these tasks any more. Many of our assumptions and expectations are based on the world of the 1950s - a world that no longer exists. It is no longer possible to pretend that marriage is the only institution in which children are born and raised, the elderly and ill are cared for, youth are supported until they gain living-wage jobs, and interpersonal obligations are incurred. We can no longer design work schedules, leave policies, and housing complexes on the assumption that every worker has a wife at home to take care of “life.” Unmarried people increasingly are likely to have care-giving obligations, whether for children, aging relatives, or a live-in partner. Meanwhile, the growing amount of time that unmarried young adults spend in school or in jobs without health insurance is creating a health-coverage gap that can be as catastrophic for young people who experience serious illness or injury as it is for elders without an able-bodied spouse to care for them.

From buying homes, to raising children, to pooling economic resources with a partner, to caring for aging relatives, individuals are increasingly taking on long-term obligations outside marriage. Marriage is no longer the only institution in which people need support mechanisms or legal rights to follow through on commitments they have made. Nor is it the only place where the state needs to step in when people try to walk away from those commitments without fairly settling up the obligations they have incurred, such as dividing joint resources or maintaining financial and emotional obligations to a child they have been raising together.

Recently Americans have been involved in a loud, acrimonious debate over whether to grant gays and lesbians access to the privileges and responsibilities of marriage. But there’s an even deeper question: whether it still makes sense to use the institution of marriage as a channel for so many interpersonal rights and responsibilities.