Married couples, whose numbers have been declining for decades as a proportion of American households, have finally slipped into a minority, according to an analysis of new census figures by The New York Times.

The American Community Survey, released this month by the Census Bureau, found that 49.7 percent, or 55.2 million, of the nation's 111.1 million households in 2005 were made up of married couples ? with and without children ? just shy of a majority and down from more than 52 percent five years earlier.

The numbers by no means suggests marriage is dead or necessarily that a tipping point has been reached. The total number of married couples is higher than ever, and most Americans eventually marry. But marriage has been facing more competition. A growing number of adults are spending more of their lives single or living unmarried with partners, and the potential social and economic implications are profound.

"It just changes the social weight of marriage in the economy, in the work force, in sales of homes and rentals, and who manufacturers advertise to," said Stephanie Coontz, director of public education for the Council on Contemporary Families, a nonprofit research group. "It certainly challenges the way we set up our work policies."

While the number of single young adults and elderly widows are both growing, Professor Coontz said, "we have an anachronistic view as to what extent you can use marriage to organize the distribution and redistribution of benefits."

Couples decide to live together for many reasons, but real estate can be as compelling as romance.

"Owning three toothbrushes and finding that they are always at the wrong house when you are getting ready to go to bed wears on you," said Amanda Hawn, a 28-year-old writer who set up housekeeping near San Francisco with her boyfriend, Nate Larsen, a real estate analyst, after shuttling between his apartment and one she shared with a friend. "Moving in together has simplified life," Ms. Hawn said.

The census survey estimated that 5.2 million couples, a little more than 5 percent of households, were unmarried opposite-sex partners. An additional 413,000 households were male couples, and 363,000 were female couples. In all, nearly one in 10 couples were unmarried. (More than one in four households consisted of people living alone).

And the numbers of unmarried couples are growing. Since 2000, those identifying themselves as unmarried opposite-sex couples rose by about 14 percent, male couples by 24 percent and female couples by 12 percent.

Matt Foreman, executive director of the National Gay and Lesbian Task Force, said gay couples were undercounted because many gay people were reluctant to disclose their sexual orientation. But he said that inhibition seemed to be fading.

"I would say the increase is due to people feeling more comfortable disclosing that they are gay or lesbian and living with a partner," he said.

The survey did not ask about sexual orientation, but its questionnaire was designed to distinguish partners from roommates. A partner was defined as "an adult who is unrelated to the householder, but shares living quarters and has a close personal relationship with the householder."

Some of the biggest gains in unmarried couples were recorded in unexpected places. In the rural Midwest, the number of households made up of male partners rose 77 percent since 2000.

The survey revealed wide disparities in household composition by place. The proportion of married couples ranged from more than 69 percent in Utah County, Utah, which includes Provo, to 26 percent in Manhattan, which has a smaller share of married couples than almost anyplace in the country. But Manhattan registered a 1.2 percent increase in married couples since 2000, in contrast to the rest of New York City and many other places.

Among counties, the highest proportion of unmarried opposite-sex partners was in Mendocino, Calif., where they made up nearly 11 percent of all households.

The highest share of male couples was in San Francisco, where, according to the census, they accounted for nearly 2 percent of all households. In Manhattan, they made up 1 percent of households. Hampshire County, Mass., home to Northampton, had the highest proportion of female couples, at 1.7 percent. Some of the highest numbers of unmarried couples were recorded in the South, which as defined by the census, has the largest population of any region.

David Blankenhorn, president of the marriage advocacy group the Institute for American Values, said married couples had become a minority largely because of the growing number of households made up of people who planned to marry or who used to be married.

Steve Watters, the director of young adults for Focus on the Family, a conservative Christian group, said that the trend of fewer married couples was more a reflection of delaying marriage than rejection of it.

"It does show that a lot of people are experimenting with alternatives before they get there," Mr. Watters said. "The biggest concern is that those who still aspire to marriage are going to find fewer models. They're also finding they've gotten so good at being single it's hard to be at one with another person."

But Pamela J. Smock, a researcher at the University of Michigan Population Studies Center, said her research ? unaffiliated with the Census Bureau ? found that the desire for strong family bonds, and especially marriage, was constant.

"Even cohabiting young adults tell us that they are doing so because it would be unwise to marry without first living together in a society marked by high levels of divorce," Ms. Smock said.

A number of couples interviewed agreed that cohabiting was akin to taking a test drive and, given the scarcity of affordable apartments and homes, also a matter of convenience. Some said that pregnancy was the only thing that would prompt them to make a legal commitment soon. Others said they never intended to marry. A few of those couples said they were inspired by solidarity with gay and lesbian couples who cannot legally marry in most states.

Jennifer Lynch, a 28-year-old stage manager in New York, said she had lived on the Lower East Side with her boyfriend, who is 37 and divorced, for most of the five years they have been a couple.

"Cohabitating is our choice, and we have no intention to be married," Ms. Lynch said. "There is little difference between what we do and what married people do. We love each other, exist together, all of our decisions are based upon each other. Everyone we care about knows this."

If anything, she added, "not having the false security of wedding rings makes us work even a little harder."

With more competition from other ways of living, the proportion of married couples has been shrinking for decades. In 1930, they accounted for about 84 percent of households. By 1990 the proportion of married couples had declined to about 56 percent.

Married couples have not been a majority of households headed by adults younger than 25 since the 1970's, but among those aged 25 to 34 the proportion slipped below 50 percent for the first time within the past five years. (Among Americans aged 35 to 64, married couples still make up a majority of all households.)

"It's partially fueled by women in the work force; they don't necessarily have to marry to be economically secure," said Andrew A. Beveridge, a demographer at Queens College of the City University of New York, who conducted the census analysis for The New York Times. "You used to get married to have sex. Now one of the major reasons to get married is to have children, and the attractiveness of having children has declined for many people because of the cost."

William H. Frey, a demographer at the Brookings Institution, attributed the accelerated trend to the lifestyles of baby boomers.

"It's the legacy of the boomers that have finally caused this tipping point," Dr. Frey said. "Certainly later generations have followed in boomer footsteps, with high levels of living together before marriage, and more flexible lifestyles. But the boomers were the trailblazers, once again, rebelling against a norm their parents epitomized.

"This would seem to close the book on the Ozzie and Harriet era that characterized much of the last century," he said.

Correction: Oct. 18, 2006

An article on Sunday about a decline in the proportion of households with married couples misstated the proportion of households consisting of people living alone. It is more than 1 in 4 - 27 percent - not 1 in 20.

Copyright 2006 The New York Times Company