When Karin Denison was in her early 20s, it seemed that all her peers were coupling up and planning to live happily ever after. She spent the summers after college driving to friends’ weddings, she recalls. In small-town Minnesota, marriage was just what people did. It was expected.

Today, almost two decades, hundreds of dates, and untold hours on OKCupid later, Ms. Denison, who moved to Boston when she was 26, lives in a far different reality.

“There are tons of single people in Boston,” she says. “You can be single in Boston and nobody really cares. I’ve never felt the pressure here to get married.”

Indeed, if there is any “normal” in the shifting, complicated world of American relationships, it arguably looks a lot more like Denison than her childhood friends who wed at 21. Last year, for the first time, the number of unmarried American adults outnumbered those who were married. One in 7 lives alone – about 31 million compared with 4 million in 1950 – and many of those are clustered in urban centers.

But even outside cities, there is a distinct rise of the “single.” Almost half of new births are to unmarried mothers. The number of parents living together but not married has tripled. And the number of American adults who have never been married is at a historic high, around 20 percent.

Meanwhile, only 30 percent of Millennials say that having a successful marriage is “one of the most important things” in life, according to the Pew Research Center, down from even the 47 percent of Generation X who said the same thing in 1997. Four in 10 Americans went ever further, telling Pew researchers in 2010 that marriage was becoming obsolete.

In short, academics say, American society is in the midst of a fundamental social and demographic shift, the “greatest social change of the last 60 years that we haven't already named and identified,” according to New York University sociologist Eric Klinenberg. [The original story did not include Mr. Klinenberg's full quote.] It is a shift that goes well beyond the dynamics of relationships, affecting everything from housing and health care to child rearing and churches.

And although single women like Denison – educated, urban, and leading a full life – are often portrayed as the poster children of this new nonmarital world order (think “Sex and the City” and writer Kate Bolick’s new book, “Spinster”), the reality is far more complex. The way Americans now couple – or don’t – offers insight into not only evolving views of marriage and family, but into the country’s growing economic, racial, and geographic divides.

“Just as marriages are no longer alike, singleness is no longer all alike,” says Stephanie Coontz, director of research and public education at the Council on Contemporary Families.

Understanding the various facets of the new Singles Nation, it turns out, is key to understanding much about America today.

• • •

There is little debate that American adults are far less likely to be married than they were two generations ago. In 1950, married couples represented 78 percent of households in the United States. In 2011, the US Census Bureau reported, that percentage had dropped to 48 percent. In 2014, the Bureau of Labor Statistics reported that 124.6 million Americans 16 years and older were single, or 50.2 percent of the population, compared with 37.4 percent of the population in 1976.

But underneath those numbers, nuances exist.

One of the big ones has to do with when Americans get married. For years, the average age at which both men and women first marry has been creeping upward, to 27 for women and 29 for men. It was 20 for women and 22 for men in 1960. In other words, there may at any given moment be more single people who have never been married, but that doesn’t mean that those singles are going to stay that way.

But this seemingly simple demographic explanation belies a huge shift in culture. Particularly for college graduates, this delay in marriage has ushered in a new phase of life that sociologists are calling “emerging adulthood” or, less charitably, “delayed adolescence.”

This is a time when people focus on their careers and their own personal fulfillment, sociologists say: They go out to dinner, work late hours, and make close groups of friends that are sometimes dubbed “urban tribes.” And while there has been some hand-wringing about this, with worries about a lack of maturity among young American adults today, a number of scholars who study singles point out that this group is the antidote to another point of cultural anxiety: the decline in community.

College-educated singles are moving into old downtown buildings and spending money in revitalizing urban centers. In cities from Denver to Detroit to Boston, they are joining everything from kickball leagues to museum boards, neighborhood associations to volunteer organizations.

“People who live alone don’t want to be alone or isolated,” Mr. Klinenberg says. “So they spend an enormous time out in public.”

It’s a point that University of California, Santa Barbara professor Bella DePaulo has been trying to make for a long time. Dr. DePaulo, who is happily single, debunks what she says are myths related to the country’s “matrimania.” Her research has found that contrary to conventional wisdom (and a number of studies) married people are no more happy and healthy as a group than their single counterparts. And it is singles, not marrieds, who are the most active in their communities.

“When people get married, they have less contact with their friends, their siblings, their neighborhood,” DePaulo says, adding that studies show this is true even with people who are married and don’t have children. “It’s just the opposite of the stereotype.”

Quite often, she says, single people realize that they enjoy living without a spouse. “People used to think of single life as where you mark time until you get married,” she says. “It’s not. It’s the real thing.”

• • •

But the definition of “single” is a bit vague. Statistically, it simply means unmarried. And that leaves plenty of room for different family structures.

DePaulo, who lives alone, is considered a single. So is Sarah Wright, the board chair of a singles’ advocacy group called Unmarried Equality, who lives with a longtime partner.

“I do not describe myself as ‘single’ because I’m not,” Ms. Wright says. “I am coupled.” When she gets government forms asking for her marital status, she crosses off all the responses and writes in “none.”

Tara Dublin of Portland, Ore., is officially single, even though she was married for years. Today, she says, “I am unmarried, unattached, and have no partner.” Still, she says, friends take umbrage when Ms. Dublin calls herself a single mom, since her ex-husband also cares for their sons.

Hugh Ryan is considered single, as well, even though he lives with two other men in New York, and the three consider themselves a family. (They recently bought a house in Brooklyn together.)

“We have the same stupid fights and the same wonderful stuff as in any relationship,” Mr. Ryan says. “There just happens to be more of it. Once you recognize that the two-parent, two-kid family that married at 22 and are together till the end of their lives is a rarity these days, everything else seems less unusual.”

Denison, for her part, describes herself as “single – sort of.” She has been in a number of relationships since she moved to Boston, some long term, some decidedly short. But she has always lived alone.

Two generations ago, this would have been highly atypical. A female college graduate getting an apartment on her own would have been seen as indecorous. But as Klinenberg points out in his book “Going Solo,” cultural attitudes have changed. Today, living on one’s own is a marker of adulthood.

While openly living with a partner outside of marriage would have been taboo – especially a same-sex partner, as in Wright’s case (not to mention a family such as Ryan’s) – today it is almost expected. The social penalties for sexual relationships outside of marriage have disintegrated, says Andrew Cherlin, a professor of sociology and public policy at Johns Hopkins University in Baltimore.

“It’s less necessary to be married than it used to be,” Dr. Cherlin says. “Before, it was not acceptable to have any other sort of adult life. Now there are alternatives – living with someone but not getting married, not having kids, having kids and not being married. There are alternative ways of forming the family.”

• • •

Denison was 35 years old when she started writing about being single. She was trying to get over a heartache, she says, so she started a blog mostly as an accountability project: If she shouted into cyberspace that she was going out on a date with someone she met on eHarmony, well, then, she’d have to follow through.

“She rents an apartment in a neighborhood of trendy condos,” Denison wrote about herself in her profile. “Her bike is vintage Raleigh. Her car is from 1991. The cat’s litter box is next to her bed and she doesn’t own a dresser.”

Soon, her “Single in the City” blogspot was drawing a small, but loyal, following. Denison wrote about dates. But she also wrote about single living – everything from riding her bike and running marathons, to the challenge of looking sexy in a parka in January, to how much she loved her young nephews.

Eventually Denison met up with some of the local women who were posting regular comments on her blog. They had many of the same experiences. They were also regularly frustrated with dating, Denison recalls.

And this, it turns out, is one of the hallmarks of dating today. Internet dating has allowed individuals to connect as never before, says Lori Gottlieb, a couples therapist and the author of “Marry Him: The Case for Settling for Mr. Good Enough.”

But it has also presented a false sense of choice: If anything comes up that seems slightly imperfect about Man or Woman A, you can sift through 5,000 other profiles to find someone better.

“It’s a very American idea that choice is freedom, freedom is choice,” Ms. Gottlieb says. “But it can really cripple you if you have too much choice.”

In Gottlieb’s opinion, many Americans have never learned how to compromise – something she sees as key for any healthy relationship.

It’s something Denison feels herself, now that she’s in her 40s and, as she puts it, “really tired of meeting new people.”

“For a long stretch, I felt like I was always very clear with being OK with casual. Like, let’s go out and have a drink and be casual. And then at a certain point I wasn’t,” she says. “But I didn’t really know how to let something develop more slowly to the point where it was what I wanted. There is a lot of instant gratification for things. Once you get in that habit, it’s hard to let things go differently.”

Still, the vast majority of people in Denison’s situation will, indeed, get married someday. According to the most recent statistics, Cherlin says, 84 percent of women with college degrees are expected to marry.

It’s one of the main points Ms. Bolick, author of “Spinster,” hopes readers will take away from her work, which tries to put the contemporary conversation about singles in a historical context.

“We’re getting married later,” she says. “This is what you’re doing, so be OK with it. Everything is OK.”

But this is a conversation, says Ms. Coontz, that centers on white, college-educated affluent people. Although there may be a perception that highly educated people are less likely to marry, the reality is that women like Denison and Bolick are far more likely to wed than their less-educated peers.

“These people are the ones who can construct a very, very rewarding single life,” Coontz says. “But they ultimately end up getting married. And they are the ones who stay married.”

For other demographics, however, the situation is far different.

• • •

Sheila Arias sighs when she talks about that central statistic of American relationships – that more than half of American adults are single. “It’s so sad, isn’t it?” she says.

Ms. Arias does not believe being single is fabulous and exciting. For her, it’s just really, really hard.

Arias, who lives in Durham, N.C., is 29 years old and has two young children. She and their father were never married, but were living together when they had their daughter, Jaslene, who is now 6 years old. They had split up and were living separately when they had 3-year-old Neoch and never got back together.

Although the father, who is in the country illegally, has some involvement with the children, Arias has full custody and is the one who most often cares for them. It’s been a struggle. She has a high school diploma but no higher education and lost her job as an interpreter when the state stopped funding the position. She moved in with her dad. While she recently started work at Head Start, which her children have attended, money is still tight.

It is a typical story. In the past decade, the number of cohabitating couples having children has increased 10-fold. But those couples tend to be less educated and far less likely to stay together than married couples. Meanwhile, the financial, logistical, and emotional toll of raising children alone, particularly for a woman without a college degree, is huge.

“The college graduates take some time, but they finally marry,” Cherlin says. “And those college graduates tend to wait until after they are married to have kids. Those without college degrees have fragile, cohabitating units and have kids within those units, leading to lots of turnover in family life.”

Although much of the press about the new Singles Nation has focused on people like Bolick and Denison, Cherlin points out that half of young adults in the US have a high school degree but not a bachelor’s degree. In 2012, 81 percent of bachelor’s degree holders between the ages of 35 and 39 had been married – about the same as in 2000. The figure for everyone else in that age group with less than a bachelor’s degree had dropped nine percentage points, to 73 percent, according to the Pew Research Center.

Part of this is economics, academics say. Fewer jobs and fewer routes toward economic stability exist for people with only high school diplomas. Many lower-income people tell researchers they can’t afford to get married. Coontz says that research also shows that many lower-income women make the calculation that marrying a man with a lower education level will end up destabilizing the home rather than helping it.

All of this ends up creating what some call a “marriage gap.” Wealthier individuals wait until they are older, have more educational degrees, and are more advanced in their careers. Then they marry other professionally successful individuals. Their relationships last longer (the divorce rate among college graduates is around 25 percent, compared with close to 50 percent for those without a college degree), and they are able to pass down both educational and financial capital to their children.

“We have two different family systems in the US,” Cherlin says.

• • •

This issue has long been a focus within the black community. According to the Centers for Disease Control and Prevention, 73 percent of black children are born to single mothers. That includes women who are living with their child’s father. Around 70 percent of black women are single. (Although, as with whites, that number doesn’t mean that they won’t eventually get married. They do – just not as often as other demographic groups.) And the question of marriage – and why it is or isn’t happening – is the regular focus of talk shows, sermons, and studies.

Many scholars point out the effect on black marriage of the so-called war on drugs, and show that the spike in the percentage of never married black men at age 35 mirrors the dramatic increase in the number of black men incarcerated between 1980 and 2000. Others blame old welfare policies that penalized families with men in the home, while others decry the decline in black church attendance. Still others note that black women have a higher average educational attainment than black men, and suggest that women are reluctant to “marry down.”

This fascinated Nika Beamon, who wrote the 2009 book “I Didn’t Work This Hard Just to Get Married: Successful Single Black Women Speak Out.” So much talk, Ms. Beamon says, swirled around black women wanting to find a “good man,” or about the “angry black woman” stereotype, that people were forgetting that many black singles were quite happy as they were. “Single women are painted as miserable,” she says. “And that is just not the picture that I have.”

Although she was looking primarily at the black community, and was aware of issues ranging from incarceration rates to the dearth of black-focused e-dating sites, Beamon’s position ended up sounding a lot like Bolick’s.

“Being single is a transitional state that most of us find ourselves in multiple times,” Beamon says. “And getting married shouldn’t be the end game – you’re going to be single for more of your life. You should probably be focused on being the best ‘you’ you can be.”

• • •

All of which raises the question: Why do people still get married at all? If being single is more common and widely accepted, and if it can be just as fulfilling as being married, why do the majority of Americans still yearn for matrimony?

The answers, scholars say, range from the practical to the religious to the cultural. As advocates such as Wright and DePaulo are quick to point out, married people enjoy a slew of legal and logistical advantages, from Social Security benefits to inheritance rules.

Indeed, a number of singles advocates have been trying to mitigate what they see as unfair practices toward unmarried adults – everything from housing discrimination (in many municipalities landlords can decide not to rent to singles) to attitudes by some employers that singles are more able to work late and take undesirable shifts than their married-with-children colleagues.

“The number of single people is growing and growing,” says DePaulo. “It has been for decades. This should mean that single people have more ... power in our society ... [That’s] not happening at all. The marriage mafia is getting even stronger.”

The growing acceptance of gay marriage, singles advocates say, has meant a joining of the political left and right in support of marriage and legal marriage protections. Part of this, DePaulo believes, is because of increasing insecurity about marriage. And part of it, she says, is comfort in the easy “marriage story.”

“It sounds so simple,” she says. “You just find the one person, you get married, and your whole life path is figured out.”

But other scholars see deeper reasons for society’s continued reverence for matrimony. Marriage, numerous studies have found, increases health, longevity, quality of life, and wealth. Those people who describe themselves as being in “good” marriages are regularly found to be happier than the rest of the population. In the US, married couples, as a group, still provide the most durable family structure for children.

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Many people also believe in marriage as a religious necessity and gift. Even for the secular, marriage as an institution has long been seen as a hallmark of social stability – something woven into the very fabric of American society.

“Marriage still remains a highly valued state,” Cherlin says. “It just doesn’t play as large a role in people’s lives as it used to.”