For the first time in modern history, more 18-to-34-year-olds live with their parents than in any other living arrangement, according to a Pew Research Center report released Tuesday.

In 2014, nearly one-third of young adults lived in their parents’ home, a bigger group than those living with a spouse or romantic partner, living alone or with roommates, or living as single parents.

While millennials moving back with their parents have been the butt of jokes and hand-wringing for several years, and the recession of 2009 played a part in their doing so, this shift spans more than one generation. It has been decades in the making, a result of deep-rooted societal transformations in education, work and family building.

Since 1880, when the Census Bureau started keeping track, the most common arrangement for young people had been to live with a spouse or a significant other. That peaked in 1960, at 62 percent. But over the past 50 years, their options have opened up, making marriage just one of several possibilities.

As a result, the portion of young Americans settling down romantically has plunged to 31.6 percent, falling to second place for the first time.

“For earlier generations of young Americans, one of the major activities that they were focused on was partnering, forming a new family, maybe with children,” said Richard Fry, the study’s author. Now, they spend more time tending to studies and work, hoping to save enough to move out on their own.

A big reason is a decline in economic opportunities. As the cost of living has escalated and wages have stagnated, mounting student debt and rising home prices create obstacles to cohabitation and marriage.

“If you’re not living with your parents, you’re living with your roommates,” said Laura Zelaya, 28, a news producer who lives with her parents in Falls Church, Va., while she saves to buy a house. Her brother and sister also came home after college. “I don’t see a lot of people my age living alone.”

The trend is led by young men, whose fortunes have been waning since the 1960s. While they have always lived with their parents in greater numbers than young women, this has been their dominant housing arrangement since 2009. In 2014, 35 percent lived with parents, while 28 percent lived with a spouse or partner. For young women, the percentages are flipped: 29 with parents and 35 with partners; the difference is explained by the fact that young women tend to marry slightly older men.

Unemployed young men are more likely to live with their parents than young men with jobs, and employment among young men has dropped significantly in recent decades.

“I moved in with my parents because I don’t really have to pay rent and I get free meals,” said Marshall Taliaferro, 25, of Leesburg, Va.

Taliaferro, who works in his father’s advertising agency and at a concert venue, says the setup is far from what he dreams of for himself.

“My ideal life is to be married, with maybe a kid or two, and at that point I would not be living with my parents; I would be living with my wife or girlfriend . . . and substantial enough pay. No parents would be lovely.”

The trend has significant economic and demographic implications. People who delay starting families could face fertility challenges down the road, and in the near term, “the spending that goes on in the formation of a household — the furniture purchases, the appliance purchases, the cable subscriptions — that isn’t happening,” Fry said.

But the shift goes beyond economics. The marriage rate began to fall in the 1960s as options for young people were widening.

“The main driving force in the past for living apart from family was getting married, and people used to marry young,” said Michael Rosenfeld, a sociology professor at Stanford University. “Part of the reason women lived with their parents was they couldn’t afford to live on their own, and there were social pressures against doing so.”

But the introduction of the birth-control pill, the fading stigmas against premarital sex and out-of-wedlock childbirth, and the entry of more women into the workforce changed the landscape.

As a result, the median age of first marriage has risen from a 1956 low of 20.1 for women and 22.5 for men to 27.1 for women and 29.2 for men.

“Getting married early has lost a lot of its motivation for young people because young people have fewer kids” — meaning they don’t have to start early — “and women don’t need a man to support them . . . so people are more picky,” Rosenfeld said. “Those who can afford to live on their own tend to prefer that.”

In fact, the portion of young people living with their parents was even higher in 1940, at 35 percent. But many more people were married then.

Karla Torres, 25, and her boyfriend would love to live on their own. They have lived on and off with her mother in Falls Church, Va., since graduating from college and plan to move back in with her mother next week.

“When we’ve been living on our own, we haven’t been able to save,” said Torres, a news producer. She hopes to go to graduate school and her boyfriend wants to travel, so the move made sense.

“There was something of, like, ‘I have a full-time job; I should be able to live on my own,’ ” she said. “But realistically, this is our best option.”

Philip Cohen, a sociology professor at the University of Maryland, said the study signals an important demographic milestone.

“I see this as part of an overall trend in an increase in family diversity and decline in the nuclear-family household,” he said.

It also reflects a change in young women’s expectations and prospects. “Young women really don’t want to be dependent on a man they’re going to marry, and also they think they might have a better selection” if they wait until their careers are launched, he said. They may be right, he said. “A large number of men say they want a wife who is a major financial contributor to the household.”

And the stigma seems to be fading.

“I was a little embarrassed; I was like, ‘Oh, my gosh, does this mean I’m a failure?’ ” said Kimberly Moser, 24, who moved in with her parents in Culver City, Calif., while she attended graduate school. “But when I tell people, I see it’s more accepted. They say, ‘Oh, yeah, that’s smart; save money.’ ”

Even so, it can restrict social lives. “Dealing with your parents, it’s hard to have people come over and do your own thing, have a party,” said Denis Burt, 26, of Ashburn, Va., who has lived with his parents aside from a couple of years during college.

It also can hamper romance. When Burt dated someone who was also living with parents, it was tricky. “You’re always trying to schedule times when you can be alone in your house.”

The trend is more pronounced among minorities, the study found, with 36 percent of young black and Hispanic people doing so.

In part, this is cultural. “My family’s Venezuelan, and I feel like it’s very normal in my culture not to leave till we get married,” Torres said, adding that her sister also lived at home until recently marrying.

But even though the percentage of minorities in the United States has risen, minorities are not driving the change. Among whites, the shift since 1960 is stark: from 19 percent living with parents then to 30 percent in 2014.

The study also found that people with lower education levels are more likely to live with parents than with romantic partners, while the more highly educated are more likely to live with romantic partners.

That does not surprise Cohen, the U-Md. professor. “Marriage has declined faster for people with low levels of education, and that has a lot to do with their ability to attain the kind of economic security to make them feel able to settle down and be excited to do so.”

For them, he said, cohabitation is not necessarily a one-way street, especially as middle-aged people are less likely to own their homes now than 20 or 30 years ago.

“The care and support flows up and down the generations, especially among poorer people,” he said. “Now it’s more likely that both generations are economically insecure, and they’re taking care of each other.”

That is not a bad thing, said Stephanie Coontz, director of research at the Council on Contemporary Families. “The kids who get this kind of support from their parents are more independent in their later years, because you’ve been able to provide them with that safety net.”

And while marriage is not dead, she said, it is no longer the main driver for young people.

“It’s not the only way that they organize their major decisions and transitions” such as buying a house, having children or forming social networks. “That could actually help in the long run, because you’re not putting all your eggs in one basket.”