Over the last four decades, the financial circumstances into which children have been born have increasingly determined where they have ended up as adults. But an expansive new study, based on billions of social media connections, has uncovered a powerful exception to that pattern that helps explain why certain places offer a path out of poverty.

For poor children, living in an area where people have more friendships that cut across class lines significantly increases how much they earn in adulthood, the new research found.

The study, published Monday in Nature, analyzed the Facebook friendships of 72 million people, amounting to 84 percent of U.S. adults aged 25 to 44.

Previously, it was clear that some neighborhoods were much better than others at removing barriers to climbing the income ladder, but it wasn’t clear why. The new analysis — the biggest of its kind — found the degree to which the rich and poor were connected explained why a neighborhood’s children did better later in life, more than any other factor.

The effect was profound. The study found that if poor children grew up in neighborhoods where 70 percent of their friends were wealthy — the typical rate of friendship for higher-income children — it would increase their future incomes by 20 percent, on average.

These cross-class friendships — what the researchers called economic connectedness — had a stronger impact than school quality, family structure, job availability or a community’s racial composition. The people you know, the study suggests, open up opportunities, and the growing class divide in the United States closes them off.

“Growing up in a community connected across class lines improves kids’ outcomes and gives them a better shot at rising out of poverty,” said Raj Chetty, an economist at Harvard and the director of Opportunity Insights, which studies the roots of inequality and the contributors to economic mobility. He was one of the study’s four principal authors, with Johannes Stroebel and Theresa Kuchler of N.Y.U. and Matthew O. Jackson of Stanford and the Santa Fe Institute.

The findings show the limitations of many attempts to increase diversity — like school busing, multifamily zoning and affirmative action. Bringing people together is not enough on its own to increase opportunity, the study suggests. Whether they form relationships matters just as much.

“People interested in creating economic connectedness should equally focus on getting people with different incomes to interact,” Professor Stroebel said.

Growing up, Jimarielle Bowie says, her family was lower-middle class. Her parents divorced, lost jobs and lost homes in the housing crisis of the late 2000s. So when she made friends in high school with girls who lived on the rich side of town, their lifestyles intrigued her. Their houses were bigger; they ate different foods; and their parents — doctors, lawyers and pastors — had different goals and plans for their children, including applying for college.

“My mom really instilled working hard in us — being knowledgeable about our family history, you have to be better, you have to do better,” said Ms. Bowie, 24, who goes by Mari. “But I didn’t know anything about the SAT, and my friends’ parents signed up for this class, so I thought I should do that. I had friends’ parents look at my personal statements.”

Ms. Bowie became the first person in her family to get a postgraduate degree. She’s now a criminal defense lawyer — a job she found through a friend of one of those high school friends.

“My experience meeting people who were more affluent, I got to get in those circles, understand how those people think,” she said. “I absolutely think it made a significant difference.”

Birds of a feather

Social capital, the network of people’s relationships and how they’re influenced by them, has long intrigued social scientists. The first known use of the phrase was in 1916, by L.J. Hanifan, a school administrator in West Virginia. Since then, researchers have found that ties to more educated or affluent people, starting in childhood, can shape aspirations, college-going and career paths.

But the new study is the first to show that living in a place that fosters these connections causes better economic outcomes, using a significantly larger data set than other studies, covering 21 billion Facebook friendships.

The researchers limited the data, which did not include names, to active Facebook users. They estimated users’ incomes based on their ZIP codes, college, phone model, age and other characteristics.

For each low-income Facebook user, the researchers determined where the person was currently living, and how many high-income friends they had. That gave them a measure of how economically connected each neighborhood was. Then they compared the new data with earlier research that used tax records to measure how much a particular neighborhood raised low-income children’s economic prospects.

The researchers were also able to link almost 20 million users to both their high school and to their parents on Facebook. Using those ties, they repeated their analysis, this time on high school connections between children of rich and poor parents, to measure the impact of relationships made early in life. They did a similar analysis for teenage Instagram users. And they built on an earlier analysis of siblings who moved at different ages to show that it was the place that made a difference, versus something about the families who moved to those places.

Each analysis had the same result: The more connections between the rich and poor, the better the neighborhood was at lifting children from poverty. After accounting for these connections, other characteristics that the researchers analyzed — including the neighborhood’s racial composition, poverty level and school quality — mattered less for upward mobility, or not at all.

“It’s a big deal because I think what we lack in America today, and what’s been dropping catastrophically over the last 50 years, is what I call ‘bridging social capital’ — informal ties that lead us to people who are unlike us,” said Robert Putnam, the political scientist at Harvard who wrote “Bowling Alone” and “Our Kids,” about the decline of social capital in the United States. “And it’s a really big deal because it provides a number of avenues or clues by which we might begin to move this country in a better direction.”

Other kinds of social capital matter, too, like rates of volunteering in a community and friendships with people from similar backgrounds. Yet the new study shows that even in places lacking in other kinds of social capital, an increase in cross-class relationships is enough to benefit children’s economic prospects. And it’s this kind of social capital that has decreased as the country has become more segregated by class. In recent decades, people have become more likely to live in neighborhoods and attend schools with people of similar economic status — behavior that social scientists say is driven by anxiety about falling down the income ladder in an age of growing inequality.

“The pressure that parents feel to try to give their kids a competitive advantage is amplified when society is unequal and there’s more to be lost,” said Jessica Calarco, a sociologist at Indiana University who studies inequality in schools and among families. “Our society is structured in ways that discourage these kinds of cross-class friendships from happening, and many parents, often white, are making choices about where to live and what extracurriculars to put their kids into that make those connections less likely to happen.”

As a result, rich people have mostly rich friends, and poor people have mostly poor friends.

Low-income people are far more likely than high-income people to make friends in their neighborhoods, the study found. But in poorer areas, there are fewer rich people nearby to befriend.

It’s human nature to befriend people who are similar, which is why most cultures have a phrase like “birds of a feather flock together,” Professor Putnam said. Even when people do form cross-class connections, there is evidence in this and other research that they gravitate toward people of the same race.

Ms. Bowie, who is Black and Japanese, said that the friends she made from wealthier families were also Black.

“Just being with Black people who had money was a culture shock,” she said. “But white people with money had a completely different lifestyle. At least with Black people, we had the same sayings, we saw the same movies, our grandparents had the same beliefs.”

The analysis did not directly measure the role of race, which was not provided in the Facebook data. (Though there are techniques researchers use to guess race, the authors of the new study did not use them.) But in more racially diverse places, the study found fewer cross-class relationships.

Race is clearly associated with levels of mobility, a variety of research has shown, including by Professor Chetty’s team. In general, Black people in segregated areas are more likely to experience concentrated poverty and have worse economic outcomes.

“There was speculation that maybe it was about differences in resources, quality of schools, social norms,” he said. “What we show here is places that have large Black populations tend to be more economically disconnected — both Black and white people living there have fewer high-income friends.”

It’s clear that other factors also influence outcomes for Black people in both segregated and integrated areas, he said, including racial discrimination in the labor market and mass incarceration.

But the researchers say their findings on the importance of cross-class relationships are true regardless of race. They found the same relationship — high economic connectedness leading to higher economic mobility — in neighborhoods that were nearly entirely white, Black or Hispanic.

‘A culture of success’

The researchers focused on high schools, one of the few settings where people of all classes make friends at similar rates, and a place where people form lifelong friendships before they start making decisions that may determine their economic trajectories.

Angelo Rodriguez High School in Fairfield, Calif., which Ms. Bowie attended, had more cross-class friendships than the average large public high school.

Fairfield, midway between Sacramento and San Francisco, is an unusually diverse area, racially and economically, and three-quarters of Rodriguez’s student body of around 2,000 are students of color. The school, which opened in 2001, had a catchment area shaped like a reverse C, drawing from neighborhoods on the far sides of town — which is how Ms. Bowie ended up commuting to a wealthier area for school. It also allows some students outside the boundary to attend.

In general, larger and more diverse schools — both economically and racially — have a smaller share of cross-class connections. It can be harder to make friends in large groups, and there are more chances to form cliques with people from similar backgrounds. But Rodriguez High nurtured cross-class friendships in ways both planned and unintentional.

“Being at Rod, you become friends with everybody,” Ms. Bowie said. “Literally that’s what that school does.”

One thing that may have helped was the school’s campus layout, with a promenade around a central library, outdoor stage and quad. That was deliberate, said John Diffenderfer, president of Aedis Architects, which designed the campus: “Accidental unstructured interactions between students was a very high priority.”

Rodriguez High has a block schedule in which classes meet for two hours each, every other day. This creates small, diverse groups that spend a lot of time together. When large institutions do this, it helps foster cross-class friendships, the research found. Separating students based on academic achievement, through gifted or international baccalaureate programs, has the opposite effect.

Extracurricular activities and interest clubs also play a big role in bringing together students from different backgrounds, said Catie Coniconde, a Rodriguez school counselor who also graduated from the school, in 2006. Half the student body is enrolled in them.

“Kids get identified by their extracurriculars, more than race or socioeconomic status,” she said. “There’s the athletes, the band kids, the kids who are interested in anime.”

Timothy Malacarne, a sociologist at Nevada State College, has found that the arts in particular seem to foster friendships across racial and economic lines. “Anytime people do something hard together, they’re more likely to feel a sense of kinship that’s not based on the identity of your racial or socioeconomic group,” he said.

Athletics is another avenue, but sports are becoming more segregated as expensive club sports have overtaken recreational or school teams. Some school districts have begun offering free transportation, physical exams and equipment to allow more students to participate.

While pursuing shared extracurricular interests, the students begin to share aspirations, Ms. Coniconde said. Scoring well on the SAT and attending a four-year college are common goals at Rodriguez, she said. The students from the wealthier part of town usually arrive with those goals, while many students from lower-income families hadn’t considered them before.

“It just seemed like a culture of success,” she said. “The four-year push was huge at Rod, and it still is to this day.”

The power of friends

These ideas can be applied beyond high schools, the researchers said.

Colleges, for instance, could place incoming students with small groups of roommates from a variety of backgrounds, instead of randomly assigning them roommates or allowing them to choose their own. Yale does this, and the small groups generally live in the same dorms all four years. Its low-income students have unusually high levels of cross-class friendships. (It has fewer total such friendships, because low-income students are a minority, though it has committed to increasing the share to one-fifth by 2025.)

Large state colleges, where students can choose their roommates or sort into fraternities or sororities or off-campus housing, tend to have fewer friendships across class lines. The University of Mississippi, where more than a third of undergraduates participate in Greek life, has one of the lowest rates of cross-class friendships of any large public university.

Cities could prioritize centrally located parks and other communal spaces, especially those that are free. Libraries could host radio studios, writing centers, cafes, maker spaces, tool libraries and other programming that appeals to wealthier residents while continuing to offer critical services to low-income residents, bringing both groups together in one physical space.

And mentorship programs can build cross-class relationships, by pairing people from different socioeconomic backgrounds for extended periods. Friends of the Children matches at-risk children with paid professional mentors who spend four hours a week with them from kindergarten through high school graduation. Becoming a Man enrolls boys in school-based counseling groups and matches them with young professionals as mentors. Both have been shown to have powerful effects on children’s outcomes.

Ms. Bowie still lives in Fairfield and remains close with both sets of friends — those from her neighborhood and those she met in high school. Their trajectories have differed. Most of her neighborhood friends went to community college, live near home and are still figuring out what to do, she said. Her high school friends left town for four-year colleges and are pursuing careers in medicine and design.

Her experience straddling both worlds, she said, has been essential for her professional success.

“I got a lot of in-depth knowledge about cultures that I wouldn’t have gotten, not in college, law school or now, because of my high school experience,” she said. “Had I not had those experiences, I might have been really culture shocked by going into these spaces as an attorney.”