For many Americans, cohabitating is a necessity, not just a preference. In decades past, many 20- and 30-somethings shared a household with their spouse—nearly half of the adult population lived with a spouse as recently as 2007—but lately, delayed marriage rates, climbing student-loan debt, and rising housing costs have led to increased numbers of doubled-up households, a term used by demographers to describe homes that include additional adults other than the householder or their partner. This includes people who live with roommates or parents. In 2015, about a quarter of Americans between the ages of 18 and 34 lived with roommates, up from 23 percent a decade prior, according to U.S. Census Bureau data. Nearly 32 percent of the overall American adult population lived in a shared household in 2017, an increase from about 29 percent in 1995, according to a Pew Research Center analysis of census data. In examining housing trends among young adults, Jonathan Vespa, a U.S. Census Bureau demographer, noted that by 2015, most adults between the ages of 18 and 34 were not living alone, or with a spouse or an unmarried romantic partner, a dramatic shift from the decade prior when most young adults in most of the country lived independently.

Vespa discovered that, on average, the type of person most likely to live with roommates is between the ages of 18 and 24, has completed some college, but is usually enrolled in school. People who are unemployed are more likely to live with roommates, Vespa found, and rent either a single-family home or an apartment together.

The trend may have been spurred by the 2008 recession, when unemployment rates peaked at 10 percent, according to the Bureau of Labor Statistics. But it seems like a recovering economy hasn’t led young people to change course. Shacking up with roommates has persisted “despite an improving labor market, despite more young grads getting jobs,” says Aaron Terrazas, a senior economist for the real-estate-and-rental website Zillow. In a survey, Zillow found that 30 percent of American adults aged 23 to 65 lived with roommates, up from 21 percent in 2005. “We thought this would be a cyclical phenomenon, but it’s turned out to be quite durable, which is quite surprising.”

One possible explanation is that many young adults and recent graduates who flock to expensive coastal cities for job opportunities find themselves shacking up with friends, or sometimes complete strangers, to cut costs. Zillow found that Los Angeles, Miami, and San Francisco were among the top cities for adults living in doubled-up households; nearly half of adults in Los Angeles lived with a non-partner. “You think of people moving to booming job centers and they don’t have a family network to rely on in these areas,” Terrazas says.

The United States has seen this phenomenon before. As people moved to cities seeking work in the 19th century, boarding houses became hubs where diverse residents—immigrants, single men and women, workers of all kinds—could live affordably and mingle with others in shared spaces. Now, as housing becomes increasingly scarce and rents continue to rise (cities like Orlando, Salt Lake City, and Knoxville are experiencing the fastest rent growth in the country), the boarding-house experience is back, just at a smaller scale. Along with it comes the proliferation of a unique sort of relationship—sharing your home life, and all its little intimacies, with someone, or multiple someones, who you aren’t related to, and who you may not even be friends with.