(Bloomberg) -- Those basement-dwelling millennialsare at it again.

In 2014, Americans 18 to 34 years old were a little bitlikelier to be living in their parents’ home than with aspouse or partner in their own household, according to aPew Research Center analysis of U.S. Census Bureaudata, released on Tuesday. It's the first time that hashappened in the modern era.

Young men have long been more likely than their female counterparts to beroommates with mom and/or dad. The share of young men living intheir parents’ homes most recently surpassed the share living withpartners in their own households in 2009, but as of 2014, thecrossover still hadn’t occurred for young women.

Still, the proportions of both male and female 18- to34-year-olds living at home are high — 35% for the men, 29% for thewomen — and have grown in recent years, while the shares of thoseliving with partners have plummeted.

A similar story emerges when the data are broken down byeducation. Young adults without bachelor’s degrees are more likelyto live in their parents' homes, which in 2008 became morecommon than residing with partners did.

Ethnicity differences

By ethnicity, living with parents overtook living with a spouse orunmarried partner in 1980 for young blacks, in 2007 for youngAmerican Indians/Alaska natives, and in 2011 for young Hispanics.Young whites and Asians/Pacific Islanders were still more likely tolive with partners in 2014.

“Trends in living arrangements for specific groups of youngadults indicate that the crossover is being driven by theexperiences of more economically disadvantaged young adults,specifically, less-educated young adults and some racial and ethnicminorities,” the report says.

Indeed, while the overall proportion of 18- to 34-year-oldsliving with their parents didn't peak in 2014 — that occurredaround 1940 — the shares of young blacks and Hispanics (aswell as young people without high school degrees) living withparents were at their highest in recorded history.

What accounts for this meeting of trend lines — the recent risein the percentage of young adults living with parents and thedecline in those living with spouses or unmarried partners?

Age for first marriage has risen

The increase in the median age at first marriage for both men andwomen plays a big part. Another likely (and related) factor is thedecline over the last several decades in both the share ofemployed young men and the level of their wages. The pictureisn’t quite as clear for young women, who have seen theirlabor-market prospects improve, but those struggling young men maynot be the most appealing partners.

The report also notes that “initially in the wake of therecession, college enrollments expanded, boosting the ranks ofyoung adults living at home. And given the weak job opportunitiesfacing young adults, living at home was part of the private safetynet helping young adults to weather the economic storm.”

But “both the upswing in living with mom and dad and thedecline in young adults partnering in their own household” havebeen “decades in the making,” said Richard Fry, the report’sauthor.

Jed Kolko, a senior fellow at the Terner Center for HousingInnovation at the University of California-Berkeley (whoconsults for Bloomberg Beta), has also looked at data on youngAmericans living with their parents over the last couple ofdecades. Though the data in the recent report go only through 2014,both Kolko and Fry observed that the share of youngadults living with parents hasn't declined morerecently. Kolko did note that “the increase in rentssince the recession has made it harder for some young people tomove out of their parents’ homes.”

Long-term shifts

So while it’s fun to deal in superlatives — the first time in themodern era! — these seem to be long-term shifts, not hordes ofrecession-wary millennials suddenly dashing from the altar to theirparents’ basements. In fact, if you go back far enough, one shiftdoesn’t even look that shifty. The share of young adults living intheir parents’ home was almost exactly the same in 1900 as it wasin 2014.

“If we look back over the last century, we can see that the rushout of the parental home was a post-World War II phenomenon,” wroteRichard Settersten of Oregon State University in a 2014 WashingtonPost article.

Millennials and their parents may also simply be morecomfortable with living together. Jeffrey Jensen Arnett, aresearch professor of psychology at Clark University who studies“emerging adulthood,” said that during 20 years of researchingthis, he has seen “an increasing acceptance that it takes longer togrow up than it used to,” adding that there’s now less stigmaattached to remaining at home with one’s parents.

“There’s a lot of good will between parents and children in thisgeneration,” Arnett said, adding that “boomers have succeeded inhaving these relationships with their children, that by the timethey’re in their twenties, it is almost like a friendship. Itwill never be quite like a friendship, but it’s a lot closer tothat than it was in previous generations.”

Arnett admires "that they have this kind of support from theirparents" and their parents "can have this kind of closerelationship before the emerging adults go off for the lasttime.”

Related: 10 U.S. cities millennials can actually afford tolive in

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