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

Ethnicity differences

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

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

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

What accounts for this meeting of trend lines — the recent rise in the percentage of young adults living with parents and the decline 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 and women plays a big part. Another likely (and related) factor is the decline over the last several decades in both the share of employed young men and the level of their wages. The picture isn’t quite as clear for young women, who have seen their labor-market prospects improve, but those struggling young men may not be the most appealing partners.

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

But “both the upswing in living with mom and dad and the decline in young adults partnering in their own household” have been “decades in the making,” said Richard Fry, the report’s author.

Jed Kolko, a senior fellow at the Terner Center for Housing Innovation at the University of California-Berkeley (who consults for Bloomberg Beta), has also looked at data on young Americans living with their parents over the last couple of decades. Though the data in the recent report go only through 2014, both Kolko and Fry observed that the share of young adults living with parents hasn't declined more recently. Kolko did note that “the increase in rents since the recession has made it harder for some young people to move out of their parents’ homes.”

Long-term shifts

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

“If we look back over the last century, we can see that the rush out of the parental home was a post-World War II phenomenon,” wrote Richard Settersten of Oregon State University in a 2014 Washington Post article.

Millennials and their parents may also simply be more comfortable with living together. Jeffrey Jensen Arnett, a research professor of psychology at Clark University who studies “emerging adulthood,” said that during 20 years of researching this, he has seen “an increasing acceptance that it takes longer to grow up than it used to,” adding that there’s now less stigma attached to remaining at home with one’s parents.

“There’s a lot of good will between parents and children in this generation,” Arnett said, adding that “boomers have succeeded in having these relationships with their children, that by the time they’re in their twenties, it is almost like a friendship. It will never be quite like a friendship, but it’s a lot closer to that than it was in previous generations.”

Arnett admires "that they have this kind of support from their parents" and their parents "can have this kind of close relationship before the emerging adults go off for the last time.”

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

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