As recently as 2000, nearly 43 percent of young adults, ages 18 to 34, were married or living with a partner. By 2014, that proportion was just 31.6 percent.
In 2000, only 23 percent of young adults were living with parents. In 2014, the figure reached 32.1 percent.
The Pew researchers point to several ongoing forces at work, including the ongoing trend of young adults postponing marriage.
The typical U.S. woman now marries at 27.1 years old, the typical man at 29.2, according to census data. That's up from record lows of 20.1 for women and 22.5 for men in 1956.
"They're concentrating more on school, careers and work and less focused on forming new families, spouses or partners and children," said Richard Fry, lead author of the report and a senior economist at the Pew Research Center. Fry said of the millennials.
Among young men, the delay in moving out on their own is partly due to declining employment and falling wages, the Pew researchers said. The share of 18 to 34 year old with jobs fell to 71 percent in 2014, the report found, from 84 percent in 1960 — the year when the proportion of young adults living outside the home peaked.
That financial burden is compounded by falling incomes, rising rents and the growth of student debt. Adjusted for inflation, wages for young men fell by a third between 2000 and 2014.