In Pittsburgh, the term "boomerang" refers to someone who left the city after college but whose deep affection for it draws them back.

For sociologists, the term refers to an increasingly common phenomenon of men and women over the age of 25 moving back in with their parents, often out of economic necessity.

A study released Thursday by the Pew Research Center in Washington, D.C., shows a recent significant bump in the number of grown people sharing a roof with their parents and the number of people living in households where three or more generations are represented.

The study, titled "The Return of the Multi-Generational Family Household," shows that since 1980, there has been an upward trend in the number of multi-generational family households, defined as households with three or more generations, households with a grown children (over 25) and parents, and households with a "skipped" generation, such as one where a grandparent is raising a grandchild.

After World War II, the portion of Americans living of multi-generational family households fell by more than half until 1980, when the trend reversed itself. From 1980 to 2008, this portion rose by a third to 16 percent of the total U.S. population, or around 49 million Americans.

One of the most significant jumps was seen from 2007 to 2008, when the number of Americans living in multi-generational households rose by 2.6 million people.

The reasons behind the rise over the past two decades are complex, said Paul Taylor, the director of the center's Social and Demographic Trends project, but it's largely rooted in the increased number of immigrants, who are more inclined to live in multi-generational households, and the rise in the average age people marry, since unmarried people are more likely to live at home.

But the most recent spike seems to be rooted in the souring economy.

"Virtually every demographic group we looked ... went up from '07 to '08, leading us to believe there is most certainly an economic element to this," Mr. Taylor said.

Mr. Taylor said that baby boomers, still the country's largest age group, are not only taking in elderly parents but also "have boomeranged back because ... they got laid off." And people in their 20s, who are the hardest hit by the Great Recession, also are contributing to the trend as they've been unable to leave home or have also been forced to move back in with their parents.

"You have a lot of people losing jobs and losing homes," he said, noting that foreclosures also seem to have played a role in the recent rise. "The family has become the ultimate social safety net."

That has been the case for the Eric and Bonnie Olson, whose two grown daughters have lived with them on and off since they finished college. Right now, their three-story Crafton residence is home to them, their 23-year-old daughter Veronica and her boyfriend, who occupy the third floor, and, until recently, 27-year-old Victoria, who still drives home every weekend from her job in West Virginia.

Mrs. Olson said she never imagined that her daughters would move home after college. Then, her daughter Victoria's job at a company that sold real estate guides was thrown into jeopardy by the housing crisis, so she moved back to Pittsburgh to become an abstractor, helping oil companies obtain land leases to drill for Marcellus shale gas. When the oil business stalled, she was out of a job, and had to give up her apartment in Squirrel Hill to move home. Last fall, she found similar work with a company in a small town in West Virginia, but she makes the two-hour drive home every weekend because there's not much to do there.

"When [Victoria] was laid off, there was just no way, it was just too uncertain to sign any lease," Mrs. Olson said. "It was a hard decision. She didn't want to come home. But we adjusted."

Veronica moved home with her boyfriend after graduating from University of Wyoming to get her master's degree in professional writing at Carnegie Mellon University. Mrs. Olson said it simply didn't make sense for her daughter, who can only work 20 hours a week, to get her own place.

Mrs. Olson said the situation is manageable for now. She said both of her daughters are responsible and work hard, so she doesn't worry about them. Still, she anticipates both will eventually leave home permanently and she's looking forward to a quieter house with a lower grocery bill.

"This isn't forever," she said. "I don't see us growing old together."

Mr. Taylor also hypothesized that the millenials, or the demographic of ages 18 to 29, are less hesitant to stay at home or move back in because studies have shown that they have far better relationships with their parents than the baby boomer generation had with theirs. So some may choose to move back in not out of just an economic imperative, but also "because Mom and Dad are buddies."

The study also showed that a significantly larger portion of blacks live in multi-generational situations than whites -- about 23 percent compared to 13 percent.

Larry Davis, dean of the University of Pittsburgh School of Social Work, said the historic rise in the number of multi-generational families among blacks has been driven by a number of factors, including the rise of single parenthood and teenage pregnancy. But this, too, is an outgrowth of economics, he said.

Black children, for example, are about three times more likely to be raised by grandparents than white children, Mr. Davis said.

"It's more of an economic factor for African-Americans than it is for other groups," Mr. Davis said.

Washington correspondent Daniel Malloy writes the "Pittsburgh On The Potomac" blog exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.